Cataclysmic Variables in Globular Clusters, the Galactic Center, and Local Space
Craig O. Heinke, Ashley J. Ruiter, Michael P. Muno, Krzysztof Belczynski
aa r X i v : . [ a s t r o - ph ] J a n A Population Explosion: The Nature and Evolution of X-ray Binaries in Diverse Environments28 Oct.–2 Nov. 2007, St. Petersburg Beach, FLR.M. Bandyopadhyay, S. Wachter, D. Gelino, & C. R. Gelino, eds.Session:
Faint Galactic XRB Populations
Cataclysmic Variables in Globular Clusters, the Galactic Center,and Local Space
Craig O. Heinke
University of Virginia, Astronomy Dept., PO Box 400325, Charlottesville VA 22903;[email protected]
Ashley J. Ruiter
Dept. of Astronomy, New Mexico State University, 1320 Frenger Mall, Las Cruces, NM88003
Michael P. Muno
Space Radiation Laboratory, California Institute of Technology, Pasadena, CA 91125
Krzysztof Belczynski
Dept. of Astronomy, New Mexico State University, 1320 Frenger Mall, Las Cruces, NM88003; Tombaugh Fellow
Abstract.
We compare the X-ray spectra and luminosities, in the 2-8 keV band, of knownand suspected cataclysmic variables (CVs) in different environments, assessing the nature of thesesource populations. These objects include nearby CVs observed with ASCA; the Galactic CenterX-ray source population identified by Muno et al.; and likely CVs identified in globular clusters.Both of the latter have been suggested to be dominated by magnetic CVs. We find that the brighterobjects in both categories are likely to be magnetic CVs, but that the fainter objects are likely toinclude a substantial contribution from normal CVs. The strangely hard spectra observed fromthe Galactic Center sources reflect the high and variable extinction, which is significantly greaterthan the canonical 6 × cm − over much of the region, and the magnetic nature of many of thebrightest CVs. The total numbers of faint Galactic Center sources are compatible with expectationsof the numbers of CVs in this field.
1. Introduction
The unprecedented spatial resolution of the
Chandra X-ray Observatory allows us to studypopulations of faint X-ray sources at distances of kiloparsecs. Large numbers of X-ray sourcesof moderate luminosities (10 < L X < ergs/s) have been discovered in several Galacticenvironments (e.g. star-forming regions, globular clusters, the Galactic Center) in recentyears. For instance, Grindlay et al. (2001); Pooley et al. (2002) and others have found largenumbers of X-ray sources in globular clusters, many of which are optically identified as CVs.Muno et al. (2003) identified a large population of ∼ λ N H columns towards the brightest CVs(Heinke et al. 2005), and the lack of dwarf nova outbursts from globular clusters (Shara et al.1996; Edmonds et al. 2003; Dobrotka et al. 2006).However, the question of whether magnetic CVs are more common than normal CVsin globular clusters or the Galactic Center has not yet been explored. We have directlycompared, for the first time, the spectra of Galactic Center X-ray sources, and likely globularcluster CVs (those with optical counterparts), with archival ASCA X-ray spectra of well-known nearby CVs. Full details will be published in Heinke et al. (2008, in prep).
2. Local CVs
We cross-correlated the CV database of Ritter & Kolb (2003) with the ASCA observationsin the HEASARC archive. We chose 20 confirmed IPs with substantial pointed ASCA obser-vations, 11 polars, 8 novalike (or outbursting dwarf nova) CVs, and 11 quiescent dwarf novaobservations (see Baskill et al. 2005; Ezuka & Ishida 1999). We used the archived spectraand responses from the ASCA archive, and extracted appropriate background spectra.We fit the ASCA spectra from 2-8 keV, in order to compare the same energy range forall our data. A power-law model with a single gaussian (representing the combination ofFe lines) generally gave acceptable fits to the data. We find (omitting 3 CVs with poorlydetermined indices) a clear difference between the fitted photon indices of the magnetic(mean Γ = 1 . σ = 0 .
33) and the nonmagnetic (mean Γ = 1 . σ = 0 .
20) ASCA CVs.We find consistency, however, between the average IP and polar spectra, and between theaverage quiescent dwarf nova spectrum and the average novalike/outbursting dwarf novaspectrum. High- B IPs and polars (e.g. AE Aqr and V884 Her) have suppressed hard X-rayradiation, producing low L X s and photon indices. Excluding these high- B systems, we donot find strong dependence of the photon index with L X within either the magnetic CVs ornonmagnetic CVs as groups.
