CHARACTERIZING OPTICAL COUNTERPARTS OF X-RAY SOURCES IN
THE CORE OF OMEGA CENTAURI
A thesis presented to the faculty of San Francisco State University In partial fulfilment of Z e \ * i The Requirements for The Degree fHY* M 8 ?
Master of Science In Physics
by
Kyle Murphy
San Francisco, California
May 2019 Copyright by Kyle Murphy 2019 CERTIFICATION OF APPROVAL
I certify that I have read Characterizing Optical Counterparts of X-Ray
Sources in the Core of Omega Centauri by Kyle Murphy and that in my opinion this work meets the criteria for approving a thesis submitted in partial fulfillment of the requirements for the degree: Master of Science in Physics at San Francisco State University.
Professor of Physics & Astronomy
•IbsepK Barranco Associate Professor of Physics & Astronomy
Huizhong Xu Associate Professor of Physics CHARACTERIZING OPTICAL COUNTERPARTS OF X-RAY SOURCES IN
THE CORE OF OMEGA CENTAURI
Kyle Murphy San Francisco State University 2019
X-ray imaging of globular clusters is a powerful tool to determine their overall X-ray emissivity as well as identifying their binary star populations that greatly influence cluster dynamics and evolution. One such cluster in which a great deal has yet to be discovered about its population of X-ray emitting binaries is Omega Centauri, the most massive (4x 106 Msun) globular cluster in the Milky Way. 67 X-ray sources have already been detected by the Chandra X-ray Observatory within its large core (rc = 3.9 pc = 155"). Identifying the optical counterparts of these X-ray sources is almost always necessary to properly classify the source of the X-ray emission. Utilizing the most comprehensive catalog of photometry and proper motions ever constructed for a globular cluster, based on over 650 exposures taken in 26 different filters by the Hubble Space Telescope’s Wide-Field Camera 3, we constructed numerous color- magnitude diagrams to search for and characterize cluster binary stars. We have identified ten new possible binary stars in the core of Omega Centauri. We also recovered seven previously known systems and provided independent confirmation that they have the properties expected of binary stars. These potential optical counterparts include five cataclysmic variables, four faint blue cataclysmic variables, five active binaries, and three red stragglers that may be associated with the metal- rich anomalous giant and subgiant branches of the cluster.
I certify that the Abstract is a correct representation of the content of this thesis. ACKNOWLEDGMENTS
I would first like to thank my incredible advisor, Dr. Adrienne Cool, whose expertise, mentorship, and encouragement made this thesis possi ble. Thank you so much for your unwavering support through each stage of the process and everything you have taught me. I would also like to thank the other members of my thesis committee, Dr. Joseph Barranco, and Dr. Huizhong Xu, for their insightful suggestions and questions. Fi nally, I wish to express my deep and sincere gratitude to my parents for their unconditional love and support. I can never thank you enough for everything you have done for me.
v TABLE OF CONTENTS
1 Introduction...... 1
1.1 Binary Stars in Globular C lusters...... 1
1.2 Omega Centauri...... 4
1.3 Searching for Optical Counterparts of X-ray Sources...... 8
2 Observations Sz D a t a ...... 11
2.1 Chandra X-ray Observations...... 11
2.2 HST Observations...... 14
3 A nalysis...... 23
3.1 Color-magnitude Diagrams of Omega Centauri...... 23
3.2 Equivalent W id th ...... 27
3.3 Classes of Optical Counterparts within Omega Centauri...... 29
4 Results...... 35
4.1 Cataclysmic Variables ...... 35
4.1.1 1 2 a ...... 35
4.1.2 1 3 a ...... 36
4.1.3 1 3 c ...... 37
4.1.4 1 3 f ...... 37
4.1.5 2 3 b ...... 38
4.1.6 2 4 c ...... 39
vi 4.1.7 2 2 h ...... 40
4.2 Faint Blue Cataclysmic V aria bles...... 41
4.2.1 2 1 b ...... 41
4.2.2 2 2 c ...... 42
4.2.3 I l f ...... 43
4.2.4 2 2 i ...... 44
4.3 Active Bin a ries...... 45
4.3.1 1 4 a ...... 45
4.3.2 2 4 e ...... 45
4.3.3 1 3 e ...... 46
4.3.4 1 4 f ...... 48
4.3.5 2 3 a ...... 48
4.3.6 2 4 h ...... 50
4.4 Red Stragglers or RGB/SGB-a S t a r s ...... 51
4.4.1 1 3 b ...... 52
4.4.2 2 2 e ...... 53
4.4.3 2 4 f ...... 53
4.4.4 l i e ...... 53
4.4.5 1 4 b ...... 54
4.4.6 2 2 b ...... 55
4.5 Blue-only and Ha-only Stars...... 56
vii 4.5.1 2 4 a ...... 57
4.5.2 2 1 c ...... 58
4.5.3 2 1 d ...... 58
4.5.4 3 3 d ...... 58
4.6 Variable Star ...... 60
4.6.1 l i b ...... 60
4.7 Foreground S t a r s ...... 62
4.7.1 2 2 a ...... 62
4.7.2 2 2 d ...... 62
4.7.3 2 2 j ...... 63
4.7.4 1 4 c ...... 64
4.8 Active Galactic N u c le i...... 65
4.8.1 2 2 f ...... 66
4.8.2 2 3 c ...... 67
4.8.3 2 4 g ...... 67
4.8.4 3 1 b ...... 68
4.9 Proper M o tio n s...... 68
4.10 Summary of Results ...... 69
5 Discussion...... 72
5.1 Future W ork...... 75
viii Bibliography LIST OF TABLES
Table Page
2.1 List of the 8 selected UVIS filters and their respective exposure times. 19
2.2 The 67 X-ray sources with their Cycle 1 and Cycle 13 fluxes in the
0.5 — 4.5 keV “medium” X-ray band, error circle radii in arcseconds,
and coverage in the eight selected UVIS filters. The error circle sizes
for the sources detected in Cycle 13 were determined by Henleywillis
et al. (2018). For the ten X-ray sources that were detected in Cycle
1 but not redetected in Cycle 13, we calculated the error circle sizes.
See Chapter 3 for further details. If the source had coverage in a filter
it was marked with a / symbol and if the source had partial coverage
it was marked with a ~ symbol (e.g. Ha656 coverage of 22h)...... 21
x 2.3 The 23 X-ray Sources in which potential optical counterparts were
identified by Cool et al. (2013) with their previous optical identifica
tions, offset from the center of the X-ray error circle in arcseconds, and
the number of images in which the counterpart was detected for each
filter. Abbreviations for the source classifications: cataclysmic vari
able (CV), possible CV (CV?), faint blue CV (fbCV), BY Draconis
binary (BYDra), blue straggler (BS), star located on or near anoma
lous subgiant or giant branch (RGB/SGB-a), Ha-bright source with
no measurable blue excess (Ha-only), foreground star (FGND), back
ground active galactic nucleus (AGN), possible AGN (AGN?), blue
source with no measurable Ha-excess (Blue-only). In filters where
the star was saturated, we placed a * symbol next to the number
of images in which that star was detected (e.g. The possible optical
counterpart of 24f in the B438 and R,606 filters)...... 22
3.1 List of X-ray sources from Cycle 1 that were not redetected in Cycle
13 with their respective offsets from the optical axis, number of net
counts, and error circle sizes...... 26
xi List of the optical counterparts we were able to confirm or not confirm along with their IDs in the Bellini et al. (2017) catalog, offset from the center of their X-ray error circles, and the number of images in which they were detected in each filter. Abbreviations for the source classifications and symbols as in Table 2.3. NV371 is a variable star from Kaluzny et al. (2004)...... LIST OF FIGURES
Figure Page
1.1 Hess diagram from Bellini et al. (2017) showing the multiple stellar
population of Omega Centauri. A Hess diagram plots the relative
density of stars at differing color-magnitude positions for a given star
cluster...... 6
1.2 Zoomed in color-magnitude diagram of the upper red giant branch
of Omega Cen from Pancino et al. (2000). Different symbols refer to
different stellar populations with the anomalous branch stars plotted
as filled triangles...... 7
2.1 Combined 222 ksec Chandra ACIS-I image of Omega Cen from Hen-
leywillis et al. (2018). The circles show 1, 2, and 3 core radii and the
crosshair is the location of the cluster center...... 12
2.2 Combined 222 ksec Chandra ACIS-I image of the core of Omega Cen.
The circle indicates the core radius and the crosshair is the location
of the cluster center...... 13
2.3 Field of views covered by each WFC3 filter from Bellini et al. (2017).
Axes are in arcseconds with respect to the center of the cluster and
the dotted circle in each panel representsthe cluster’s core radius. . . 15
xiii 2.4 Tricolor image of the inner region of Omega Cen from Bellini et al.
