arXiv:1008.0647v1 [astro-ph.HE] 3 Aug 2010 norudrtnigo irqaasoe h atfew past stone the over stepping microquasars important of decades. an understanding our become jets, in has radio parsec-scale X-1 resolved, ultra- Circinus its an make with of With would star which jet. neutron 1 relativistic XRB, accreting known the Figure only of the this arcseconds (see ev- few jets detected curve ultra-relativistic a (2004) from within light motion al. superluminal et X-ray of Fender al. idence 2003), et year al. (Tudose peak et 40+ the Parkinson scales at the Additionally, multiple ob- therein). of by have on references observations inflated and emission 2008, radio nebula jet More-recent radio re- served parsec-scale X-1 Circinus jets. a that curved of noticed inside first sides (1993) al. et Stewart .H Sell H. P. ahstsIsiueo ehooy abig,M 23,U 02139, MA Cambridge, Technology, of Institute sachusetts USA 02138, MA Cambridge, Nether- Street, the Nijmegen, GL NL-6500 9010, lands Box P.O. Nijmegen, Netherlands The Utrecht, CA, 3584 2, Netherlands nelaan The Groningen, AV 9700 800, Box P.O. Romania Bucharest, RO-077125 405, Atomistilor Romania Bucharest, RO-040557 5, Argint de Netherlands The Dwingeloo, AA UK 1BJ, SO17 Southampton ton, USA 53706, WI Madison, Madison, mtra,SinePr 0,19X mtra,The Amsterdam, 1098XH 904, Park Netherlands Science Amsterdam, P University USA Laboratory, 16802, Davey PA 525 University, State vania rpittpstuigL using 2021 typeset 17, Preprint January version Draft icnsX1hsbe xesvl tde tX-ray at studied extensively been has X-1 Circinus (XRB). binary X-ray unusual an is X-1 Circinus ASCSAEBPLRXRYSOK RDCDB OEFLJET POWERFUL BY PRODUCED SHOCKS X-RAY BIPOLAR PARSEC-SCALE 10 9 8 7 6 5 4 3 2 1 12 11 avr–mtsna etrfrAtohsc,6 Garden 60 Astrophysics, for Center Harvard–Smithsonian University Radboud IMAPP, Astrophysics, Sorbon- of Research, Department Space for Institute Netherlands Groningen, Astrophysics, SRON, of and University Institute, Physics Astronomical Atomic Kapteyn for Center Cutitul Academy, Research Romanian the of Institute 7990 Astronomical 2, Postbus Astronomy, Wisconsin- Radio for Southamp- Institute of Netherlands of University Astronomy, and University Physics of School Astronomy, of Department al nttt o srpyisadSaeRsac,Mas- Research, Space and Astrophysics for Institute Kavli srnmclIsiueAtnPneok nvriyof University Pannekoek, Anton Institute Astronomical Pennsyl- The Astrophysics, and Astronomy of Department nepe h pcr rmtesok scoe ycrto em synchrotron w cooled very as not shocks either are the jets from observa the other ∼ spectra that and The This the suggesting angle, galaxies. interpret region. a radio opening into the powerful running wide sp in of are a seen are X-1 images is Circinus and from radio as angles jets medium, high-resolution the opening new that wide suggests in show regions seen outflows jets bipolar radio The X-1. Circinus aigti n fafwmcousr ihadrc esrmn fit of emission. measurement X-ray direct headings: large-scale a Subject stationary with microquasars exhibits few that a microquasar of one this making erpr h icvr fmlisaeXryjt rmteaccret the from jets X-ray