Chandra X-Ray Observations of the X-Ray Faint Elliptical Galaxy NGC 4697

By C. Sarazin, J. Irwin, J. Bregman. 2001

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A Chandra ACIS S3 observation of the X-ray faint elliptical galaxy NGC 4697 resolves much of the X-ray emission (61% of the counts from within one effective radius) into 90 point sources, of which ~80 are low-mass X-ray binaries (LMXBs) associated with this galaxy. The dominance of LMXBs indicates that X-ray faint early-type galaxies have lost much of their interstellar gas. On the other hand, a modest portion of the X-ray emission from NGC 4697 is due to hot gas. Of the unresolved emission, it is likely that about half is from fainter unresolved LMXBs, while the other half (~23% of the total count rate) is from interstellar gas. The X-ray-emitting gas in NGC 4697 has a rather low temperature (kT = 0.29 keV). The emission from the gas is very extended, with a much flatter surface brightness profile than the optical light, and has an irregular, L-shaped morphology. The physical state of the hot gas is uncertain; the X-ray luminosity and extended surface brightness are inconsistent with a global supersonic wind, a partial wind, or a global cooling inflow. The gas may be undergoing subsonic inflation, rotationally induced outflow, or ram pressure stripping. X-ray spectra of the resolved sources and diffuse emission show that the soft X-ray spectral component, found in this and other X-ray faint ellipticals with ROSAT, is due to interstellar gas. The cumulative LMXB spectrum is well fitted by thermal bremsstrahlung at kT = 8.1 keV, without a significant soft component. NGC 4697 has a central X-ray source with a luminosity of LX = 8 × 1038 ergs s-1, which may be due to an active galactic nucleus and/or one or more LMXBs. At most, the massive black hole (BH) at the center of this galaxy is radiating at a very small fraction (≤4 × 10-8) of its Eddington luminosity. Three of the resolved sources in NGC 4697 are supersoft sources. In the outer regions of NGC 4697, seven of the LMXBs (about 20%) are coincident with candidate globular clusters, which indicates that globulars have a high probability of containing X-ray binaries compared to the normal stellar population. We suggest that all of the LMXBs may have been formed in globulars. The X-ray to optical luminosity ratio for the LMXBs in NGC 4697 is LX(LMXB, 0.3-10 keV)/LB = 8.1 × 1029 ergs s-1 L, which is about 35% higher than the value for the bulge of M31. Other comparisons suggest that there are significant variations (factor of ≳2) in the LMXB X-ray-to-optical ratios of early-type galaxies and spiral bulges. The X-ray luminosity function of NGC 4697 is also flatter than that found for the bulge of M31. The X-ray luminosities (0.3-10 keV) of the resolved LMXBs range from ~5 × 1037 to ~2.5 × 1039 ergs s-1. The luminosity function of the LMXBs has a "knee" at 3.2 × 1038 ergs s-1, which is approximately the Eddington luminosity of a 1.4 M☉ neutron star (NS). This knee appears to be a characteristic feature of the LMXB population of early-type galaxies, and we argue that it separates BH and NS binaries. This characteristic luminosity could be used as a distance estimator. If they are Eddington limited, the brightest LMXBs contain fairly massive accreting BHs. The presence of this large population of NS and massive BH stellar remnants in this elliptical galaxy shows that it (or its progenitors) once contained a large population of massive main-sequence stars.

Subfields: Astronomy and Astrophysics, Instrumentation

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Citations in the corpus

The Ultraluminous X-Ray Source Population from the Chandra Archive of Galaxies (2004, background): “Of order 100 X-ray sources per LB=10 43 ergs s−1 are routinely detected above a few 1037 ergs s−1 in elliptical galaxies (e.g., NGC 4697, Sarazin et al. 2001) and in the bulges of spiral galaxies (e.g., NGC 3031, Swartz et al. 2003).”
X-Ray Luminosity Function and Total Luminosity of Low-Mass X-Ray Binaries in Early-Type Galaxies (2003, background): “With its sub-arcsecond spatial resolution and unprecedented sensitivity (Weisskopf et al. 2000), Chandra has provided incontrovertible proof of the existence of populations of LMXBs in all E and S0 galaxies (e.g., Sarazin et al. 2001; Angelini et al. 2001; Kim and Fabbiano 2003, hereafter KF03).”
X-Ray Thermal Coronae of Galaxies in Hot Clusters: Ubiquity of Embedded Mini-Cooling Cores (2006, methodology): “Galaxy coronae with temperature of< 0.4 keV are indeed reported in poor environments (e.g.,∼ 0.25 keV gas in NGC 4697, Sarazin et al. 2001;∼ 0.3 keV gas in Centaurus A, Kraft et al. 2003), but the X-ray gas luminosities of these galaxies are all low.”
Low-mass X-ray binaries as a stellar mass indicator for the host galaxy (2004, methodology): “…K-band magnitude obtained from the near-infrared growth curve fit as described in subsection 3.2; References for X-ray data: (a) – Maccarone et al. ( 2003), (b) – Sarazin et al. ( 2001), (c) – Trinchieri & Goudfrooij ( 2002), (d) – Finoguenov & Jones ( 2002), (e) – Blanton et al. (2001), (f) –…”
X-Ray Spectral Properties of Low-Mass X-Ray Binaries in Nearby Galaxies (2002, background): “1 2 dex ranging from 1.5–2.0 has been obtained (Sarazin et al. 2001; Randall et al. 2001; Kim & Fabbiano 2002; Irwin, Sarazin, & Bregman 2002).”
THE HERSCHEL REFERENCE SURVEY: DUST IN EARLY-TYPE GALAXIES AND ACROSS THE HUBBLE SEQUENCE (2011, background): “NGC4697 is an X-ray-faint galaxy given its optical luminosity, which may be a result of severe loss of interstellar gas via stripping or outflows (Sarazin et al. 2001).”
X-Ray Emission from the Hot Interstellar Medium and Southwest Radio Lobe of the Nearby Radio Galaxy Centaurus A (2003, background): “129 characteristics of dark matter halos can be deduced (Loewenstein & Mushotzky 2001) and by resolving out the LMXB population in nearby objects (Blanton, Sarazin, & Irwin 2001a; Sarazin et al. 2001).”
The ACS Virgo Cluster Survey. I. Introduction to the Survey (2004, background): “…with the Chandra X-Ray Observatory have shown that many early-type galaxies are surrounded by numerous low-mass X-ray binaries, roughly half of which are associated with GCs (e.g., Sarazin et al. 2001; Angelini et al. 2001; Maccarone et al. 2003; Jordán et al. 2004a, hereafter Paper III).”