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ALABAMA University Libraries THE UNIVERSITY OF ALABAMA University Libraries The X-Ray-Faint Early-Type Galaxy NGC 4697 Jimmy A. Irwin – University of Michigan Craig L. Sarazin – University of Virginia Joel N. Bregman – University of Michigan Deposited 09/14/2018 Citation of published version: Irwin, J., Sarazin, C., Bregman, J. (2000): The X-Ray-Faint Early-Type Galaxy NGC 4697. The Astrophysical Journal Letters, 544(2). DOI: 10.1086/317196 © 2000. The American Astronomical Society. All rights reserved. Printed in U.S.A. THE ASTROPHYSICAL JOURNAL, 544:293È301, 2000 November 20 ( 2000. The American Astronomical Society. All rights reserved. Printed in U.S.A. THE X-RAYÈFAINT EARLY-TYPE GALAXY NGC 4697 JIMMY A. IRWIN,1,2 CRAIG L. SARAZIN,3 AND JOEL N. BREGMAN1 Received 2000 March 7; accepted 2000 June 27 ABSTRACT We analyze archival ROSAT HRI, ROSAT PSPC, and ASCA data of the X-rayÈfaint early-type galaxy NGC 4697. The joint ROSAT PSPC]ASCA spectrum is Ðtted by a two-component thermal model, a MEKAL model withkT \ 0.26`0.04 keV with low metallicity and a bremsstrahlung \ `3.0 MEKAL ~0.03 model withkTBREM 5.2~1.6 keV. A similar model was found to Ðt the spectra of another faint early- type galaxy (NGC 4382) and the bulge of M31. We interpret this soft emission as a combination of emission from a soft component of low-mass X-ray binaries (LMXBs) and from a low-temperature inter- stellar medium, although the relative contributions of the two components could not be determined. Twelve point sources were identiÐed within 4@ of NGC 4697, of which 11 are most likely LMXBs associ- ated with the galaxy. The soft X-ray colors of four of the LMXBs in NGC 4697 support the claim that LMXBs possess a soft spectral component. Finally, we present a simulation of what we believe the Chandra data of NGC 4697 will look like. Subject headings: binaries: close È galaxies: elliptical and lenticular, cD È galaxies: ISM È X-rays: galaxies È X-rays: ISM È X-rays: stars 1. INTRODUCTION roughly with the optical luminosity of the galaxy (note, however, that Buote & Fabian 1998 found no formal need It is well established that early-type galaxies exhibit a for a hard component in the X-rayÈbrightest galaxies of large range of X-rayÈtoÈoptical luminosity ratios, L X/L B. their ASCA sample). The luminosity of this hard com- Although there is a strong correlation between the X-ray ponent was small compared to the soft, gaseous component P and optical luminosities of elliptical and S0 galaxies (L X in the X-rayÈbrightest galaxies but increased in importance L 1.7ÈL3.0 ; Canizares, Fabbiano, & Trinchieri 1987; White B B asL X/L B decreased. However, the ASCA observations did & Davis 1997; Brown & Bregman 1998), a large dispersion not resolve this component into discrete sources because of exists in this relation. Two galaxies with similar blue lumi- the insufficient spatial resolution of the instrument. nosities might have X-ray luminosities that di†er by as Previous studies have found that the spectra of the X- much as a factor of 100 (Canizares et al. 1987; Fabbiano, rayÈfaintest early-type galaxies are signiÐcantly di†erent Kim, & Trinchieri 1992; Brown & Bregman 1998). than the spectra of gas-rich X-rayÈbright early-type gal- Hot (D107 K) gas is believed to be responsible for the axies. Einstein data revealed that the X-rayÈfaintest galaxies bulk of the X-ray emission inhigh-L X/L B galaxies (e.g., exhibited a strong very soft X-ray excess (Kim, Fabbiano, & Forman, Jones, & Tucker 1985). Another possible source of Trinchieri 1992). Subsequent ROSAT PSPC and ASCA X-ray emission in early-type galaxies is from discrete stellar studies found that the spectra of these galaxies were best sources, primarily low-mass X-ray binaries (LMXBs). Esti- Ðtted with a two-component model consisting of the pre- mates for the stellarL X/L B value are uncertain and vary by viously mentioned hard D5È10 keV component (generally a factor of 10 among various studies (Forman et al. 1985; attributed to the integrated LMXB emission) and a very Canizares et al. 1987). Although stellar X-ray sources are soft D0.3 keV component whose origin is uncertain not expected to contribute signiÐcantly to the 0.1È2.0 keV (Fabbiano, Kim, & Trinchieri 1994; Pellegrini 1994; Kim et X-ray emission in gas-rich, X-rayÈbright galaxies, this claim al. 1996). cannot be made with any certainty for X-ray-faint (low One recently proposed solution to the very soft X-ray L X/L B) galaxies. In these galaxies it is possible that stellar excess problem in X-rayÈfaint galaxies is that