3. Globular Cluster CVs
For comparison with the ASCA observations and Galactic Center sources, we perform homo-geneous X-ray spectral fitting of 23 (optically) identified CVs in globular clusters (47 Tuc,NGC 6397, ω Cen, NGC 6752, and M4), with >
90 counts above 2 keV. We compare the fittedpower-law photon indices and estimated X-ray luminosities of the globular cluster CVs withthe observed CV populations in Figure 1a. The locations of globular cluster X-ray sourcesin this plot suggest that some (the brightest and hardest) are likely magnetic systems, whilemany others are likely nonmagnetic systems. http://physics.open.ac.uk/RKcat/ einke et al.; CVs in Globulars and the Galaxy 3 Figure 1.
Left:
X-ray luminosities vs. photon index measurements for various classesof nearby CVs vs. those measured for globular cluster X-ray sources optically identifiedas CVs.
Right:
Histograms of the photon indices of magnetic, nonmagnetic, and globularcluster CVs. The bottom panel also shows the best-fit distribution of photon indexproduced by scaling the histograms of magnetic and nonmagnetic CV photon indices tomatch the globular cluster CVs.
We selected an X-ray luminosity range (10 to 2 × ergs/s) that includes all globu-lar cluster CVs in our list. In this range we find 25 magnetic systems and 12 nonmagneticsystems with ASCA spectra. We show histograms of the photon indices of magnetic, non-magnetic, and cluster CVs in Fig. 1b. We scaled the histograms of magnetic and nonmag-netic CV indices to match the histogram of globular cluster CV indices. We find a best fit of39 +12 − (1 σ )% magnetic systems, i.e. 5 to 12 of the 23 confirmed globular cluster CVs, with therest being nonmagnetic systems. Since these systems have been identified in a nonuniformway, with strong X-ray selection, it is likely that the fraction of magnetic CVs in globularclusters is lower than this value. We do not find evidence that the fraction of magnetic CVs(polars and intermediate polars) in globular clusters is higher than the ∼
10% estimated forthe field (Liebert et al. 2003).
4. Galactic Center Sources
Muno et al. (2004) characterize the Galactic Center sources, finding typical X-ray luminosi-ties of 3 × –10 ergs/s, and very hard X-ray spectra, with equivalent photon indicesgenerally between 1 and -1. This is rather harder than the typical globular cluster CV orfield CV, or even the magnetic CVs in either location (Heinke et al. 2006). Muno et al.(2004) pointed out that selection effects (the high extinction and diffuse background) proba-bly had a role in the hardness of these sources. To test whether the Galactic Center sourceswere consistent with a combination of magnetic and nonmagnetic CVs, or even with justmagnetic CVs, we undertook MARX simulations in which we added sources of known prop-erties to 414 ks (2/3 of the total used by Muno et al. (2003); the 3 longest observations,einke et al.; CVs in Globulars and the Galaxy 4
28 30 32 34050010001500 log(Lx) erg/s wdwdwdevwdms 28 30 32 340200040006000 log(Lx) erg/s wdwdwdevwdms
Figure 2.
Left:
Histograms of L X (2-8 keV) from StarTrack population synthesis formagnetic CVs. Blue: white dwarf–white dwarf systems, green: white dwarf–main se-quence systems, red: white dwarf–evolved star systems. Black vertical line: rough lowerlimit of Muno observations. Right:
Same as left, but for nonmagnetic CVs. to reduce computing time) of the real
Chandra observations of the Galactic Center, randetection algorithms and measured the colors of the detected fake sources.For our simulated source population, we choose a population synthesis model usingthe StarTrack code (Belczynski et al. 2008) as implemented in Ruiter et al. (2006), withupdates to compute the X-ray luminosities of magnetic and nonmagnetic CVs using theprescription of Patterson & Raymond (1985). We show the luminosity functions for magneticand nonmagnetic systems in Figure 2. We assume that 10% of the total CV population aremagnetic systems.For the spectra of the simulated CVs, we choose absorbed power-law spectra with singlegaussians to represent the Fe K line complex, with average energies and equivalent widthsbased on the ASCA fits. Our absorption includes both photoelectric absorption (using theXSPEC model phabs ) and scattering by dust (using P. Predehl’s XSPEC model scatter ,Predehl et al. (2003)). For nonmagnetic systems, we use an average photon index of 1.97.For magnetic systems, we produce simulated systems with photon indices of 0.72, 1.22, and1.72, with a distribution set by the results from the ASCA magnetic CV spectra. The onlyparameter we adjust to match the observations is the extinction. We compare the simulatedsystems to the real sources, extracted in the same way from the same Galactic Center data.We compare the medium and hard colors (defined by Muno et al. 2003, as (h-s)/(h+s),where the medium color uses the 2.0-3.3 and 3.3-4.7 keV bands, and 3.3-4.7 and 4.7-8 forthe hard color) and measured photon fluxes (see Figure 3).We find that the real data can be reasonably described with our model only if a higher N H of 10 cm − (plus dust scattering) absorbs the majority of the sources, rather than thecanonical N H = 6 × . Figures 3 (color-flux) and 4 (color-color) show samples of ourresults, using only N H = 6 × (left panels) or 10 cm − (right panels). It can be seeneinke et al.; CVs in Globulars and the Galaxy 5 -1-0.500.51-1-0.500.51 Figure 3. Photon flux vs. hard color for Galactic Center sources (black), and for oursimulations with N H = 6 × (left, magenta) or 10 cm − (right, red). Definitionsof photon flux and hard color are the same as in Muno et al. (2004). Triangles indicatesimulations of magnetic systems, open pentagons nonmagnetic systems.Figure 4. Medium color vs. hard color for Galactic Center sources (black) and for oursimulations with N H = 6 × (left, magenta) or 10 cm − (right, red). Definitions ofcolors as in Muno et al. (2003); triangles indicate simulations of magnetic systems andopen pentagons nonmagnetic systems. einke et al.; CVs in Globulars and the Galaxy 6 Figure 5.