(2017), made using the image stacks of the bluest (F225W), the red
dest (F160W), and an intermediate filter (F814W). The scale (10")
and orientation are labeled in the bottom right corner...... 16
2.5 The transmission curves of the eight selected filters as a function of
wavelength...... 18
3.1 Example of a CV spectrum from Shafter et al. (1983). The spectrum
shows a number of the common emission lines of CVs including the
prominent Ha emission line at 6562A...... 24
3.2 Selection of CMDs for X-ray Source 13c. The stars within the error
circle of the source are marked as red dots and the stars just outside
of the error circle, up to 1.5x the error circle radius, are shown as
cyan dots. The optical counterpart is represented as a red triangle in
the diagrams...... 30
3.3 Selection of CMDs for X-ray Source 21b. Symbols as in Figure 3.2. . 31
3.4 Selection of CMDs for X-ray Source 14a. Symbols as in Figure 3.2. . 32
3.5 Selection of CMDs for X-ray Source lie. Symbols as in Figure 3.2. . . 34
4.1 Selection of CMDs for X-ray Source 12a. Symbols as in Figure 3.2. . 36
4.2 Selection of CMDs for X-ray Source 13a. Symbols as in Figure 3.2. . 37
4.3 Additional CMDs for X-ray Source 13c. Symbols as in Figure 3.2. . . 38
4.4 Selection of CMDs for X-ray Source 13f. Symbols as in Figure 3.2. . . 39
xiv 4.5 Selection of CMDs for X-ray Source 22h. Symbols as in Figure 3.2. . 40
4.6 Additional CMDs for X-ray Source 21b. Symbols as in Figure 3.2. . . 41
4.7 Selection of CMDs for X-ray Source 22c. Symbols as in Figure 3.2. . . 42
4.8 Selection of CMDs for X-ray Source Ilf. Symbols as in Figure 3.2. . . 43
4.9 Selection of CMDs for X-ray Source 22i. Symbols as in Figure 3.2. . . 44
4.10 Additional CMDs for X-ray Source 14a. The stars within the error
circle of the source are marked as red dots and the stars just outside
of the error circle, up to 1.5x the error circle radius, are shown as
cyan dots. The optical counterpart is represented as a red triangle in
the diagrams and the counterpart’s close neighbor is marked as a red
square...... 46
4.11 Selection of CMDs for X-ray Source 24e. Symbols as in Figure 3.2. . . 47
4.12 Selection of CMDs for X-ray Source 13e. Symbols as in Figure 4.10. . 48
4.13 Selection of CMDs for X-ray Source 14f. Symbols as in Figure 3.2. . . 49
4.14 Selection of CMDs for X-ray Source 23a. Symbols as in Figure 3.2. . 50
4.15 Selection of CMDs for X-ray Source 24h. Symbols as in Figure 3.2. . 51
4.16 Selection of CMDs for X-ray Source 13b. Symbols as in Figure 3.2. . 52
4.17 Selection of CMDs for X-ray Source 24f. Symbols as in Figure 3.2. . . 54
4.18 Additional CMDs for X-ray Source lie. Symbols as in Figure 3.2. . . 55
4.19 Selection of CMDs for X-ray Source 14b. Symbols as in Figure 3.2. . 56
4.20 Selection of CMDs for X-ray Source 22b. Symbols as in Figure 3.2. . 57
xv 4.21 Selection of CMDs for X-ray Source 21d. Symbols as in Figure 3.2. 59
4.22 Selection of CMDs for X-ray Source 33d. Symbols as in Figure 3.2. 60
4.23 Selection of CMDs for X-ray Source lib . Symbols as in Figure 3.2. 61
4.24 Selection of CMDs for X-ray Source 22a. Symbols as in Figure 3.2. 63
4.25 Selection of CMDs for X-ray Source 22d. Symbols as in Figure 3.2. 64
4.26 Selection of CMDs for X-ray Source 22j. Symbols as in Figure 3.2. . 65
4.27 Selection of CMDs for X-ray Source 14c. Symbols as in Figure 3.2. . 66
4.28 Selection of CMDs for X-ray Source 22f. Symbols as in Figure 3.2. . 67
4.29 Proper motions for 245,443 stars within the core of Omega Cen mea
sured by Bellini et al. (2014). The red circle of radius 3 milliarcsec-
onds per year separates the vast majority of the stars that are likely
members of the cluster from the field stars. The red triangles repre
sent the four non-member stars found within the X-ray error circles
of 14c, 22a, 22d, and 22j...... 69
5.1 X-ray flux and luminosity as a function of offset from the center of
the cluster. Abbreviations for the source classifications in the legend
as in Table 2.3. Solid symbols mark likely cluster members and open
symbols show optical counterparts outside of the cluster. The small
colored symbols represent sources with less secure characterizations. 73
xvi X-ray color-magnitude diagram for the sources with non-zero fluxes in both the hard and soft bands. Abbreviations for the source clas sifications in the legend as in Table 2.3 and symbols as in Figure
5.1......
xvii 1
Chapter 1
Introduction
1.1 Binary Stars in Globular Clusters
Globular clusters are dense, spherical star systems made of hundreds of thousands to millions of old, low metallicity Population II stars that are bound together by gravity and are typically found in galactic halos. Traditionally, the stars within globular clusters were believed to have all formed at roughly the same time from the same molecular cloud. Therefore, all of the stars in a cluster should be the same age and variations in luminosity and temperature between the stars are the result of different initial masses. However, the discovery of multiple stellar populations within most globular clusters have complicated this view by suggesting that clusters likely experience several epochs of star formation.
A comprehensive study of the stellar populations in globular clusters must in clude its binary star systems. Binary stars play a critical role in the dynamics and 2
evolution of globular clusters. Binaries in globular clusters are created through two distinct channels: primordial and dynamical. Primordial binaries originate from the formation of the cluster and dynamical binaries formed later through stellar inter
actions, primarily in the dense environments of cluster cores (Ivanova et al. 2006;
Hong et al. 2017).
Over time, globular clusters will tend towards a state of energy equipartition in which higher mass stars have lower velocities on average as a result of transferring their kinetic energy to lower mass stars through stellar interactions. These high mass stars will “sink” towards the center of the cluster in a process known as mass segregation. The net loss of kinetic energy from the central region of the cluster that results from mass segregation will lead to the gravitational collapse of the core.
However, many globular clusters observed in our Galaxy do not have collapsed cores
(Meylan & Heggie 1997). Binary stars are instrumental in preventing collapse by
stirring up the other stars in the center of the cluster. Binary systems will effectively
act as a single object with a combined mass of the two stars and congregate near the core of the cluster through mass segregation. In the dense cluster core, stellar
interactions will cause binaries to lose potential energy by becoming more tightly
bound and transferring their energy to passing stars in the form of kinetic energy.
This mechanism can generate high enough velocities among the stars in the cluster’s
central region to halt core collapse (Hut et al. 1992; Heggie & Hut 2003).
One effective way of searching for cluster binaries is through X-ray imaging. X- 3
ray binaries in globular clusters consist of two main classes: accreting binaries and close detached binaries with high coronal activity known as active binaries (AB). The brightest X-ray sources in globular clusters are accreting neutron stars known as low- mass X-ray binaries (LMXBs). These binaries are overabundant in dense globular clusters by a factor of ~ 100 relative to the Galaxy, strongly suggesting a dynamical origin (Ivanova et al. 2008). If the neutron stars are actively accreting they can reach X-ray luminosities of Lx > 1036 erg s_1. Lower luminosity accreting binaries in globular clusters are primarily cataclysmic variables (CV) in which a white dwarf accretes matter from its main sequence (MS) companion forming an accretion disk around the white dwarf. The high temperature of the accretion disk results in a great deal of X-ray emission with X-ray luminosities of < 3 x 1030 — 1033 erg s-1.
Active binaries experience rapid rotation induced by tidal locking, which couples the rotation and orbital periods of the component stars. The rapid rotations result in an enhanced solar-dynamo which produces strong magnetic fields, increased coronal activity and associated X-ray emission. BY Draconis stars (BY Dra) are the MS variety of ABs with X-ray luminosities of Lx< 3 x 1029 — 2.5 x 103° erg s-1 (Dempsey et al. 1997). RS Canum Venaticorum systems (RS CVn), in which one or both stars in the binary is a subgiant, have X-ray luminosities of Lx ~ 1031 erg s-1 (Dempsey et al. 1993).
There is an additional category of binary found within globular clusters called red stragglers that are as of yet poorly understood. Red stragglers (also known as 4
sub-subgiants) are generally defined to be redder than MS stars and fainter than subgiants. These stars cannot be explained by single-star evolution theory and are likely binaries that have experienced mass transfer and/or significant dynamical exchanges (Leiner et al. 2017; Mathieu et al. 2003). Red stragglers are often detected in clusters as X-ray sources with luminosities similar to RS CVn systems on the order of Lx = 1030 — 1031 erg s-1 (Geller et al. 2017), which is consistent with them being coronally active binaries.
Identifying and characterizing the X-ray binary populations of globular clusters is also necessary to determine their overall X-ray emissivity, which is the cumulative
X-ray luminosity of the cluster per unit stellar mass. Globular clusters appear to have fewer X-ray binary stars compared to the field and other types of star clusters
(Haggard et al. 2009; van den Berg 2012). This lack of X-ray emission may be a result of the destruction of binaries in dense environments through stellar interactions.
1.2 Omega Centauri
Omega Centauri (Omega Cen) is a particularly interesting target in the search for binary stars systems in globular clusters. Omega Cen contains several million stars and it is the most massive globular cluster in our Galaxy (4 x 106 Msun; D ’Souza
& Rix 2013). The cluster possesses a large core (rc = 155" = 3.9 pc at a distance of 5.2 kpc; Harris 2010) and a high stellar interaction rate (Pooley et al. 2003).
Omega Cen’s substantial mass and high interaction rate is expected to produce a 5
large number of binaries in the cluster. It was the first globular cluster to show signs of multiple stellar populations (Freeman & Rodgers 1975; Norris & Da Costa
1995) including a double main sequence (Anderson 2002; Bedin et al. 2004; Piotto et al. 2005), multiple red giant branches (Lee et al. 1999), and subgiant branches
(Ferraro et al. 2004; Villanova et al. 2007). There is an unusually large range of chemical compositions between the various stellar populations. For example, the anomalous red giant and subgiant branches (RGB/SGB-a) contain stars with 10 times the [Fe/H] metallicity of the dominant population of giants and subgiants in the cluster (Pancino et al. 2000; Sollima et al. 2005; Johnson & Pilachowski 2010).
These diverse stellar populations are evidence of multiple epochs of stellar for mation in Omega Cen’s past. Due to its chemical complexity, it has been suggested that Omega Cen is not in fact a globular cluster but the remnant of a dwarf galaxy that merged with the Milky Way (Bekki & Freeman 2003; Lee et al. 2009). This possibility, along with its high velocity dispersion, has made Omega Cen a leading candidate for harboring an intermediate-mass black hole (IMBH). IMBHs are a class of black holes in the 102 — 105 Msun mass regime that would fall between stellar mass black holes (5 - 100 Msun) and supermassive black holes (> 106 M s^ ) found in the center of galaxies. In contrast to stellar mass black holes and supermassive black holes, there is a lack of strong observational evidence for IMBHs and so far none have been confirmed. Miller & Hamilton (2002) demonstrated that binaries have a significant effect on the formation of IMBHs in globular clusters. Three-body 6
16
10
05 0 05 1 15 2 25
m F275W m F336W
Figure 1.1: Hess diagram from Bellini et al. (2017) showing the multiple stellar population of Omega Centauri. A Hess diagram plots the relative density of stars at differing color-magnitude positions for a given star cluster. 7
B-I 2 3
Figure 1.2: Zoomed in color-magnitude diagram of the upper red giant branch of Omega Cen from Pancino et al. (2000). Different symbols refer to different stellar populations with the anomalous branch stars plotted as filled triangles. 8
interactions involving binaries may eject lower mass black holes out of the cluster, but more massive black holes (> 5 0 Mgun) may sink to the center and merge with other objects to form an IMBH.
There have been several claims of dynamical evidence, or lack thereof, for an
IMBH in Omega Cen. Noyola et al. (2008) reported a clear rise in velocity dispersion in the center of the cluster that implied a central black hole mass of 4 x 104 Msun.