multi-scale of discovery the report We 1 60y.Ti losu ocntantejtpwrt e3 be to power jet the constrain to us allows This yr. 1600 .Heinz S. , .N Brandt N. W. 1. A 1 INTRODUCTION T .E Calvelo E. D. , E tl mltajv 11/10/09 v. emulateapj style X S:jt n uflw,Xry:bnre,Xry:idvda (Circ individual X-rays: binaries, X-rays: outflows, and jets ISM: 11 .A Nowak A. M. , 2 .Tudose V. , rf eso aur 7 2021 17, January version Draft TRCRIU X-1 CIRCINUS STAR 10 .Wijnands R. , 3,4,5 ABSTRACT ark, SA .Soleri P. , acdCDIaigSetoee AI)aor the aboard Observatory (ACIS) X-ray Spectrometer Chandra Imaging CCD vanced al. wide et a Iaria of kpc; middle (4.1–11.8 the estimates in distance 2005). is 5. kpc which of 7.8 Section 2007), of range in distance implica- al. a results et physical assume (Jonker our we the Letter, summarize discusses this we emission Throughout 4 Finally, the in Section of observations tions. analysis our 3. initial outline Section an We present in and (2007). 2 found al. Section emission et X-ray Heinz diffuse by the of observation in- imaging 2009), this al. with et available (Soleri was observation information HRC-I strument. spectral ks spec- no 50 emission but subsequently a the was in of emission seen diffuse nature signal– Similar insufficient the troscopically. had constrain radio observation to arcminute-scale the to–noise emis- However, the X-ray with diffuse jets. coincident faint, 2005, roughly discovered in observa- taken (2007) sion gratings ks) al. 50 longest et (zero-order, the source Heinz point in the Finally, of tion shifts. included, no uncertainties partic- star. systematic that find con- with neutron analyzed and, also the set also data (2008) of is ular al. motion emission et Schulz orbital the Moreover, simple in- as with this unique sistent However, not through 2006) is emission. al. terpretation have et line Lopez precess- (2008) X-ray e.g., highly-inclined al. SS433, Doppler-shifted to et a (similar Iaria of jet ing detection 2002). possible Schulz Brandt & claimed & (Brandt were Schulz that outflows 2000; profiles high-velocity line as Multiple P-Cygni interpreted X-ray star. variable neutron revealed accreting an the Chandra al. is that et establish object firmly Linares now by compact flare, again recent seen observed a and during originally (1986) (2010) al. bursts, et Tennant X-ray by Type-I wavelengths. eosre icnsX1o h 3ci fteAd- the of chip S3 the on X-1 Circinus observed We deep follow-up a on report we Letter, this In 12 6 .vndrKlis der van M. , .P Fender P. R. , × rtnsosrain ftepitsuc have source point the of observations gratings 10 35 sinaddrv oln g of age cooling a derive and ission interstellar the with shock terminal r s erg l olmtdo rcsig We precessing. or collimated ell in niaeta h eshave jets the that indicate tions n eto trXrybinary, X-ray star neutron ing e oe n h nyknown only the and power jet s 2. − opooyo h emission the of morphology 2 1 .G Jonker G. P. , tal onietwt the with coincident atially OBSERVATIONS 12 . .Casella P. , P RMTENEUTRON THE FROM S jet n20 a ,i contin- a in 1, May 2009 on . nsX-1) inus 2 × 10 7,8,9 2 37 .S Schulz S. N. , r s erg − 1 , Chandra 10 , 2