Left:
Locations of Chandra X-ray sources from Muno et al. (2004), coded bythe N H value in fits to thermal plasma spectra. Green: N H < × , red: 6 − × ,magenta: 10 − × , blue: > × . Right:
K-band image of Galactic Center,from the 2MASS survey. that the right panels exhibit much better qualitative matches to the data. Similar agreementcan be reached using broader ranges of N H with an average extinction of 10 cm − .The interstellar absorption towards the Galactic Center is known to be inhomogeneousand filamentary. The variations in this absorption have a strong effect on the numbersand hardness of the galactic center X-ray sources seen at different positions. This canbe qualitatively seen in Figure 5, which compares a K-band image of the Galactic Centerfrom the 2MASS survey with the positions and fitted N H values of Galactic Center X-raysources. Current near-IR observations of the Galactic Center field (e.g. Gosling et al. 2006)will improve our understanding of the effects of extinction upon X-rays from the GalacticCenter.The total number of CVs in the Galactic Center may be inferred from our results(with caveats, particularly the variability of extinction in the region). The total number ofsimulated CVs required to match the numbers of observed sources, using a single extinctionof N H = 10 cm − , is about 7000. Following Muno et al. (2003), and using an estimate of1 × − CVs pc − (Grindlay et al. 2005) in local space, we estimate a total CV number inthe Galactic Center of 5000. This numerical agreement indicates that CVs are indeed themajor contributor to the Galactic Center X-ray sources, and suggests that these CVs arenot significiantly different in their X-ray properties or formation mechanisms from CVs inour galactic neighborhood.
5. Conclusions
Studies of faint (10 < L X < ergs/s) hard X-ray sources in globular clusters and theGalactic Center have identified them as primarily CVs, on both observational and theoretical(lack of a sufficiently numerous alternative population) grounds. They have also suggestedeinke et al.; CVs in Globulars and the Galaxy 7that many or most of them are magnetic systems, particularly intermediate polars. We havecompared the spectra of nearby CVs, observed with ASCA, to the low-count spectra or colorsof globular cluster CVs and Galactic Center X-ray sources. We find that significant (althoughpoorly constrained) fractions of the observed cluster and galactic center populations areconsistent in their X-ray spectra and fluxes with nonmagnetic CVs. For the Galactic Center,we require a somewhat higher average extinction than typically assumed ( N H = 10 insteadof 6 × cm − ). We do not find evidence for significant differences between the X-rayproperties and fraction of magnetic systems of CVs in local space, vs. those of CVs inglobular clusters or the Galactic Center. Acknowledgments.
We thank R. E. Taam and Koji Mukai for useful con-versations. COH has been supported by the Lindheimer Fellowship at North-western University, and Chandra grant G07-8078X at the Univ. of Virginiawhile doing this work. MPM has been supported by a Hubble Fellowship, whileKB has been supported by a Tombaugh Fellowship. AJR was supported by aChandra Theory Grant and is thankful to the Dept. of Physics and Astronomyat Northwestern University for their hospitality. This publication makes use ofdata products from the Two Micron All Sky Survey, which is a joint project ofthe University of Massachusetts and the Infrared Processing and Analysis Center,funded by the National Aeronautics and Space Administration and the NationalScience Foundation.