Anderson & van der Marel (2010) were unable to confirm these results using their own proper motions data and found that Noyola et al. (2008) misidentified the center of Omega Cen. Baumgardt (2017) compared velocity dispersion profiles of
50 Galactic globular clusters to N-body simulations with varying central black holes masses. Omega Cen was the only cluster in which its velocity dispersion profile provided strong evidence for a central IMBH. However, a sensitive X-ray search of the cluster found no evidence of X-ray emission from any of the proposed centers of
Omega Cen (Haggard et al. 2013). This means that if an IMBH does exist in the cluster, it must be experiencing very little or inefficient accretion.
1.3 Searching for Optical Counterparts of X-ray Sources
X-ray imaging is a powerful tool to identify binary star systems in globular clusters.
It is almost always necessary to find the optical counterparts of X-ray sources to properly classify the source of the emission. The search for optical counterparts within Omega Centauri began after the detection of five X-ray sources in or towards 9
the cluster during the Einstein Observatory Imaging Proportional Counter (IPC) survey of globular clusters (Hertz & Grindlay 1983). Of the five Einstein IPC sources detected (named A-E) the three farthest from the cluster’s core were later found to be foreground stars. Sources “A” and “D” were determined to be dwarf M stars with emission lines (dMe) by Cool et al. (1995b) using the more accurate positions provided by the Einstein High Resolution Imager (HRI). Source “E” was tentatively identified as a foreground K star by Margon & Bolte (1987). Source “C” , which fell in the cluster’s core, was later resolved into three distinct sources by Verbunt &
Johnson (2000) with the ROSAT High Resolution Imager (HRI). Carson, Cool &
Grindlay (2000) searched for optical counterparts within the ~ 10" radii X-ray error circles of the three core sources using observations from the Hubble Space Telescope’s
(HST) Wide Field Planetary Camera 2 (WFPC2). Hubble was launched into low
Earth orbit in 1990 and is able to take exceptionally high-resolution images with its
2.4 meter primary mirror due to the lack of atmospheric distortions. Hubble’s high spatial resolution (~ 0.05") allows for the discovery of optical counterparts of X-ray binaries within the crowded cores of globular clusters.
The HST observations of the three R 05A TH R I sources revealed two UV-bright and Ho bright stars that fell within their own respective X-ray error circles and were identified as probable CVs by Carson, Cool & Grindlay (2000). A third tentative
CV identification was made for a star that showed no UV excess but was Ho bright.
The fifth and final Einstein IPC source ( “B” ), located 1.7 rc from the cluster center, 10
was characterized as a quiescent neutron star (qNS) by Rutledge et al. (2002). The optical counterpart of the qNS was discovered by Haggard et al. (2004) using the
Advanced Camera for Surveys (ACS) on HSand X-ray positions from the Chandra
X-ray Observatory. Chandra was launched in 1999 into a highly elliptical geocentric orbit with a period of 64 hours. Chandra’s superior spatial resolution and associated high sensitivity allowed it to be the first X-ray telescope to detect the low luminosity sources characteristic of many X-ray binaries. In addition to finding the qNS optical counterpart, Haggard et al. (2004) recovered the three optical counterparts identified by Carson, Cool & Grindlay (2000) and confirmed them as likely CVs as well as detecting three possible BY Dra stars.
A great deal of progress was made in the discovery of optical counterparts in
Omega Cen by Cool et al. (2013) using HST ACS observations with a ~ 10' x 10' field of view that centered on the core of the cluster and covered 109 previously detected Chandra X-ray sources. Cool et al. (2013) reported 59 optical counterpart candidates for the sources including 27 CVs, 3 BY Dra stars, a blue straggler, the qNS previously found by Haggard et al. (2004), 7 ABs that fall on or near the metal-rich anomalous giant and subgiant branches (RGB/SGB-a) of Omega
Cen, 3 foreground stars, and 11 active galactic nuclei (AGN). We seek to confirm these previous characterizations and find new optical counterparts within the core of Omega Cen using new optical and X-ray data from HST and Chandra. 11
Chapter 2
Observations & Data
2.1 Chandra X-ray Observations
We utilize two epochs of X-ray observations of Omega Centauri made with Chandra’s
Advanced CCD Imaging Spectrometer (ACIS - I). The first epoch of observations
(Cycle 1), which took place from January 24-25 2000, resulted in a 69 ksec exposure of the cluster in Chandra’s “very faint” mode. Haggard et al. (2009) detected 180
X-ray sources with a limiting flux of ~ 4.3 x 10-16 erg cm-2 s-1 (Lx ~ 1030 erg s-1).
The observations had a 17' x 17' (2048 x 2048 pixels with 1 pix = 0.492") field of view (FOV) that covered over 3 core radii of the cluster.
The second epoch of Chandra observations (Cycle 13) occurred on April 16-
17 2012 with a combined exposure time of 222 ksec. Henleywillis et al. (2018) identified 233 X-ray sources, 95 of which were detected for the first time. The Cycle
13 observations overlapped with 88% of Cycle l ’s FOV and were able to redetect 12
Figure 2.1: Combined 222 ksec Chandra ACIS-I image of Omega Cen from Henley- willis et al. (2018). The circles show 1, 2, and 3 core radii and the crosshair is the location of the cluster center.
138 of the 180 previously identified X-ray sources. Despite having a much longer exposure time, 36 Cycle 1 sources that fell within Cycle 13’s FOV were not recovered in the later observations. This is likely due to the flux variability of the kinds of
X-ray sources that would be found in or towards the cluster such as CVs, active binaries, background AGN, and foreground stars. Henleywillis et al. (2018) also estimated that of the 233 sources detected in Cycle 13, 60 ± 20 are likely members of the cluster and ~ 30 lie within the core of Omega Centauri. 13
Figure 2.2: Combined 222 ksec Chandra ACIS-I image of the core of Omega Cen. The circle indicates the core radius and the crosshair is the location of the cluster center. 14
2.2 HST Observations
In 2009, the Wide Field Camera 3 (WFC3) was installed in the Hubble Space Tele scope during its fourth service mission. The calibration team of the instrument decided that the core of Omega Cen would be one of its targets for detector calibra tion. As a result, over 650 exposures in 26 different filters were taken of the core of the cluster through WFC3’s Ultraviolet-VISible (UVIS) and InfraRed (IR) channels from 2009 to 2013. WFC3’s UVIS channel has a total FOV of 162" x 162" with a pixel scale of 0.04" and the IR channel has a FOV of 123" x 136" with a pixel scale of 0.13". See Figure 2.3 for the total FOV of each of the 26 filters used during the observations.
Bellini et al. (2017) combined these individual exposures to create the most com prehensive catalog of photometry ever constructed for a globular cluster. The cata log contains 478,477 sources and their photometry is measured using three different methods using the FORTRAN code KS2 (Anderson et al. 2008, Also see Appendix A of Libralato et al. 2018). Each method is optimized for a distinct magnitude range.
Method one is designed for the brightest stars in the cluster with instrumental mag nitudes brighter than ~ —11. Instrumental magnitudes are uncalibrated apparent magnitudes that are useful for comparing objects in the same image. They can be defined by minst = —2.5 log(f) when f is the intensity of the source in counts.
Method two works best for stars of intermediate brightness between ~ —11 and
~ — 8 and method three is superior at measuring stars at the bottom of the main 15
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L H j . ^ 0 0 0) • : m \ r &■': -150 3 - 150 - , i , , l , , i , : , i , . 1 , , i , r M I m | M i M ii 1 i i 1 i i 1 i T. . . . ____ * . M 111 j 1111 + i i I i i I i i I i i c0 i 1 l'lM M i r 'Mil : F7?5w : F814W : F85toLP : : f9u>3n : -1 5 0 0 150 -150 0 150 : F098M o 150 150
0 0 ♦ ♦ 150 ♦ ♦ ♦ 150 1 I 1 . L .. i i 1 , 1 1 i i i i 1 i . i i ri i 11 . 11 ■ i i ; . i . . I , , i . r , i , . i ., i , ! i i i t I i i i > r 150 0 -1 5 0 150 0 -1 5 0 150 0 -150 150 0 -150 150 0 -150 150 0 -150 150 0 -150 ARA (arcsec)
Figure 2.3: Field of views covered by each WFC3 filter from Bellini et al. (2017). Axes are in arcseconds with respect to the center of the cluster and the dotted circle in each panel represents the cluster’s core radius. 16
Figure 2.4: Tricolor image of the inner region of Omega Cen from Bellini et al. (2017), made using the image stacks of the bluest (F225W), the reddest (F160W), and an intermediate filter (F814W). The scale (10") and orientation are labeled in the bottom right corner.
sequence and white dwarfs fainter than ~ —8.
The photometric catalog was also compared to a compilation of high precision proper motion (PM) measurements published in Bellini et al. (2014). The PM mea surements began with exposures from Hubble’s Advanced Camera for Surveys (ACS) in 2002 (GO-9442, PI: Cool) and continued on with a number of the UVIS exposures included in the photometric catalog, creating a time baseline of over 10 years for a majority of the stars. Over 270,000 objects, which are overwhelmingly stars but 17
also include a number of AGN, were found in common between the photometric and
PM catalogs. For a further discussion of the photometry methods used to create the catalog and how the proper motions were measured, see Bellini et al. (2017).
Of the 26 filters available in the catalog we used eight of the UVIS filters in our
analysis. The filters we selected were F225W, F275W, F336W, F438W, F606W,
F656N, F658N, and F814W (hereafter referred to as UV225, UV275, U336) B438, R6065
Hci656, Ha658 , and Isi4 respectively). These filters were chosen to optimize the
chances of detecting optical counterparts by providing six wide filters (UV225, UV275,
U336, B438, R.606 and U14) with long exposure times to produce deep imaging of the
cluster and two narrow filters (Ha656 and Ha^ss) that cover the Ha Balmer emission
line of hydrogen. There is a strong correlation between X-ray emission and the
Ha line. Table 2.1 contains a list of the eight filters and their respective exposure
times and Figure 2.5 shows the transmission curves of each filter as a function of
wavelength. We also possess “stacked” images of the cluster in each filter that
combines all of the individual exposures and “subtracted” images which show the
residual flux of the stars after the point sources were removed from the stacked
images. 67 of the X-ray sources detected in Cycle 1, Cycle 13, or both epochs fall
within or towards the core of the cluster and have coverage in at least one of the
eight UVIS filters. Table 2.2 lists the fluxes, X-ray error circle sizes, and the filter
coverage for all of the X-ray sources.