20

40 Inner Emission

-57:10:00

Dec (J2000) 20

40 Caps

48 46 44 42 15:20:40 38 36 34 RA (J2000) Fig. 2.— Exposure-corrected and azimuthally-smoothed, Fig. 1.— Reduced event file showing the point source and the background-subtracted X-ray image. Overlaid are contours from the higher resolution radio image (levels: -1, 2, 4, 8, 16, 32 times bipolar “caps” created by the outflow. Overlaid are contours from −1 ′′ ′′ the lower resolution radio image (logarithmically spaced from 1.9 rms noise of ∼ 6 µJy beam ; beam size: 3. 02 × 2. 21; D. E. −1 to 25 mJy beam ; beam size: 18.′′1 × 16.′′5) from Tudose et al. Calvelo et al., in preparation). The point source, inner emission, (2006). and the X-ray caps described in Sections 3.1–3.3 are all evident here and well-matched to the radio contours. uous 99 ks exposure. Data were taken in timed exposure age (Figure 2). Background surface brightness profiles mode and telemetered in Faint mode. Data reduction were constructed by azimuthally averaging over nested and analysis were completed using CIAO and Sherpa partial annuli in the NE and SW quadrants (visually versions 4.2, XSpec 12.5.1, and ACIS Extract version chosen to exclude the excess diffuse emission) and then 2010-02-26 (AE; Broos et al. 2010). Compared to pre- subtracted from the exposure-corrected image. vious gratings and HRC-I observations, this observation Finally, we note that on the largest scales, we find proved about an order of magnitude more sensitive to roughly axisymmetric soft emission from what we in- diffuse flux not only because of the long exposure and terpret as the dust-scattering halo, centered on the bi- the lack of gratings, but also because the contaminating nary, best visible in the adaptively-smoothed image in emission from the point source was at an exceptionally Figure 3. An in-depth discussion of the dust-scattering low level (Section 3.1). halo will be presented in a follow-up paper. Radio comparison data were constructed from two Image analysis and X-ray–radio comparisons are pre- separate observations. Standard calibrations using the sented below, ordered from smaller to larger scales. All Miriad software (Sault et al. 1995) were applied to both well-determined background/foreground point sources sets of observations. A low angular resolution radio im- were masked and excluded from our analysis. age was derived from data taken on 2001 August 3 dur- 3.1. ing an 11 hr run at 1.4 GHz with the Australia Tele- The Point Source scope Compact Array (ATCA) in 1.5A array configura- We caught Circinus X-1 at the lowest observed flux to −11 −2 −1 tion. This observation and the resulting images were pre- date (F0.5−8keV ≃ 1 × 10 erg cm s ). Even so, the sented in Tudose et al. (2006) and are shown as contours source is still significantly piled up with a pileup fraction in Figure 1. of ∼ 40%. We will present detailed model fits to the A higher angular resolution radio image (Figure 2) point source spectrum in a follow–up paper. was created from ∼ 80 hr of observations spread from 2009 December 30 to 2010 January 8, using the ATCA- 3.2. An Arcsecond Jet Compact Array Broadband Backend in 6A configuration Because the point source was so faint, the point spread at 5.5 GHz (D. E. Calvelo et al., in preparation). function (PSF) from the XRB only dominates the in- ′′ 3. ner ∼ 5 . This enabled us to search for jet emission on ANALYSIS scales of a fraction of a parsec, much smaller than what The reduced event file of the X-ray observation is had been possible in Heinz et al. (2007) and Soleri et al. shown in Figure 1 with a contour overlay of the low- (2009). We found two sources within ∼ 15′′ of the point resolution radio image of the large-scale radio nebula of source, which are both evident in Figure 2: Circinus X-1. This image clearly shows the existence of two bright emission regions ∼ 30′′ from the point source. 1. In the W-NW direction (between position angles In order to highlight the morphology of the diffuse 0◦ and +45◦, measured counterclockwise from due emission closer to the point source and to allow detailed W), we find a clear surface brightness enhancement comparison with the high-resolution radio image, we cre- separated from the XRB by ∼ 5′′ and extending out ated an exposure-corrected, background-subtracted im- to ∼ 8′′. The emission appears to be resolved and 3

