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Register a new userFading of the X-ray flux from the black hole in the NGC 4472 globular cluster RZ 2109
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Thomas J. Maccarone
Publication date
2010
fields
Physics
and research's language is
English
Authors
Thomas J. Maccarone
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We present the results of new X-ray observations of XMMU 122939.7+075333, the black hole (BH) in the globular cluster RZ 2109 in the Virgo Cluster galaxy NGC 4472. A combination of non-detections and marginal detections in several recent Swift and Chandra observations show that the source has varied by at least a factor of 20 in the past 6 years, and that the variations seem not just to be flickering. This variation could be explained with changes in the absorption column intrinsic to the source no larger than those which were previously seen near the peak of the 1989 outburst of the Galactic BH X-ray binary V404 Cyg. The large amplitude variations are also a natural expectation from a hierarchical triple system with Kozai cycles -- the mechanism recently proposed to produce BH-white dwarf (WD) binaries in globular clusters. On the other hand, variation by such a large factor on timescales of years, rather than centuries, is very difficult to reconcile with the scenario in which the X-ray emission from XMMU 122939.7+075333 is due to fallback of material from a tidally destroyed or detonated WD.
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We present the results of long-term monitoring of the X-ray emission from the ultraluminous X-ray source XMMUJ122939.9+075333 in the extragalactic globular cluster RZ2109. The combination of the high X-ray luminosity, short term X-ray variability, X-ray spectrum, and optical emission suggest that this system is likely an accreting black hole in a globular cluster. To study the long-term behavior of the X-ray emission from this source, we analyze both new and archival Chandra and XMM-Newton observations, covering 16 years from 2000 to 2016. For all of these observations, we fit extracted spectra of RZ2109 with xspec models. The spectra are all dominated by a soft component, which is very soft with typical fit temperatures of T $simeq$ 0.15 keV. The resulting X-ray fluxes show strong variability on short and long timescales. We also find that the X-ray spectrum often shows no significant change even with luminosity changes as large as a factor of five.
We report the discovery of a candidate stellar-mass black hole in the Milky Way globular cluster M62. We detected the black hole candidate, which we term M62-VLA1, in the core of the cluster using deep radio continuum imaging from the Karl G. Jansky Very Large Array. M62-VLA1 is a faint source, with a flux density of 18.7 +/- 1.9 microJy at 6.2 GHz and a flat radio spectrum (alpha=-0.24 +/- 0.42, for S_nu = nu^alpha). M62 is the second Milky Way cluster with a candidate stellar-mass black hole; unlike the two candidate black holes previously found in the cluster M22, M62-VLA1 is associated with a Chandra X-ray source, supporting its identification as a black hole X-ray binary. Measurements of its radio and X-ray luminosity, while not simultaneous, place M62-VLA1 squarely on the well-established radio--X-ray correlation for stellar-mass black holes. In archival Hubble Space Telescope imaging, M62-VLA1 is coincident with a star near the lower red giant branch. This possible optical counterpart shows a blue excess, H alpha emission, and optical variability. The radio, X-ray, and optical properties of M62-VLA1 are very similar to those for V404 Cyg, one of the best-studied quiescent stellar-mass black holes. We cannot yet rule out alternative scenarios for the radio source, such as a flaring neutron star or background galaxy; future observations are necessary to determine whether M62-VLA1 is indeed an accreting stellar-mass black hole.
We present the discovery of [OIII] 5007 emission associated with the black hole X-ray binary recently identified in a globular cluster in the Virgo elliptical galaxy NGC 4472. This object is the first confirmed black-hole X-ray binary in a globular cluster. The identification of [OIII] 5007 emission from the black-hole hosting globular cluster is based on two independent fiber spectra obtained at the VLT with FLAMES, which cover a wavelength range of 5000-5800 Angstrom at a spectral resolution of about 6000. In each of these spectra we find an emission line at 5031.2 Angstrom with an uncertainty of several tenths of an Angstrom. These are consistent with [OIII] 5007 emission at the 1475 +/- 7 km/s radial velocity of the globular cluster previously determined from an analysis of its absorption lines. This agreement within the small uncertainties argues strongly in favor of the interpretation of the line as [OIII] 5007 emission from the black-hole hosting globular cluster. We also find that the emission line most likely has a velocity width of several hundred km/s. Such a velocity width rules out a planetary nebula explanation for the [OIII] 5007 emission and implicates the black hole as the source of the power driving the nebular emission.
We present new Chandra observations of the outer halo of the giant elliptical galaxy NGC 4472 (M49) in the Virgo Cluster. The data extend to 130 kpc (28), and have a combined exposure time of 150 ks. After identifying optical counterparts using the Next Generation Virgo Cluster Survey to remove background active galactic nuclei and globular cluster (GC) sources, and correcting for completeness, we find that the number of field low-mass X-ray binaries (LMXBs) per unit stellar V-band light increases significantly with galactocentric radius. Because the flux limit of the complete sample corresponds to the Eddington limit for neutron stars in NGC 4472, many of the ~90 field LMXBs in this sample could host black holes. The excess of field LMXBs at large galactocentric radii may be partially caused by natal kicks on black holes and neutron stars in binary systems in the inner part of the galaxy. Furthermore, since the metallicity in the halo of NGC 4472 strongly decreases towards larger galactocentric radii, the number of field LMXBs per unit stellar mass is anti-correlated with metallicity, opposite to what is observed in GCs. Another way to explain the spatial distribution of field LMXBs is therefore a reversed metallicity effect, although we have not identified a mechanism to explain this in terms of stellar and binary evolution.
We present a study of the X-ray spectral properties of the highly variable X-ray emitting black hole in a globular cluster in the elliptical galaxy NGC 4472. The XMM-Newton spectrum of the source in its bright epoch is well described by a multiple blackbody model with a characteristic temperature $kT_{in}approx$ 0.2 keV. The spectrum of an archival Chandra observation of the source obtained 3.5 years before the XMM data gives similar estimates for the blackbody parameters. We confirm that the fainter interval of the XMM-Newton observation has a spectrum that is consistent with the brighter epoch, except for an additional level of foreground absorption. We also consider other possible mechanisms for the variability. Based on the timescale of the X-ray flux decline and the estimated size of the X-ray emission region we argue that an eclipsing companion is highly unlikely. We find the most likely means of producing the absorption changes on the observed timescale is through partial obscuration by a precessing warped accretion disk.
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