Of the 67 X-ray sources that fall within or towards the core of the cluster, 23 18
F225W F275W F336W
20f
I *
Wavelength (nm) F438W F606W F656N
1/ty/^WvyVx
450J 500 550 600 650 700 750 Wavelength (nm)
F658N F814W
Figure 2.5: The transmission curves of the eight selected filters as a function of wavelength. 19
Filter Central Wavelength Width Exposure Time F225W 235 nm 46.7 nm 27450 sec F275W 271 nm 39.8 nm 27600 sec F336W 337 nm 51.1 nm 13060 sec F438W 432 nm 61.8 nm 11900 sec F606W 594 nm 218 nm 2523 sec F656N 656 nm 1.8 nm 2500 sec F658N 658 nm 2.8 nm 1400 sec F814W 782 nm 153 nm 1355 sec
Table 2.1: List of the 8 selected UVIS filters and their respective exposure times. have potential optical counterparts that were identified by Cool et al. (2013) that we seek to independently confirm. Table 2.3 lists their previous optical identifications, the number of images in which the counterpart was detected for each filter and the offset from the center of the X-ray error circle. 20
Source Cycle 1 fx Cycle 13 fx Error UV225 UV275 U336 B438 R«06 Ha656 Ha658 1814 ID (10-16 erg cm-2 s_1) (10-16 erg cm-2 s) (") 11a 2.9 4.3 0.49 / / / / / / / / lib 36 79.6 0.33 / / / / / / / / 11c 8.5 - 0.51 / / / / / / / lid - 6 0.46 / / / / / / / / lie - 6.3 0.43 / / / / / / / / Ilf - 5 0.49 / / / / / / / / 12a 210 16 0.36 / / / / / / / / 12b 69 56.4 0.33 / / / / / / / / 12c - 6.6 0.41 / / / / / / / / 13a 69 914 0.29 / / / / / / / / 13b 21 34.2 0.33 / / / / / / / / 13c 630 1238.5 0.29 / / / / / / / / 13d 13 16 0.36 / / / / / / / / 13e 12 6.9 0.42 / / / / / / / / 13f 12 38.7 0.34 / / / / / / / / 13g 5.1 7.4 0.41 / / / / / / / / 14a 5.1 - 0.56 / / / / / / / / 14b 4.7 - 0.56 / / / / / / / / 14c 29 19.7 0.36 / / / / / / / / 14d 12 21.2 0.39 / / / / / / / / 14e - 3.3 0.55 / / / / / / / / 14f - 9.5 0.43 / / / / / / / / 21a 22 29.6 0.43 / / / / / 21b 6.1 - 0.67 / / / / / / / 21c 6.4 5 0.56 / / / / / / / / 21d 23 27.1 0.38 / / / / / / / / 21e 15 17.7 0.39 / / / / / / / 21f - 8.9 0.48 / / / / / / / 21g - 4 0.58 / / / / / / / 22a 4.8 5.2 0.52 / / / / / / 22b 7.9 - 0.51 / / / / / / / 22c 56 88.5 0.32 / / / / / / / 22d 16 9.5 0.44 / / / / / / 22e 35 17.8 0.39 / 22f 21 28.2 0.36 / / / / / / 22g - 2.2 0.72 / / / / / / 22h - 6.1 0.49 / / ✓ / / « / 22i - 3.5 0.53 / / / / / / / 22j - 5 0.46 / / / / / / / 22k - 2.2 0.65 / / / / / / 221 - 8.4 0.40 / / / / / / / / 23a 39 24.3 0.36 / / / / / / / / 23b 10 7.9 0.47 / / / / / / 23c 13 - 0.50 / / / / / / / / 23d - 5.2 0.47 / / / / / / 23e - 4.2 0.55 / / / / / / / 23f - 4.6 0.55 / / / / / / / / 21
source Cycle 1 fx Cycle 13 fx Error uv225 uv275 U336 B438 Rboe Ha?656 Ha658 1814 ID (10-16 erg cm-2 s-1 ) (10“ 16 erg cm-2 s) n 23g - 6.7 0.46 / / / / / / / / 23h - 4.4 0.55 / / / / / / / 24a 7.5 - 0.59 / / / / / / / / 24b 5.1 8.6 0.50 / / / / / / / 24c 18 32.3 0.40 / / 24d 8.1 9.2 0.49 / / / / / / / / 24e 2.8 - 1.14 / / / / / / / 24f 7 11.1 0.45 / / / / / / / / 24g 48 40.9 0.39 / / / / / / 24h 10 - 0.54 / / / / / / / / 24i - 6.7 0.49 / / / / / / / 24j - 8.9 0.45 / / / / / / / / 24k - 2.4 0.81 / / / / / / / 31b 8.2 8.9 0.56 / / / / / / 31d 43 36.2 0.42 / / / / / / 32i - 1.7 0.83 / / / / / / 33d 21 22.9 0.39 / / / / / / 33g 5.7 4.4 0.59 / / / / / / / 34c 3.8 - 1.08 / / 34e - 8.9 0.51 / / / / / /
Table 2.2: The 67 X-ray sources with their Cycle 1 and Cycle 13 fluxes in the 0.5 — 4.5 keV “medium” X-ray band, error circle radii in arcseconds, and coverage in the eight selected UVIS filters. The error circle sizes for the sources detected in Cycle 13 were determined by Henleywillis et al. (2018). For the ten X-ray sources that were detected in Cycle 1 but not redetected in Cycle 13, we calculated the error circle sizes. See Chapter 3 for further details. If the source had coverage in a filter it was marked with a / symbol and if the source had partial coverage it was marked with a « symbol (e.g. Hq:656 coverage of 22h). 22
iource Previous Offset uv225 UV275 U336 B438 R-606 Ha656 Ha658 1814 ID Optical ID (") 12a CV 0.29 17 19 20 20 48 3 6 22 13a CV 0 .1 0 14 16 22 18 44 3 2 19 13b RGB/SGB-a 0.31 16 20 29 23 51 4 6 24 13c CV 0.16 13 15 21 18 41 2 5 19 13f CV? 0.29 23 26 32 30 54 5 6 30 14a BYDra 0.42 26 29 36 33 61 5 6 33 2 1 b fbCV 0.14 6 9 9 9 11 1 0 8
2 1 c Ha-only - 0 0 0 0 0 0 0 0 2 1 d Ha-only 0.51 8 11 11 11 12 2 3 10 2 2 a FGND 0.37 7 7 8 8 8 0 0 8 2 2 c fbCV 0.16 5 6 8 8 17 1 0 7 2 2 d BS 0 .1 0 2 2 2 2 4 0 0 2 2 2 e RGB/SGB-a 0.16 0 0 0 0 7 0 0 0
to to AGN 0.03 5 6 7 8 12 0 0 7
23b CV? - 0 0 0 0 0 0 0 0
23c AGN - 0 0 0 0 0 0 0 0 24a Blue-only - 0 0 0 0 0 0 0 0
24c CV - 0 0 0 0 0 0 0 0 24e BYDra 0.81 1 1 1 1 2 1 0 1 24f RGB/SGB-a 0.85 8 9 11 1 0 * 1 2 * 1 1 8
24g AGN? - 0 0 0 0 0 0 0 0 31b AGN - 0 0 0 0 0 0 0 0 33d Ha-only 0.44 1 1 3 3 3 0 0 3
Table 2.3: The 23 X-ray Sources in which potential optical counterparts were iden tified by Cool et al. (2013) with their previous optical identifications, offset from the center of the X-ray error circle in arcseconds, and the number of images in which the counterpart was detected for each filter. Abbreviations for the source classifications: cataclysmic variable (CV), possible CV (CV?), faint blue CV (fbCV), BY Draconis binary (BYDra), blue straggler (BS), star located on or near anomalous subgiant or giant branch (RGB/SGB-a), Ha-bright source with no measurable blue excess (Ha- only), foreground star (FGND), background active galactic nucleus (AGN), possible AGN (AGN?), blue source with no measurable Ha-excess (Blue-only). In filters where the star was saturated, we placed a * symbol next to the number of images in which that star was detected (e.g. The possible optical counterpart of 24f in the B438 and Reoe filters). 23
Chapter 3
Analysis
3.1 Color-magnitude Diagrams of Omega Centauri
Constructing color-magnitude diagrams (CMD) is one of the most established and effective ways to study stellar populations. CMDs can also be used to search for binary star systems in globular clusters (Romani & Weinberg 1991; Sollima et al.
2007; Milone et al. 2016). A CMD plots the apparent magnitudes of stars as a func tion of their color index, which is defined as the difference in apparent magnitude between two different filters. We can identify stars as potential optical counterparts to X-ray sources by their location on the CMDs. For example, CVs appear promi nently to the blue of the MS and are typically bright in the Ha filter relative to the R6o6 filter compared to MS stars due to the presence of Balmer emission lines
(Williams 1983; Echevarria 1988; Cool et al. 1995a). An example of a CV spectrum is shown in Figure 3.1. The Ha filter is fully contained within the R6q6 filter so we 24
Figure 3.1: Example of a CV spectrum from Shafter et al. (1983). The spectrum shows a number of the common emission lines of CVs including the prominent Ha: emission line at 6562A.
assume that this wide filter is equivalent to the flux of the continuum around the
Ha line. A star with Balmer emission lines will appear to the left of the MS in a
Ha - R CMD as a result of the increased flux in the Ha filter. Blue or UV excess is a result of high temperatures in the inner part of the CV’s accretion disk. The Ha emission is also thought to come from the inner accretion disk.
Active Binaries appear to the red of the MS and have small H-alpha excesses as a result of their high coronal activity (Chevalier &; Ilovaisky 1997; Taylor et al.
2001). An unresolved binary whose mass ratio is close to unity will have a combined color that is redder than the brighter component of the system and a luminosity greater than an individual star. As a result, the binary system appears above and to the right of the MS (Hurley & Tout 1998). 25
We searched for optical counterpart candidates within the error circles of the
X-ray sources identified in Cycle 1 and Cycle 13 Chandra data. The error circle sizes for the sources detected in Cycle 13 were determined by Henleywillis et al.