it might result X-ray sources are the dominant X-ray emission mechanism. from the very same collection of LMXBs responsible for the For this to occur, the gas lost from stellar mass loss must be hard emission (Irwin & Sarazin 1998a, 1998b). Little is removed from the galaxy by galactic winds, by ram pressure known about the very soft X-ray properties of LMXBs since stripping from ambient intracluster or intragroup gas, or nearly all Galactic examples lie in directions of high Galac- possibly both (see, e.g., Ciotti et al. 1991; Mathews & Brigh- tic hydrogen column densities, so their soft X-ray emission enti 1998). is heavily absorbed. However, two nearby Galactic LMXBs Evidence is mounting that LMXB emission is detectable that lie in directions of low Galactic hydrogen column in nearly all early-type galaxies. Matsumoto et al. (1997) density (Her X-1 and MS 1603]2600) exhibit signiÐcant found a hard (D5È10 keV) spectral component in 11 of 12 very soft X-ray emission (Vrtilek et al. 1994; Hakala et al. early-type galaxies observed with ASCA, which scaled 1998). But perhaps the strongest evidence comes from the bulge of M31. M31 is close enough that most(Z75%) of its bulge X-ray emission was resolved into point sources with 1 Department of Astronomy, University of Michigan at Ann Arbor, 501 the ROSAT PSPC and HRI (Primini, Forman, & Jones East University Avenue, Ann Arbor, MI 48109-1090; jirwin= 1993; Supper et al. 1997), a majority of which are probably astro.lsa.umich.edu, jbregman=umich.edu. 2 Chandra Fellow. LMXBs. Both individually and cumulatively, these point 3 Department of Astronomy, University of Virginia, P.O. Box 3818, sources in the bulge of M31 have X-ray spectral properties Charlottesville, VA 22903-0818; cls7i=virginia.edu. very similar to the integrated emission from X-rayÈfaint 293 294 IRWIN, SARAZIN, & BREGMAN Vol. 544 early-type galaxies in the ROSAT band (Irwin & Sarazin tained at least 25 counts so that the s2 test is a valid indica- 1998a, 1998b). A joint ROSAT PSPC]ASCA study found tor of goodness of Ðt. For all the joint Ðts, we linked the the X-ray spectrum of the bulge of M31 to be almost identi- normalizations of the PSPC and GIS but let the SIS nor- cal to that of the X-rayÈfaint early-type galaxy NGC 4382 malization vary to account for the fact that some of the over the 0.2È10 keV band (Kim et al. 1996; Irwin & emission that fell on interchip boundaries of the SIS was Bregman 1999b). The bulge of the Sa galaxy NGC 1291 also lost. exhibits a very soft component and has X-ray colors identi- 3. SPECTRAL MODELING AND LUMINOSITIES OF THE cal to the bulge of M31, although it is unresolved with GLOBAL SPECTRUM ROSAT because of its distance. In both bulges, the mea- suredL X/L B value is comparable to those of the X-rayÈ Using XSPEC Version 10.0, we Ðrst attempted to Ðt a faintest early-type galaxies. This suggests that in addition to single-component thermal model to the PSPC and ASCA providing the required spectral X-ray characteristics, data separately. When the PSPC spectrum was Ðtted alone LMXBs are luminous and/or numerous enough to produce with a MEKAL model with a variable absorption com- 2\ the required amount of X-ray emission in the X-rayÈfaintest ponent a good Ðt was obtained(sl 1.07/58 degrees of early-type galaxies as well as Sa spiral bulges. No additional freedom) for kT \ 0.43È0.56 keV and Z \ 0.02 (90% con- very soft X-ray emission source seems to be required. Ðdence level), in good agreement with the analysis of the In this paper we analyze long ROSAT PSPC, ROSAT same data by Davis & White (1996). A signiÐcantly di†erent HRI, and ASCA GIS and SIS observations of the X-rayÈ result was found for this same model when just the ASCA faint elliptical galaxy NGC 4697. At a distance of 15.9 Mpc spectra were analyzed; the best-Ðt values were kT \ 2.2È3.9 2\ (Faber et al. 1989; assuming a Hubble constant of 50 km keV and Z \ 0.20, withsl 1.44/53 degrees of freedom. s~1 Mpc~1), NGC 4697 is one of the closest normal early- Clearly, signiÐcantly di†erent conclusions would be drawn type galaxies. Previous work has indicated that this galaxy from the spectra if data from only one of the satellites were has a below average but not extremely lowL X/L B value and analyzed. ROSAT band colors typical of those of other X-rayÈfaint This result illustrates the necessity of using both ROSAT early-type galaxies (Irwin & Sarazin 1998b). Jointly Ðtting PSPC and ASCA data when analyzing the spectrum of X- the ROSAT PSPC and ASCA spectra allows us to place rayÈfaint early-type galaxies.
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