these sources are beyond the scope of this Letter and will be presented in a follow-up paper. 3.3. Extended Diffuse Emission: X-ray Caps The most obvious features in Figures 1-3 are the two diffuse emission regions between ∼ 20′′ and 50′′ from Circinus X-1 in the NW and SE directions. The po- sitions of both regions are consistent with the diffuse emission tentatively reported in Heinz et al. (2007) and Soleri et al. (2009). Based on their morphological ap- pearance (NW cap appears slightly concave) and their placement away from the binary along the jet axis, we will refer to both regions as “caps” throughout the rest of this Letter. However, the geometry of these regions could be complicated by projection effects, and other in- terpretations are possible. The caps have distinctly different X-ray colors from both the much redder large-scale diffuse background (dominated by the dust-scattering halo) and the dis- tinctly bluer PSF, as can be seen from the color image in Figure 3. The two caps have very similar surface bright- nesses (∼ 2 × 10−16 erg cm−2 s−1 arcsec−2) and appear to be similar in their angular extent (NW: −5◦–65◦; SE: ◦ ◦ Fig. 3.— Adaptively smoothed three-color image ∼ 4.′5 × 4.′5 185 –245 ). However, they are asymmetric in their ra- (red: 0.5–2.9 keV, green: 2.9–4.2 keV, and blue: 4.2–7.0 keV). The dial extent (NW: ∼ 22′′–50′′, SE: ∼18–45′′). The inner large-scale dust-scattering halo from the point source (red) and the NW jet emission (Section 3.2) has similar position and hard wings of the PSF (blue) are clearly distinct in color from the cap emission. North is up and east is left. opening angles to the NW cap. The correspondence between the high-resolution radio contours (≥ 2σ) and the X-ray cap emission in Figure 2 is is coincident with a source in the high-resolution striking. While the X-ray image has significantly higher radio image. No IR counterpart is found in ei- angular resolution than the radio, it is clear that the X- ther 24 µm MIPSGAL13 or 3.6–8µm GLIMPSE14 ray caps align closely with the extended jet emission. Spitzer images. Given that the source is extended The diffuse emission to the SE of the XRB in the radio and aligned with the large-scale radio jet and with image shows a sharp surface brightness drop at the posi- the X-ray caps reported below, we interpret this tion of the cap and what has been interpreted as a bend source as genuine jet emission. in the jet direction at this position, trailing to the south 2. In the SE direction (at position angle 230◦), we (referred to as “knot B” in Tudose et al. 2006). We see detect a bright, unresolved point source at ∼ 13′′ no clear corresponding surface brightness enhancement from the XRB. This emission is coincident with a to this trail in the X-rays. However, our sensitivity is point-like radio source. It is spatially coincident compromised by the fact that the read streak runs right with a very bright IR point source, with no opti- along this feature. cal counterpart detected in an archival 5 minute We used partial annuli regions to extract the cap spec- HST/WFPC I exposure. tra. Because the background is dominated by the dust- scattering halo, the surface brightness of which depends From the FIRST (Faint Images of the Radio Sky on the angular distance to Circinus X-1, we carefully at Twenty-Centimeters) log N–log S, we estimate selected background partial annuli at the same radial ex- that the probability of finding a background radio tent as the source regions. We recover a total of 6240 point source of the same or larger flux within 13 source counts over 2860 expected background counts in arcseconds from Circinus X-1 is ∼ 1%. In addi- both cap regions combined (0.5–9.5 keV). The resulting tion, given the IR source density in the region, the spectra are shown in Figure 4. likelihood of finding an IR background source with Initial extractions of each of the caps indicated that equal or larger flux coincident with the radio and the spectral parameters are statistically indistinguish- X-ray point source within the MIPS PSF is also of able. Therefore, we jointly fit the spectra. We restricted order 1%. Neither scenario can be ruled out sta- our spectral analysis to 0.5–9.5 keV. Quoted uncertain- tistically. Thus, we cannot draw any conclusions ties correspond to 90% confidence intervals. The spectra about the nature of the SE point source without are well fit by an absorbed powerlaw with a neutral hy- further observational evidence, e.g., from optical +0.30 22 −2 drogen column of NH =2.24−0.28 ×10 cm and a pho- spectroscopy, variability, or proper motion. +0.23 2 ton index, Γ = 1.99−0.21 (χ = 243.1 over 252 degrees A detailed PSF subtraction of the binary emission of freedom), giving 0.5–9.5 keV cap fluxes of 3.2 × 10−13 (complicated by effects of pileup) and spectral fits of erg cm−2 s−1 (NW) and 2.7 × 10−13 erg cm−2 s−1 (SE). The caps can also be fit by a thermal (APEC) model 13 http://mipsgal.ipac.caltech.edu/ 2 14 (χ = 237.7 over 251 degrees of freedom). The required http://www.astro.wisc.edu/sirtf/ absorbing column is very similar to the power-law case 4

10-2 compared to the travel time through the X-ray emission region to blend the jet emission (as seen in the arcsecond radio jets of SS433 Hjellming & Johnston 1981). ◦ The claim of a highly relativistic jet (Γjet ∼ 16, θ . 5 ; -1 10-3 Fender et al. 2004) coupled with these new observations keV