(2018). For the ten X-ray sources in the core that were detected in Cycle 1 but not redetected in Cycle 13, we calculated 95% error circles, in which the true position of the source should fall within the error circle 95% of the time, using the empirical formula in Hong et al. (2005)
0.1" 1 Perr — 0.25 + 1 + logiQ (Cn + 1) logio (cn + l). (3.1) ■^offset -^offset + 0.03" + 0.0006" .loglO (Cn + 2)_ _log10 (cn -I- 3)_ where D0f[set is the offset from the source to the aim point of the observation in arcminutes and cn is the number of net counts. T>0ffset was determined by calculating the angular distances between the X-ray sources and the time-averaged location of the optical axis of the observations, which were included in the header of the event file as RA_PNT and DEC_PNT. For cn, we utilized the number of counts detected in the “medium” X-ray band (0.5 — 4.5 keV) listed in Haggard et al. (2009) for each source.1
We evaluated the potentially interesting objects that fell within the X-ray error
1In Hong et al. (2005), the net count c7l is the number reported by wavedetect, which is a routine in the Chandra Interactive Analysis of Observations (CIAO) software package that searches for X-ray point sources and provides their properties. 26
X-ray Source •^offset Cn 1p err 11c 1.314' 8 0.517"
14a O CO -si 5 0.549" 14b 0.297' 5 0.54" 21b 1.892' 6 0.65" 22b 1.509' 8 0.535" °o 23c 1.662' 12 O 24a 1.5' 7 0.56" 24e 2.195' 3 1.02" 24h 2.565' 4 0.974" 34c 2.565' 4 0.974"
Table 3.1: List of X-ray sources from Cycle 1 that were not redetected in Cycle 13 with their respective offsets from the optical axis, number of net counts, and error circle sizes. circles in a number of different ways. First, we checked the reliability of the photom etry to see if the counterpart candidates were impacted by bright nearby neighbors or diffraction spikes. Next, we examined the subtracted images to look for extended sources in which flux was left behind when the point source was subtracted. Ex tended sources, most easily recognized from residual flux in the subtracted images, are indicative of AGN. Finally, we examined the stars visually by blinking between filters to confirm blue/UV or Ha excess. This is effective if the color excesses are
~ 0.5 magnitudes or greater. For smaller excesses, we relied on the photometry alone and checked for signs that any problems had arisen in the fitting. 27
3.2 Equivalent Width
We also calculated the strength of the Ho; emission line of the potential optical counterparts in terms of equivalent width (EW). The equivalent width is a measure of emission line flux relative to the continuum. We can express the EW is terms of known and measured quantities including the widths of filters and the apparent magnitudes of the possible optical counterparts (Haggard 2004). If there was no Ha emission line, the difference in magnitude between the Ha656 and R.606 filters would be
mHa6S6,i - = -2-5 log ^3'2^
where AAj/a656 is the width of the Ha656 filter (18A) and AAj^06 is the width of the
R.606 filter (2182A). When the emission line is present, flux is added to the continuum through both filters. The difference in magnitude with the presence of Ha emission is therefore
o cl ( 56 + EWHa\ tr> oA m = -2.5 log ( AA^ +gH/ga ) (3-3) 28
where EW//a represents the additional flux from the emission line. The difference between these two expressions is
A (jT lH a 656 ^i?606) (^Ha656i2 ^i?606il)
= -2 .5 log ( A W + ™ H?) + 2.5log ( AARg06 + EWHq J (3.4) 'I I gWgq AXholqsq = -2 .5 log 2£a- 1 + AAHfi
Therefore, i I m x * 10? = ____^^Ho:656 (3.5) 1 I ^■^606 where
A [m656 P = (3.6) -2 .5
Rearranging the equation,
1+^AAHa656 ^ = 10P+(lr^ \AAH606 / )xl0P (37) results in the following expression for the equivalent width of the Ha emission line
e w Hc, = AAf;r(10P ~ 1}\ (3-8) X- M 2 f * 10P) 29
The typical Ha equivalent width of CVs is EW //Q > 1 0 A (Echevarria 1988). For
ABs, the typical Ha emission line strength is EW A (Chevalier & Ilovaisky
1997), meaning that this class of binary will have relatively small H-alpha excesses.
3.3 Classes of Optical Counterparts within Omega Centauri
Here we present prototypes of the four classes of optical counterparts we will be looking for. The full results of the search are presented in Chapter 4. First, we considered the blue and H-alpha bright stars that have been characterized as CVs.
X-ray source 13c is one of the original three sources for which Carson, Cool & Grind lay (2000) identified an optical counterpart and it is the brightest X-ray source in
Omega Cen. We utilized the same naming convention from Haggard et al. (2009) for the X-ray sources. The first character of the source name is equal to the ra dial offset from the cluster center rounded to the nearest arcminute and the second character is a number between 1 and 4 that represents the quadrant in which the source falls. The third and final character in the source name is a letter assigned in the order of increasing, counterclockwise azimuthal angle for each quadrant. Figure
3.2 shows the U336—B438 and Ha656— Reo6 versus B438 color-magnitude diagrams for the stars within the error circle of 13c marked as red dots and the stars just outside of the error circle, up to 1.5x the error circle radius, as cyan dots. The optical counterpart, represented as a red triangle in the diagrams, shows signifi cant UV excess and it is consistently blue compared to the MS in all of the other U 336 — B 438 Ha656 — ^606
Figure 3.2: Selection of CMDs for X-ray Source 13c. The stars within the error circle of the source are marked as red dots and the stars just outside of the error circle, up to 1.5x the error circle radius, are shown as cyan dots. The optical counterpart is represented as a red triangle in the diagrams.
CMDs (all of the available CMDs for the X-ray sources in the core can be found at http://www.physics.sfsu.edu/~cool/omegaCen/). The optical counterpart is also clearly Ha bright in the Ha656— R-606 CMD. The counterpart was also easily visually confirmed as blue and Ha bright.
In Cool et al. (2013), a number of very faint blue stars were found in the X-ray error circles that were not detected in the Ha filter. These stars occupy the same region of the CMDs as white dwarfs and without the associated X-ray emission one would likely conclude that is what they are. However, Cool et al. (2013) calculated 31
18
20
00 2 22 00 24
26
28 -2 0 2 4 6 -2 -1 0 U336 ” 1814 H0656 “ ^606
Figure 3.3: Selection of CMDs for X-ray Source 21b. Symbols as in Figure 3.2.
the probability of a white dwarf landing in an X-ray error circle by chance to be
1.8%. In addition, the absence of an Ho: detection does not necessarily mean the stars are not Ha bright, they may be too faint to be detected in the filter. They concluded that these stars likely represented the optical counterparts of the X-ray sources and were labeled as faint blue cataclysmic variables (fbCV). One example of an fbCV in the core of Omega Cen is 21b. The counterpart is consistently blue of the MS including in the U336—Isi4 versus B438 CMD shown in Figure 3.3. The star was again not detected in the Ha filter and therefore does not appear in the Ha656—
R606 versus B438 CMD in Figure 3.3. As a result, we are still unable to confirm the counterpart’s status as a CV. UV275 — ^606 Ha656 — ^606
Figure 3.4: Selection of CMDs for X-ray Source 14a. Symbols as in Figure 3.2.
A number of candidate ABs were identified by Cool et al. (2013) in Omega Cen as well. One example of a previously proposed AB in the core is the possible BY
Dra star within the error circle of X-ray source 14a. The counterpart again appears to the red of the MS in the WFC3 CMDs including in the UV275—R.606 versus R606
CMD in Figure 3.4. The Hq:656— R-606 versus B438 CMD also shows that the star is slightly H-alpha bright. We therefore independently confirm this object as a possible
BY Dra star in Omega Cen.
Cool et al. (2013) also found a number of potential red stragglers within the error circles of the X-ray sources of Omega Cen. Surprisingly, most of the red straggler candidates were found on or near the metal-rich anomalous giant and 33
subgiant branches of the cluster. X-ray sources were at least five times more common among the RGB/SGB-a stars compared to the normal giant and subgiant branch stars. They concluded that these stars were the likely sources of the X-ray emission considering their low probability (~ 2 x 1 0 -5 ) of falling in the error circles by chance.
In addition, these stars showed signs of Ha emission which is characteristic of stars with enhanced coronal activity. However, without metallicity measurements, it is not known whether these stars are true members of the anomalous branch or appear in that region of the CMDs for some other reason. The best example of this class of object in the WFC3 data is a possible red straggler associated with the X-ray source lie that we have newly identified. Figure 3.5 shows the U336—Isi4 and Ha656—R606 versus B438 CMDs where the counterpart is to the red of the anomalous branch and is clearly Ha bright. The Ha excess was also confirmed visually. 34
U336 — 1814 Ha656 — ^606
Figure 3.5: Selection of CMDs for X-ray Source lie. Symbols as in Figure 3.2. 35
Chapter 4
Results
4.1 Cataclysmic Variables
Cool et al. (2013) were able to identify five UV/blue and Ha bright optical coun terparts as likely CVs within the core of Omega Cen. Three of these CVs represent the counterparts of the first X-ray sources detected in the core of the cluster (now labeled 1 2 a, 13a, and 13c) that were first identified by Carson, Cool & Grindlay
(2 0 0 0 ). Cool et al. (2013) also tentatively characterized an additional counterpart as a CV in the error circle of X-ray source 13f which showed clear UV/blue excess but whose Ha brightness was uncertain.
4.1.1 12a
The previously identified counterpart of X-ray source 1 2 a shows significant UV/blue and Ha excess in the UV225—B438, U336—B438 and Ha656—Reo6 versus B438 CMDs 36
in Figure 4.1. The counterpart’s UV/blue and Ha excesses were greater than 0.5 magnitudes so we were able to confirm the excesses visually as well. We can confirm the previously identified CV as the optical counterpart to source 1 2 a.
UV225 — B438 U336 — B438 H 0^656 — ^606
Figure 4.1: Selection of CMDs for X-ray Source 1 2 a. Symbols as in Figure 3 .2 .
4.1.2 13a
In the error circle of source 13a, the Carson et al. (2000) counterpart is clearly
UV/blue and Ha bright in the UV225-B 438, U336-B 438, and Ha656-R -606 versus B438
CMDs in Figure 4.2. We also confirmed their UV/blue and Ha excesses visually.
We can confirm the previous CV characterization for the optical counterpart of 13a. -2 0 2 4 6 -2 -1 0 1 2 -2 -1 0
U V 2 2 5 — ^ 4 3 8 U336 — B438 H « 6 5 6 — ^ 6 0 6
Figure 4.2: Selection of CMDs for X-ray Source 13a. Symbols as in Figure 3.2.
4.1.3 13c
The optical counterpart of 13c shows significant UV/blue and Ha excesses in the
CMDs that were also confirmed visually. In Figure 4.3 we present CMDs for the source in addition to those shown in Figure 3 .2 and confirm the counterpart’s pre vious characterization as a CV.
4.1.4 13f
The optical counterpart of the X-ray source 13f was tentatively identified as a CV by
Cool et al. (2013). The counterpart was given the label of “CV?” due to its uncertain
Ha brightness. Figure 4.4 shows the UV225—B438, U336—B438 and Ha656—R606 versus 38
Figure 4.3: Additional CMDs for X-ray Source 13c. Symbols as in Figure 3 .2 .
