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XMM-Newton Study of the Draco Dwarf Spheroidal Galaxy ⋆

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XMM-Newton Study of the Draco Dwarf Spheroidal Galaxy ⋆ A&A 586, A64 (2016) Astronomy DOI: 10.1051/0004-6361/201526233 & c ESO 2016 Astrophysics XMM-Newton study of the Draco dwarf spheroidal galaxy Sara Saeedi1, Manami Sasaki1, and Lorenzo Ducci1,2 1 Institut für Astronomie und Astrophysik, Kepler Center for Astro and Particle Physics, Eberhard Karls Universität Tübingen, Sand 1, 72076 Tübingen, Germany e-mail: [email protected] 2 ISDC Data Center for Astrophysics, Université de Genève, 16 Chemin d’Ecogia, 1290 Versoix, Switzerland Received 31 March 2015 / Accepted 2 October 2015 ABSTRACT Aims. We present the results of the analysis of five XMM-Newton observations of the Draco dwarf spheroidal galaxy (dSph). The aim of the work is the study of the X-ray population in the field of the Draco dSph. Methods. We classified the sources on the basis of spectral analysis, hardness ratios, X-ray-to-optical flux ratio, X-ray variability, and cross-correlation with available catalogues in X-ray, optical, infrared, and radio wavelengths. Results. We detected 70 X-ray sources in the field of the Draco dSph in the energy range of 0.2−12 keV and classified 18 AGNs, 9 galaxies and galaxy candidates, 6 sources as foreground stars, 4 low-mass X-ray binary candidates, 1 symbiotic star, and 2 binary system candidates. We also identified 9 sources as hard X-ray sources in the field of the galaxy. We derived the X-ray luminosity function of X-ray sources in the Draco dSph in the 2−10 keV and 0.5−2 keV energy bands. Using the X-ray luminosity function in the energy range of 0.5−2 keV, we estimate that ∼10 X-ray sources are objects in the Draco dSph. We have also estimated the dark matter halo mass that would be needed to keep the low-mass X-ray binaries gravitationally bound to the galaxy. Key words. galaxies: individual: Draco dwarf spheroidal galaxy – X-rays: galaxies – X-rays: binaries 1. Introduction (Evans et al. 2010) before the XMM-Newton observations were performed. All of the X-ray source studies using the ROSAT or Dwarf spheroidal galaxies (dSphs) are the most numerous galax- Chandra observations were aimed at classifying active galactic ies in the Local Group. So far, 19 dSphs satellite galaxies of nuclei (AGNs) in the field of the Draco dSph (e.g. Gibson & the Milky Way and 12 of M31 have been detected (Metz & Brandt 2012; Suchkov et al. 2006). In addition, the background Kroupa 2007). The dSphs are approximately spherical, have − X-ray emission of this galaxy in XMM-Newton and Chandra no gas or recent star formation, and are usually 1 2orders observations has been studied in order to find evidence for the of magnitude fainter than the faintest spiral galaxies known decay of dark matter (e.g. Riemer-Sørensen & Hansen 2009; (Nucita et al. 2013). They are the least luminous galaxies and ff Malyshev et al. 2014). Recently, two groups reported the de- their nature is fundamentally di erent from spiral and elliptical tection of a line at 3.5 keV based on observations of galaxies galaxies (Kormendy 1985). The absolute magnitude of dSphs ∼ − ∼ − and galaxy clusters, which was interpreted as a dark matter de- is 8 12 mag and their optical radius is 0.5 3 kpc (Irwin & cay line (Bulbul et al. 2014; Boyarsky et al. 2014). Lovell et al. Hatzidimitriou 1995). It has been suggested that dwarf galax- (2015) have performed simulations of the flux of the dark matter ies are the galaxies that are the most dominated by dark mat- decay line and suggest that an observation of Draco dSph with ter since their stellar velocity dispersion is significantly larger XMM-Newton with a total exposure time of 1.3 Ms is necessary than would follow from the virial theorem for the luminous mass to detect such a line. (Mashchenko et al. 2006). This paper reports the results of the study of five The Draco dSph (RA = 17h20m19s, Dec = 57◦5455, XMM-Newton observations of the Draco dSph and is organised J2000) has a major and minor diameter of 35.5 and 24.5cor- as follows. In Sect. 2 we describe the data reduction and analy- responding to 0.85 and 0.59 kpc (Falco et al. 1999) at a distance ± ∼ 10 sis of the XMM-Newton observations. In Sect. 3 we present the of 82.4 5.8 kpc (Kinemuchi et al. 2008). The age is 10 yr different methods used to classify the X-ray sources. In Sect. 4 (Aparicio et al. 2001; Bellazzini et al. 2002) and the metallic- we explain the properties and the classification of the detected ity of the galaxy is Z = 0.0004 (Girardi et al. 2004), which is ∼ ff sources. In Sect. 5 we discuss the X-ray luminosity function and 0.03 times the solar value (Ca au et al. 2010). The King model the possible presence of low-mass X-ray binaries in the Draco (King 1962) fits the stellar density profile with a core radius of dSph. 7.7 and a tidal radius of 40.1(Odenkirchen et al. 2001). Deep photometric surveys show that central stellar density increases inward from ∼2 to 0.5(Ségall et al. 2007). 