-1 would require that such a jet has a precession cone that is within 5◦ of the line–of–sight such that the jet

counts s sometimes points very close to the line–of–sight. Such 10-4 a geometry would imply a physical scale of ∼ 20 pc from cap to cap. For comparison, this is an order of magnitude larger than the projected distance between 10-5 the hotspots of XTE J1550–564 (Corbel et al. 2002)

2 and H1743–322 (Corbel et al. 2005) but would still fit 1 comfortably within the radio nebula surrounding SS433 χ 0 -1 (Dubner et al. 1998). -2 Below, we will discuss two possible interpretations of 1 2 5 the radiative origin of the cap emission. Energy (keV) 4.1. Cooled Synchrotron Model Fig. 4.— Top: spectrum of the two large-scale caps and best-fit absorbed power-law fit (red: NW cap; green: SE cap). Bottom: The most likely interpretation of the spectra is syn- deviations from the best-fit model. chrotron emission. In this case, the emission arises +0.21 22 −2 with NH =1.93−0.18 ×10 cm . This is consistent with from the reaccelerated jet particles entering the termi- the fact that there is no obvious line emission present in nal shock, as is observed in the hot spots of many FR II the spectra, the APEC fit requires a low abundance of radio galaxies (e.g., Meisenheimer et al. 1989). The X- +0.38 +2.2 Z = 0.41−0.27 and a high temperature of kT = 6.6−1.8 ray emission in this case should be co-spatial with the keV. When forced to solar abundance, the fit requires radio, which appears consistent with the radio contours even higher temperatures. The 0.5–9.5 keV cap fluxes in Figure 2. are 3.2 × 10−13 erg cm−2 s−1 (NW) and 2.7 × 10−13 erg The radio synchrotron spectrum of the large-scale neb- −2 −1 ula has a typical spectral index of αR = 0.53 (with cm s (SE). α Fν ∝ ν− R ) (Tudose et al. 2006), consistent with the 4. DISCUSSION standard power-law slope of first-order Fermi accelera- The limb-brightened morphology of the X-ray image tion. For the canonical model of continuously injected (which has significantly higher angular resolution than power-law electrons one would expect a spectral index either radio image) suggests that the emission from the of αX = αR +0.53 = 1.03 above the cooling break at two caps originates at a shock in the outflow from Circi- frequency νb, perfectly consistent with the observed X- nus X-1, seen in projection. This suggestion is supported ray spectral index of αX = 0.99. From extrapolation of by a sharp drop in surface brightness in radio and X-ray the radio and X-ray spectra, we estimate the break fre- just outside of the caps and by the fact that the SE ra- quency from uncooled to cooled synchrotron emission to 16 dio jet seems to be changing direction at the location of be νb ∼ 2 × 10 Hz, which is uncertain by about an the X-ray cap. The fact that the caps appear inside the order of magnitude when uncertainties in the radio and large-scale radio nebula would be a result of foreshort- X-ray spectral indices are used primarily because the X- ening, since the jet axis is likely inclined with respect ray and radio power-law slopes are extrapolated over a to the line of sight (though the actual inclination is un- very large range of frequencies. known). The outflow itself appears largely X-ray dark Taking the emission regions to be spherical to low- (except for the inner NW diffuse radio–X-ray feature), est order, we estimate the equipartition magnetic field similar to the X-ray cavities observed in many clusters strength to be Beq ∼ 50 µG, which gives a minimum to- with central radio galaxies. tal internal energy for each emission region of ∼ 8 × 1045 The caps span projected half–opening angles of 35◦ erg. From the estimate of the break frequency, we esti- ◦ (NW) and 30 (SE). This implies that either: mate a cooling time, τcool ∼ 1600 yr. A different emission geometry will change these numbers by factors of order 1. the outflow is, in fact, much wider than what would unity. typically be considered a jet and might thus be bet- Together, these numbers provide a robust lower limit 35 −1 ter characterized as a non-thermal wind, or on the total jet power of Pcir > 3 × 10 ergs , needed just to put the synchrotron emitting particles in place 2. the jets are precessing with a fairly wide open- (not including any plasma currently in the jet or the en- ing angle, as is the case for SS433 (Margon 1984), ergy needed to inflate the large-scale radio nebula). De- and/or parting from equipartition can only increase this number. 3. the jet or precession cone axis is very close to the This limit is independent of the assumed geometry and line of sight, causing significant foreshortening of size of the emission regions. an intrinsically narrow jet. Conversely, we can place a rough upper limit on the outflow power from the fact that the large-scale radio The wide opening angle of the inner NW jet would be nebula must be at least as old as τcool. Using the same consistent with (1) and (3), but to be consistent with (2), arguments as presented in Heinz (2002) and Tudose et al. it would require a precession period of the jet that is short (2006) for the expansion of the radio nebula we find that 5