B438 CMDs for the source. The counterpart to source 13f is consistently blue and
Ho bright in the CMDs. The counterpart’s UV and Hct excesses were also easily identified visually. We can confirm that the previously identified counterpart for 13f is, in fact, a CV.
4.1.5 23b
The counterpart identified by Cool et al. (2013) as a CV within the error circle of
X-ray source 23b was not detected in the images and therefore was not listed in the Bellini et al. (2017) catalog despite coverage in several filters. Using the finding UV 225 — ^438 U 336 — B 438 H0f656 — ^606
Figure 4.4: Selection of CMDs for X-ray Source 13f. Symbols as in Figure 3.2.
charts from Cool et al. (2013)*, we located the counterpart in the image stacks and were able to confirm that it was blue and UV bright visually. Unfortunately, a cosmic ray fell on top of the proposed counterpart in the Ha656 image stack so we were unable to conclude if it appeared Ha bright visually as well.
4.1.6 24c
The star identified as a likely CV candidate by Cool et al. (2013) within the error circle of source 24c was not included in the Bellini et al. (2017) catalog and only had coverage in the R606 and Ha656 filters. However, we were able to confirm the
'http: / /www. physics. sfsu.edu/~cool/omegaCen/CH ARTS/ 40
counterpart as Ha bright visually using those filters.
4.1.7 22h
In the error circle of X-ray source 22h, there is a star that appears consistently blue of the MS as seen in the UV225—B438 and U336—B438 versus B438 CMDs shown in
Figure 4.5. However, the counterpart fell just outside of the Ha656 filter FOV and therefore does not appear in the Ha656—R-606 versus B438 CMD in Figure 4.5. Due to the absence of coverage in Ha, we are only able to tentatively label this new counterpart as a CV.
14
16
18
20 m00 GO 22
24
26
28
UV225 _ B438 U336 - B438 Ha656 - ^606
Figure 4.5: Selection of CMDs for X-ray Source 22h. Symbols as in Figure 3.2. 41
4.2 Faint Blue Cataclysmic Variables
Cool et al. (2013) were able to identify two potential fbCVs in the core of the cluster that showed significant UV/blue excess but were not detected in the Ha filter.
4.2.1 21b
The counterpart to X-ray source 2 1 b appears significantly to the blue of the MS in the CMDs including the U336—B438 and B438—I8J4 versus B438 CMDs shown in
Figure 4.6 and was not detected in the Ha filter. We have confirmed the fbCV characterization of the counterpart within the error circle of source 2 1 b.
Figure 4.6: Additional CMDs for X-ray Source 21b. Symbols as in Figure 3 .2 . 42
4.2.2 22c
The previously identified counterpart of X-ray source 2 2 c is consistently blue and
UV bright. Figure 4.7 shows the UV225—B438 and U336—B438 versus B438 CMDs for
2 2 c. Unlike the other fbCV candidates, this counterpart was detected once in the
Ha!656 filter where it appears Ha faint in the Ha65 6 ~ ^ 606 versus B438 CMD in Figure
4.7. However, we are unable to rule out the candidate as a possible fbCV based on a single, poor quality Ha measurement.
UV225 - B438 *-*336 - B438 Ha656 - ^606
Figure 4.7: Selection of CMDs for X-ray Source 2 2 c. Symbols as in Figure 3 .2 . 43
4.2.3 Ilf
For the source Ilf, there is a potential counterpart that lies near the white dwarf se quence on the CMDs. Figure 4.8 shows the UV225—B438, U336—B438 and Ha656—R606 versus B438 CMDs for Ilf. It appears consistently blue of the MS and Ha faint in the CMDs. Like the counterpart of 22c, its position on the Ha656—R-606 versus B438
CMD is based on poor quality Ha656 photometry so it is still possible that the counterpart is actually Ha bright. We have characterized this new counterpart as a possible fbCV.
UV225 - B438 U336 — B438 H0656 “ ^606
Figure 4.8: Selection of CMDs for X-ray Source Ilf. Symbols as in Figure 3.2. 44
4.2.4 22i
A potential counterpart to the X-ray source 22i falls near the white dwarf sequence in all of the CMDs including the UV225—B438 and U336—Isi4 versus B438 CMDs in Figure 4.9. The counterpart was not detected in the Ha656 filter, therefore it does not appear in the Ha656—R606 versus B438 CMD. We have newly identified the counterpart of source 22i as an fbCV candidate.
UV225 — B438 B 336 1814 HQ656 R 606
Figure 4.9: Selection of CMDs for X-ray Source 22i. Symbols as in Figure 3.2. 45
4.3 Active Binaries
Cool et al. (2013) identified two possible BY Dra stars within the core of Omega
Cen that appeared to the red of the MS and were slightly Ha bright.
4.3.1 14a
The previously identified optical counterpart of X-ray source 14a has a close neigh bor which is marked as a red square in the U336—B438 and U336—I8i4 versus B438
CMDs in Figure X .1 0 . It is possible that the counterpart is stealing the flux of its brighter neighbor to appear slightly Ha bright in the Ha656—R606 versus B438
CMD. The counterpart appears ~ 0 .2 to the left of the MS and the neighbor lies
~ 0 .0 2 magnitudes to the right of the MS. Without taking its close neighbor into account, we calculated the equivalent width of the counterpart using Equation 3.8 to be EWjfa ~ 3.6A. If we assume that the counterpart is stealing the ~ 0 .0 2 mag nitudes of Ha656 flux from its brighter neighbor, it would still have an EW #a ~
1.3A. We can confirm that the counterpart’s H-alpha excess is consistent with a BY
Dra star.
4.3.2 24e
The potential BY Dra star identified by Cool et al. (2013) as the counterpart to
X-ray source 24e appears to the red of the MS in the CMDs but we could not U336 — B438 U336 — 1814 HQT656 — R606
Figure 4.10: Additional CMDs for X-ray Source 14a. The stars within the error circle of the source are marked as red dots and the stars just outside of the error circle, up to 1.5x the error circle radius, are shown as cyan dots. The optical counterpart is represented as a red triangle in the diagrams and the counterpart’s close neighbor is marked as a red square.
confirm the star’s Ha excess. The UV225—B438, U336—Isi4 , and Ha656—R606 versus
B438 CMDs are shown in Figure 4.11. The X-ray source 24e was very faint in the
Cycle 1 data and it was not redetected in Cycle 13. We were unable to confirm the
BY Dra characterization of the star within the error circle of source 24e.
4.3.3 13e
Just outside of the error circle of X-ray source 13e, there is a star that was only detected in a handful of filters. Its classification as the optical counterpart is less 47
14
16 -
28 -2 0 2 4 6 - 2 0 2 4 6 - 2 -1 0 UV 225 “ B438 U336 - >814 Ha656 - ^606
Figure 4.11: Selection of CMDs for X-ray Source 24e. Symbols as in Figure 3.2.
certain as a result of it falling outside of the 95% X-ray error circles. However, the star appears to the red of the MS and is slightly Ha bright in the U336—Isi4 >
B438—1814, and Ha656—R-606 versus B438 CMDs shown in Figure 4.12. The potential counterpart also has a close neighbor but it lies on the MS in the Hai656—R-606 versus
B438 CMD so it is unlikely that the counterpart is stealing flux from it to appear
Ha bright. We estimated the equivalent width of the Ha emission line to be EW^a
~ 7.5A for this counterpart. We have tentatively classified this star as a new AB candidate. 48
U336 — 1814 B438 — 1814 H0656 — ^606
Figure 4.12: Selection of CMDs for X-ray Source 13e. Symbols as in Figure 4.10.
4.3.4 14f
In the error circle of X-ray source 14f, there is a potential counterpart that appears
red of the MS and is slightly Ha bright in the U336—B438, U336—Isi4 , and Ha656—R-606
versus B438 CMDs in Figure 4.13. We calculated the equivalent width of the coun terpart to be EW Ha — 6A. We have confirmed the optical counterpart as a possible
new AB.
4.3.5 23a
For X-ray source 23a, there is a star that lies on the border of the X-ray error circle
that falls consistently to the red of the MS in the CMDs and appears Ha bright. -2 -1 0 1 2 0 2 4 6 -2 -1 0 U 336 “ B438 U 336 “ >814 Ha656 “ ^606
Figure 4.13: Selection of CMDs for X-ray Source 14f. Symbols as in Figure 3.2.
The UV225—B438, U336—1814, and Hq:656—R606 versus B438 CMDs for the source are shown in Figure 4.14. We determined the equivalent width of the counterpart to be EW/fa — 5.9A, which is typical for a BY Dra star. We have characterized the counterpart as a new AB candidate. 50
UV225 — B438 U336 — 1814 HC656 — ^606
Figure 4.14: Selection of CMDs for X-ray Source 23a. Symbols as in Figure 3.2.
4.3.6 24h
Finally, within the error circle of X-ray source 24h a star lies to the red of the
MS and is slightly Ha bright. Figure 4.15 shows the UV225—B438, U336—B438, and
Ha656—R-606 versus B438 CMDs for 24h. The star has an H-alpha equivalent width of
EW Ha — 4.9A. We have identified this new possible AB as the optical counterpart of source 24h. Figure 4.15: Selection of CMDs for X-ray Source 24h. Symbols as in Figure 3.2.
4.4 Red Stragglers or RGB/SGB-a Stars
In the core of Omega Cen, Cool et al. (2013) identified three optical counterparts that lie on or near the anomalous giant and subgiant branches of the cluster. In the absence of metallicity measurements, they were unable to determine whether these stars represented a new class of binary system associated with the anomalous branch or if they were red stragglers. 52
4.4.1 13b
The star within the error circle of X-ray source 13b was previously included in this category by Cool et al. (2013) but it does not lie on the anomalous branch. The
UV225—B438 and U336— 1814 versus B438 CMDs in Figure 4.16 show that the star is a member of one of the other more metal-poor subgiant branches. In addition, the
Ha656—R-606 versus B438 CMD does not establish the star as Ha bright. Therefore, this star is unlikely to be the optical counterpart of 13b.
14
16
18
20 00 nm GO 22
24
26
28
UV225 - B438 U336 — U14 Ha656 — Reo6
Figure 4.16: Selection of CMDs for X-ray Source 13b. Symbols as in Figure 3.2. 53
4.4.2 22e
The previously identified counterpart of source 22e only had coverage in the R,606 filter so we were unable to construct CMDs to determine its Ha brightness or see if it fell on the anomalous branch. Therefore, we are unable to confirm this star as the optical counterpart to X-ray source 22e.