2. Reduction and data analysis In X-ray surveys, the Draco dSph was first observed by the We analysed the public archival XMM-Newton (Jansen et al. ROSAT all-sky survey (Voges et al. 1999, 2000)andChandra 2001) observations of the Draco dSph. Table 1 summarises Based on observations obtained with XMM-Newton,anESAsci- the details of the observations. All observations were pointed ence mission with instruments and contributions directly funded by approximately at the centre of the galaxy (RA = 17h20m12.40s, ◦ ESA Member States and NASA. Dec =+57 54 55. 3, J2000). In all observations, the European Article published by EDP Sciences A64, page 1 of 23 A&A 586, A64 (2016) Table 1. XMM-Newton observations of the Draco dSph. ID † Observation ID Observation date EPIC-pn EPIC-MOS1 EPIC-MOS2 mode/filter T.exp [ks]∗ mode/filter T.exp [ks]∗ mode/filter T.exp [ks]∗ OBS1 0603190101 2009-Aug.-04 FF/Thin1 17.0 FF/Thin1 18.6 FF/Thin1 18.6 OBS2 0603190201 2009-Aug.-06 FF/Thin1 18.0 FF/Thin1 19.4 FF/Thin1 16.4 OBS3 0603190301 2009-Aug.-08 FF/Thin1 12.5 FF/Thin1 12.7 FF/Thin1 12.8 OBS4 0603190401 2009-Aug.-20 FF/Thin1 3.1 FF/Thin1 9.3 FF/Thin1 7.2 OBS5 0603190501 2009-Aug.-28 FF/Thin1 18.0 FF/Thin1 19.6 FF/Thin1 19.6 Notes. (†) ID numbers used in the text; () FF: full frame; (∗) T.exp: exposure time after screening for high background. Photo Imaging Cameras EPIC-pn (Strüder et al. 2001)and 43 EPIC-MOS1/2(Turner et al. 2001) were operated in the full- 41 42 frame mode with a time resolution of 73.4 ms and 2.6 s, 25 respectively. 52 7 The data analysis was performed using the XMM-Newton 6563 1 45 Science Analysis System (SAS ) version 11.0.1. For each obser- 64 59 17 vation, we produced EPIC-pn and EPIC-MOS event files via the 6 emchain and epchain tools. Good-time intervals (GTIs) were 24 12 20 35 determined for each EPIC by screening for time intervals with 68 29 high-energy background caused by soft proton flares. In OBS4, 70 47 58 40 28 9 the MOS data were broken into two parts due to the flares. We 51 48 3 1 merged the event files and extracted GTIs. Owing to the long 30 13 56 49 39 duration of the high background proton flares in this observa- 67 61 38 23 tion, the exposure time is significantly lower than in the other 54 5 3331 4 observations (see Table 1). 60 37 16 2 57 27 10 69 19 14 For each observation, the data were split into five energy 44 21 bands: B1 (0.2−0.5 keV), B2 (0.5−1.0 keV), B3 (1.0−2.0keV), 53 11 8 B4 (2.0−4.5 keV), and B5 (4.5−12 keV). Single to quadruple 62 22 32 26 ≤ 66 46 pixel events (PATTERN 12) have been used for all five energy 15 bands of the EPIC-MOS data. In the EPIC-pn data, only single- 34 pixel events (PATTERN = 0) were used for the first energy band, 36 18 50 and for the other energy bands single and double-pixel events 55 (PATTERN ≤ 4) were used. We also applied FLAG = 0tore- ject all events that were close to CCD gaps or on bad pixels. We Fig. 1. Logarithmically scaled mosaic three-colour image of the produced images in five energy bands for each observation and XMM-Newton observations in the field of the Draco dSph. The images EPIC. In addition, we produced mosaic images that were a com- are smoothed with a 2D Gaussian with a kernel radius of 1.5 pixels. − bination of all observations using EPIC images, in all sub-bands, The colours represent the energy range of red: 0.3 1.0 keV, green: . − . − . − . 1 0 2 0 keV, and blue: 2 0 12 keV. Detected sources are marked with and in the total energy band of 0 3 12 0 keV. A three-colour im- the number of sources in final catalogue (see Appendix A). age of the mosaic images is shown in Fig. 1. In the process of source detection, we ran the SAS task edetect-chain for each observation in five energy bands for position of the new detections and the final source list was cre- the three EPIC detectors separately and also for the total energy ated for each EPIC with a higher threshold for the maximum de- band mosaic image.
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