37 −1 3 −1 Pcir . 2 × 10 ergs , though this limit is not as robust infer a shock velocity of ∼ 2.4 × 10 km s . From the since the magnetic field could well be out of equiparti- projected distance, we can infer an estimated travel time tion and the nebula could be significantly elongated along of ∼ 500/sin(θ) years out to the observed shock positions the line–of–sight, both of which would increase the total for each of the caps. Again approximating the emission power. region as spherical, we estimate the mass and the total These limits on Pcir confirm that the accreting com- (thermal plus kinetic) energy of the shocked material for pact object in this system is driving powerful jets into each of the caps to be ∼ 2 × 1032 g and ∼ 2 × 1049 erg, the interstellar medium (ISM). Taking the high 42 year respectively. Combined with the shock travel age, this average luminosity of Circinus X-1 at about 80% of gives a minimum outflow power for both caps combined 38 −1 −1 the Eddington luminosity for a 1.4 M⊙ neutron star of Pcir > 6 × 10 × sin (θ) erg s , just to supply (Parkinson et al. 2003) at face value would imply that the X-ray emitting material (not including the putative the long-term average jet power is between 2% and 10% power required to inflate the large-scale radio nebula). of the current average radiative power. Unless the inclination angle is very small, this power is Given that the average long-term accretion rate cannot exceedingly large. Even for inclinations as low as those greatly exceed the Eddington rate, we can derive a robust inferred from the relativistic jet claimed in Fender et al. lower limit on the efficiency with which accretion power (2004), the minimum average power would still be Pcir > 2 is converted into outflow power of ηjet = Pcir/mc˙ > 5 × 1037 ergs−1. m˙ Edd 0.034% hm˙ i (see also Heinz et al. 2007 and Soleri et al. While we cannot completely rule out a thermal ori- 2009). gin, the very high power required by the thermal model combined with the perfect spectral correspondence to a classic broken synchrotron spectrum and the spatial cor- 4.2. Thermal Model respondence with the radio suggest that the synchrotron model is the more natural choice. Given the equally good fit we achieved with a ther- mal emission model, it is worth discussing the alterna- 5. SUMMARY tive scenario that the X-ray caps are the shocked ISM We have presented an initial analysis of a deep Chan- pushed ahead of the expanding radio outflow (similar to dra imaging observation of Circinus X-1. We detect two the shells around many X-ray cavities in galaxy clusters diffuse X-ray caps that are likely the terminal shocks of and around the microquasar Cygnus X-1; Gallo et al. powerful jets running into the ISM. In addition, we find 2005). an arcsecond-scale outflow between the XRB and one of In the thermal model the X-rays should be located fur- the X-ray caps, coincident with the radio jet. These dis- ther out from the binary than the radio emission. How- coveries make Circinus X-1 the first microquasar with ever, the high-resolution radio data indicate that the ra- both an X-ray jet and two stationary X-ray shocks, and dio and the X-ray emission are co-spatial. This requires one of only a handful of microquasars with a direct esti- a very low inclination angle in order to align the two mate of the jet power. regions by projection (which is consistent with the incli- nation required by the ultra-relativistic flow claimed in Fender et al. 2004). S.H. and P.S. acknowledge support from NASA grant From the best-fit temperature of 6.6 keV, we directly GO9-0056X.

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