4.4.3 24f
The potential RGB-a star that was previously identified as the counterpart of X- ray source 24f is shown in the U336—Isi4 , B438—Igi4 , and Ha656 —R.606 versus B438
CMDs in Figure 4.17. The star is saturated in the B438 and R606 filters, falls outside of the error circle, and it is not noticeably Ha bright based on the single Ha656 measurement in the catalog. Therefore, we are unable to confirm this star as the optical counterpart to source 24f.
4.4.4 lie
The error circle of X-ray source lie contains a star that shifts between being red and blue of the RGB-a branch in the various CMDs. Figure 4.18 shows the UV225—B438,
U336—B438 , and B438—I8i4 versus B438 CMDs for the source. The star is likely a red straggler because it does not consistently fall on the RGB/SGB-a branch. However, metallicity measurements would be needed to confirm this characterization. U336 - 1814 B438 — 1814 H0656 — ^606
Figure 4.17: Selection of CMDs for X-ray Source 24f. Symbols as in Figure 3.2.
4.4.5 14b
A potential optical counterpart that fell slightly outside of the 95% X-ray error circle of X-ray source 14b falls consistently on the anomalous branch and appears slightly
Ha bright in the CMDs. We calculated the H-alpha equivalent width to be EW #Q
~ 1.8A. The UV225-B438, U336 B438, and and Ha656- R 6o6 versus B438 CMDs for
14b are shown in Figure 4.19. We have newly identified this RGB-a star as the possible optical counterpart of source 14b. 55
14
16
18
20 moo 00 22
24
26
28
Figure 4.18: Additional CMDs for X-ray Source lie. Symbols as in Figure 3 .2 .
4.4.6 22b
For X-ray source 2 2 b, a star slightly outside of the X-ray error circle appears near the turnoff of the anomalous branch in many but not all of the CMDs. An example of each can be seen in the UV225—B438 and U336—B438 versus B438 CMDs in Figure
4.20. The star also appears slightly Ha bright as seen in the Hof656— ^ 606 versus
B438 CMD in Figure 4.20. We estimated the equivalent width of the counterpart to be EW #q ~ 2.7h. We have determined that this newly identified red straggler or
RGB/SGB-a star is the possible optical counterpart to source 2 2 b. UV225 “ B438 U336 - B438 H0656 - ^606
Figure 4.19: Selection of CMDs for X-ray Source 14b. Symbols as in Figure 3.2.
4.5 Blue-only and Ho-only Stars
In Cool et al. (2013), there were a number of potential optical counterparts that were
labeled “Blue-only” or “Ha-only” . Unlike the fbCVs, the blue-only stars were bright enough to be detected in the Ha filter but showed no signs of H-alpha emission.
Ha-only stars appeared Ha bright but showed no blue excess. Therefore, these
counterparts were the least secure out of all of the categories included in Cool et al.
(2013). One blue-only star and three Ha-only stars that were previously identified
as possible optical counterparts fell within the core of Omega Cen. UV225 “ B438 U336 ~ B438 Ha656 - ^606
Figure 4.20: Selection of CMDs for X-ray Source 22b. Symbols as in Figure 3.2.
4.5.1 24a
In Cool et al. (2013), the blue-only star found within the error circle of source 24a was only slightly blue and was neither He* faint or He* bright. The star was very faint and appeared at the bottom of the CMDs in a region that is populated with background stars. They concluded that the most likely explanation for the potential counterpart was that it was a background star that landed in the X-ray error circle by chance. This blue-only star was not measured in the Bellini et al. (2017) catalog and in the images, the star didn’t show blue or Ha excess visually. We are unable to confirm this star as the optical counterpart to X-ray source 24a. 58
4.5.2 21c
Cool et al. (2013) identified an Ha-only star within the error circle of X-ray source
21c. The star was not detected in their B filter and was classified as only potentially
Ha bright. The star was also not included in the Bellini at al. (2017) catalog.
Visually, the star is not a convincing candidate in the blue/UV or Ha filters and we are therefore unable to confirm the star as a potential counterpart to source 21c.
4.5.3 21d
Within the error circle of source 21d, Cool et al. (2013) identified a potentially Ha bright star that fell on the MS in their B—R CMD. The possible counterpart is very close to its neighbor and now appears Ha faint in the Ha656—R606 versus B438 CMD in Figure 4.21. The star also doesn’t show blue excess as seen in the UV225—B438 and U336—B438 versus B438 CMDs in Figure 4 .2 1 . We are not able to confirm this star as the optical counterpart of source 2 1 d.
4.5.4 33d
Finally, the Ha-only optical counterpart of X-ray source 3 3 d identified by Cool et al.
(2013) appeared on the MS of their B—R CMD and was slightly Ha bright. The star now falls outside of the error circle of source 33d. The counterpart appears to be a
MS star from the CMDs. Figure 4.22 shows the UV225—B438 and U336 —B438 versus
B438 CMDs for source 3 3 d. In addition, the X-ray source was not covered in the 59
n 1 4
28 -2 0 2 4 6 -2 -1 0 1 2 - 2 -1 0 UV225 — B438 U336 ~ B438 HQ656 — ^606
Figure 4.21: Selection of CMDs for X-ray Source 2Id. Symbols as in Figure 3.2.
Hct656 filter so we cannot confirm the star’s potential Ha brightness and therefore its status as the optical counterpart of 33d. Figure 4.22: Selection of CMDs for X-ray Source 33d. Symbols as in Figure 3.2.
4.6 Variable Star
Kaluzny et al. (2004) performed a photometric survey in the field of Omega Cen that identified 117 new variable stars. They also determined that nine of these variable stars were the optical counterparts of known X-ray sources located outside the core of the cluster.
4.6.1 lib
Within the error circle of X-ray source lib , there is a star that was classified as a variable star by Kaluzny et al. (2004) and assigned the label NV371. Using proper 61
motions data, the star was given a 100% probability of membership by van Leeuwen et al. (2000). It was determined to be the likely counterpart of source lib by
Henleywillis et al. (2018) after searching the variable star catalog for matches to the 95 X-ray sources that they newly identified. The star appears near the metal- poor RGB and is slightly Ho; bright. We calculated the Ho: equivalent width of the counterpart to be EW #a ~ 3A. Figure 4.23 shows the U336—I8i4 , B438—1814, and
Hq!656—R-606 versus B438 CMDs for the source. We can confirm that this variable star is the likely optical counterpart of the X-ray source lib .
14
16
18
20 00 no GO 22
24
26
28 ^336 ~ 1814 B438 “ 1814 HOf656 “ ^606
Figure 4.23: Selection of CMDs for X-ray Source lib . Symbols as in Figure 3.2. 62
4.7 Foreground Stars
Several of the X-ray sources found towards the core of Omega Cen are most likely foreground stars with active coronae. With limited access to proper motions data,
Cool et al. (2013) only identified a single foreground star within the core of the cluster based on its location on the CMDs.
4.7.1 22a
A star within the error circle of X-ray source 2 2 a was previously characterized as a
foreground star by Cool et al. (2013). The star appears significantly to the red of the
MS in some of the CMDs, including the U336—B438, U336—Isi4 , and B438—I8i4 versus
B438 CMDs shown in Figure 4.24. The X-ray source did not fall within the FOV of
the Hof656 filter so we were unable to determine if the star is Ha bright. However,
using proper motions data from Bellini et al. (2014), we were able to confirm that
this star is not a member of the cluster.
4.7.2 22d
A star characterized by Cool et al. (2013) as a blue straggler in the error circle of
X-ray source 22d was shown to be a non-member by Deveny et al. (2016). The star
appears significantly to the red of the MS in the CMDs but the source didn’t fall
within the Ha656 FOV so we are unable to determine its Ha brightness. Figure 4.25 Figure 4.24: Selection of CMDs for X-ray Source 2 2 a. Symbols as in Figure 3 .2 .
shows the UV225-B 438, U336-B 438, and U336-I 814 versus B438 CMDs for 2 2 d. We can confirm that this star is the likely counterpart of source 2 2 d.
4.7.3 22j
A star within the error circle of source 2 2 j appears significantly to the red of the MS in many of CMDs. The star also shows clear Ha excess with an equivalent width of
EW Ha — 10.6A. Figure 4.26 shows the U336- B 438, U336- l 8i4 and Ha656-R 606 versus
B438 CMDs for source 2 2 j. Finally, we determined that the star is not a member of the cluster using the Bellini et al. (2014) proper motions data. Therefore, we can confirm that this Ha bright foreground star is the likely optical counterpart to 22j. 64
14 * ■ 16
jjr
18 -<■ ► i a J u , W Y:.
20 00oo 1 CO • i k m 22 '' '
24
26 . j j p l ? ■ ' I P
28 —i— -j---- 1_____ 1---- 1_ -2 0 2 4 6 -2 -1 0 1 2 -2 0 2 4 6 UV225 “ B438 U336 - B438 U336 “ 1814
Figure 4.25: Selection of CMDs for X-ray Source 22d. Symbols as in Figure 3.2.
4.7.4 14c
Finally, a possible optical counterpart to the X-ray source 14c appears above the red giant and subgiant branches and is Ha faint in the CMDs. Figure 4.27 shows the UV225-B 438, U336- I 8i4 and Ha656-R 606 versus B438 CMDs for 14c. We have confirmed this star as an additional non-member of the cluster using the Bellini et al. (2014) catalog of proper motions data. Figure 4.26: Selection of CMDs for X-ray Source 22j. Symbols as in Figure 3.2.
4.8 Active Galactic Nuclei
As a result of Omega Cen’s large core and Chandra’s sensitivity, we expect that a substantial fraction of the X-ray sources within the cluster to be background AGN.
Cool et al. (2013) identified four probable AGN within the core of the cluster from their extension in the subtracted images and their positions on the CMDs. They found that the AGN candidates appeared to the blue of the MS and were Ha faint. UV225 — B438 U336 — 1814 H0656 — ^606
Figure 4.27: Selection of CMDs for X-ray Source 14c. Symbols as in Figure 3.2.
4.8.1 22f
The object identified by Cool et al. (2013) as the optical counterpart to X-ray source
22f appears to the blue of the MS in most of CMDs including the UV225—B438,
U336—B438, and U336— 1814 versus B438 CMDs shown in Figure 4.28. However, the source did not fall within the Hq:656 filter FOV so we were unable to confirm if the object was Ha faint. The counterpart did show clear signs of extension in the subtracted images. Therefore, we are able to confirm the AGN characterization of this optical counterpart. 67
Figure 4.28: Selection of CMDs for X-ray Source 22f. Symbols as in Figure 3.2.
4.8.2 23c
Another previously characterized AGN within the error circle of X-ray source 23c was not included in the Bellini et al. (2017) catalog. Visually, the object appears
Ha faint but not particularly blue. We were able to identify signs of extension in the subtracted images. We confirm this probable AGN as the optical counterpart of source 23c.
4.8.3 24g
The object identified as a possible AGN within the source 24g was not included in the Bellini et al. (2017) catalog and there were no noticeable peaks in the images 68
at the locations marked on the Cool et al. (2013) finding charts. In addition, there were no signs of extension within the X-ray error circle. We were unable to confirm this object as the optical counterpart to X-ray source 24g.
4.8.4 31b
Finally, the possible AGN that was previously identified as the optical counterpart of X-ray source 31b was not included within the Bellini et al. (2017) catalog. There were also no noticeable peaks in the images at the position marked by the Cool et al. (2013) finding charts. We were unable to identify extension in the subtracted images and therefore unable to confirm the counterpart’s previous characterization as an AGN.
4.9 Proper Motions
The proper motions for 245,443 stars within the core of Omega Cen measured by
Bellini et al. (2014) are shown in Figure 4.29. The stars clustered together in the center of Figure 4.29 represent the likely members of the cluster and the red triangles in the upper left quadrant of the plot mark the four non-member stars we identified in the error circles of the X-ray sources 14c, 22a, 22d, and 22j. In addition, we were able to determine that the proposed counterparts for sources lib , lie, 14b, 14f, 22b,
22h, and 24h had proper motions consistent with cluster membership. -10 - 5 0 5 10 ARA, mas / year
Figure 4.29: Proper motions for 245,443 stars within the core of Omega Cen mea sured by Bellini et al. (2014). The red circle of radius 3 milliarcseconds per year separates the vast majority of the stars that are likely members of the cluster from the field stars. The red triangles represent the four non-member stars found within the X-ray error circles of 14c, 22a, 22d, and 22j.
4.10 Summary of Results
Within the core of the Omega Cen, 67 X-ray sources fell within the field of view of
one of the eight filters we selected from the Bellini et al. (2017) photometric catalog.
Using this new and independent data set, we were able to confirm the previous
characterization of four CVs, two fbCVs, one AB, one variable star, two foreground
stars, and two AGN. We were unable to confirm two CVs, one AB, three stars 70
on or near the RGB/SGB-a branch, one “Blue-only” star, three “H-alpha-only” stars, and two AGN identified by Cool et al. (2013) as the optical counterparts of their respective X-ray sources. Finally, we have identified twelve new possible optical counterparts including one CV, two fbCVs, four ABs, and three potential red stragglers or RGB/SGB-a branch stars, and two foreground stars. We present the list of all of the optical counterparts we were able to confirm or not confirm along with their IDs in the Bellini et al. (2017) catalog, offset from the center of their X-ray error circles, and the number of images in which they were detected in each filter in Table 4.1. 71
Source Optical Bellini Offset u v 225 u v 275 U336 B438 Rfr06 Ha656 Ha658 1814 ID ID ID (") lib NV371 74036 0.32 13 16 16 2 5 2 6 15 lie New RGB/SGB-a 165196 0.30 27 28 31 31 57 4 6 30 Ilf New fbCV 66767 0.35 13 16 16 16 28 2 0 15 12a Confirmed CV 152600 0.29 17 19 20 20 48 3 6 22 13a Confirmed CV 338248 0.10 14 16 22 18 44 3 2 19 13b Unconfirmed RG B/SG B-a 351954 0.31 16 20 29 23 51 4 6 24 13c Confirmed CV 396641 0.16 13 15 21 18 41 2 5 19 13e New AB? 369617 0.48 0 0 26 22 44 3 5 23 13f CV? CV 300860 0.29 23 26 32 30 54 5 6 30 14a Confirmed BYDra 270659 0.42 26 29 36 33 61 5 6 33 14b New RGB/SGB-a 247948 0.75 26 31 37 34 62 5 6 34 14c New FGND 246850 0.41 27 31 14 34* 55* 5 6 22 14f New AB 160876 0.25 5 19 21 19 39 3 5 20 21b Confirmed fbCV 63178 0.14 6 9 9 9 11 1 0 8 21c Unconfirmed Ha-only - - 0 0 0 0 0 0 0 0 21d Unconfirmed Ha-only 58008 0.51 8 11 11 11 12 2 3 10 22a Confirmed FGND 43230 0.37 7 7 8 8 8 0 0 8 22b New RG B/SG B-a 104617 0.69 12 15 16 16 23 2 0 15 22c Confirmed fbCV 129914 0.16 5 6 8 8 17 1 0 7 22d BS FGND 207656 0.10 2 2 2 2 4 0 0 2 22e Unconfirmed RG B/SG B-a 302965 0.16 0 0 0 0 7 0 0 0 22f Confirmed AGN 314002 0.03 5 6 7 8 12 0 0 7 22h New CV? 166688 0.49 1 1 1 1 4 0 0 1 22i New fbCV 262553 0.22 8 8 11 11 14 1 0 10 22j New FGND 283809 0.11 7 5 12 12 4 2 0 11 23a New AB 435075 0.36 5 8 12 9 20 2 6 10 23b Unconfirmed CV? - - 0 0 0 0 0 0 0 0 23c Confirmed AGN - - 0 0 0 0 0 0 0 0 24a Unconfirmed Blue-only - - 0 0 0 0 0 0 0 0 24c Unconfirmed CV - - 0 0 0 0 0 0 0 0 24e Unconfirmed BYDra 313622 0.81 1 1 1 1 2 1 0 1 24f Unconfirmed RG B/SG B-a 262268 0.85 8 9 11 10* 12* 1 1 8 24g Unconfirmed AGN? - - 0 0 0 0 0 0 0 0 24h New AB 220295 0.03 4 7 10 9 17 2 3 8 31b Unconfirmed AGN - - 0 0 0 0 0 0 0 0 33d Unconfirmed Ha-only 443266 0.44 1 1 3 3 3 0 0 3
Table 4.1: List of the optical counterparts we were able to confirm or not confirm along with their IDs in the Bellini et al. (2017) catalog, offset from the center of their X-ray error circles, and the number of images in which they were detected in each filter. Abbreviations for the source classifications and symbols as in Table 2.3. NV371 is a variable star from Kaluzny et al. (2004). 72
Chapter 5
Discussion
Of the 67 X-ray sources within the core of Omega Cen, we have newly identified or confirmed 24 possible optical counterparts. In Figure 5.1, we plot the fluxes and luminosities of the sources in the medium X-ray band (0.5 — 4.5 keV erg cm-2 s-1) as a function of their offset from the cluster center in core radii. For the sources detected in Cycle 13, we used the fluxes reported by Henleywillis et al. (2018) and for the sources only detected in Cycle 1, we used the fluxes reported by Haggard et al. (2009). Sources with proposed optical counterparts are indicated with colored symbols. Solid symbols signify likely cluster members and open symbols mark non members of the cluster. The brightest CVs, which are shown as large blue triangles in Figure 5.1, all fall within the inner half of the core. Their concentration in the cluster center suggests a dynamical origin. We were less likely to identify optical counterparts for fainter X-ray sources and sources with greater distances from the center of the cluster. X-ray sources with greater offsets often had less coverage in 73
the various filters which made their optical counterparts more difficult to identify.
The Hci656 filter, in particular, had a relatively small field of view and did not cover a number of sources near the edge of the core.
A A CV r 10"13 A ▼ fbCV iH 1 rH * AB CO i • RGB/SGB-a CM to 1032 - 1 Variable v*.Ch ♦ E FGND u
CD □ _ o H 1 - O AGN i »« » » 8* s ♦ T CD CD • s LD ▲ • D 1031- • n • o □ • in 1 A LO o O • . :D • : 10-15 1 • * «.• m * • • o • ▼ • 1030 : **
• T------1------1------1------1------T 0.0 0.2 0.4 0.6 0.8 1.0 1.2 e I rc
Figure 5.1: X-ray flux and luminosity as a function of offset from the center of the cluster. Abbreviations for the source classifications in the legend as in Table 2.3. Solid symbols mark likely cluster members and open symbols show optical counterparts outside of the cluster. The small colored symbols represent sources with less secure characterizations.
We can further examine the X-ray sources within the core of Omega Cen by constructing an X-ray color-magnitude diagram. In Figure 5.2, we show the fluxes and luminosities of the sources versus their X-ray color, which is defined as the log of the ratio of their fluxes in the soft (0.5 — 1.5 keV) and hard (1.5 — 6.0 keV) bands. 74
Harder sources will appear on the left side of the diagram and softer sources will appear on the right. Accreting sources often have hard X-ray colors and coronally active sources are typically softer (Heinke et al. 2005). The CVs and the AGN in
Figure 5.2 have relatively hard X-ray colors which are expected for accreting sources.
The ABs have a large range in color but they are softer on average compared to the
CVs. The foreground stars appear to be softer which is consistent with them been likely coronal sources. Surprisingly, the potential red stragglers or RGB/SGB-a stars appear to be harder X-ray sources which is in contrast with them being coronally active binaries.
▲ ▲ CV -10"13 ▲ v fbCV rH 1 rH * AB i
It - • • 10~15 1 o * A *• □
• X LO * •
□ * o •
- j ’£ •
# % io30- / ▼ • • • -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 I Og (XSOft/X/jard)
Figure 5.2: X-ray color-magnitude diagram for the sources with non-zero fluxes in both the hard and soft bands. Abbreviations for the source classifications in the legend as in Table 2.3 and symbols as in Figure 5.1. 75
5.1 Future Work
We plan to investigate the significance of the non-detections of optical counterparts within the core of the cluster. It appears that Omega Cen has fewer CVs than the field (Henleywillis et al. 2018). However, it is possible that faint CVs are present but we are not detecting them due to insufficient X-ray or optical data. It would be especially useful to determine the limiting magnitude of the H«6 5 6 filter in the Bellini et al. (2017) catalog. With this information we can determine if the fbCV candidates are, in fact, too faint to be detected in the Ha656 filter even with Ha emission lines.
We will also investigate the effects of non-simultaneous measurements in different filters. In addition, it would aid our understanding of the red straggler candidates if metallicity measurements were taken to determine if they are members of the anomalous branch. Finally, we plan to search for optical counterparts of the X-ray sources newly detected by Henleywillis et al. (2018) using archival Hubble ACS data. 76
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