Hintze Centre for Astrophysical Surveys

Abstract:

We present a statistical study of the low-mass X-ray binary (LMXB) populations of three nearby, old elliptical galaxies: NGC 3379, NGC 4278, and NGC 4697. With a cumulative 1 Ms Chandra ACIS observing time, we detect 90-170 LMXBs within the D25 ellipse of each galaxy. Cross-correlating Chandra X-ray sources and HST optical sources, we identify 75 globular cluster (GC) LMXBs and 112 field LMXBs with LX > 1036 erg s-1 (detections of these populations are 90% complete down to luminosities in the range of 6 × 1036 to 1.5 × 10 37ergs-1). At the higher luminosities explored in previous studies, the statistics of this sample are consistent with the properties of GC-LMXBs reported in the literature. In the low-luminosity range allowed by our deeper data (LX < 5 × 1037 erg s-1), we find a significant relative lack of GC-LMXBs, when compared with field sources. Using the co-added sample from the three galaxies, we find that the incompleteness-corrected X-ray luminosity functions (XLFs) of GC and field LMXBs differ at 4σ significance at LX < 5 × 1037 erg s-1. As previously reported, these XLFs are consistent at higher luminosities. The presently available theoretical models for LMXB formation and evolution in clusters are not sophisticated enough to provide a definite explanation for the shape of the observed GC-LMXB XLF. Our observations may indicate a potential predominance of GC-LMXBs with donors evolved beyond the main sequence, when compared to current models, but their efficient formation requires relatively high initial binary fractions in clusters. The field LMXB XLF can be fitted with either a single power-law model plus a localized excess at a luminosity of (5-6) × 1037 erg s-1, or a broken power law with a similar low-luminosity break. This XLF may be explained with NS-red-giant LMXBs, contributing to 15% of total LMXBs population at 5 × 1037 erg s-1. The difference in the GC and field XLFs is consistent with different origins and/or evolutionary paths between the two LMXB populations, although a fraction of the field sources are likely to have originated in GCs. © 2009 The American Astronomical Society. All rights reserved.

Abstract:

In light of the recent suggestion that the nearby eclipsing binary star system V Puppis has a dark companion on a long orbit, we present the results of radio and X-ray observations of it. We find an upper limit on its radio flux of about 300 μJy and a detection of it in the X-rays with a luminosity of about 3 × 1031 erg s-1, a value much lower than what had been observed in some of the low angular resolution surveys of the past. These data are in good agreement with the idea that the X-ray emission from V Puppis comes from mass transfer between the two B stars in the system, but can still accommodate the idea that the X-ray emission comes from the black hole accreting stellar wind from one or both of the B stars. © 2009 RAS.

Abstract:

We present observations of early-type galaxies NGC 524 and 2549 with laser guide star adaptive optics (LGS AO) obtained at GEMINI North telescope using the Near-infrared Integral Field Spectrograph (NIFS) integral field unit (IFU) in the K band. The purpose of these observations is to determine high spatial resolution stellar kinematics within the nuclei of these galaxies and, in combination with previously obtained large-scale observations with the SAURON IFU, to determine the masses (M•) of the supermassive black holes (SMBH). The targeted galaxies were chosen to have central light profiles showing a core (NGC 524) and a cusp (NGC 2549), to probe the feasibility of using the galaxy centre as the natural guide source required for LGS AO. We employ an innovative technique where the focus compensation due to the changing distance to the sodium layer is made 'open loop', allowing the extended galaxy nucleus to be used only for tip-tilt correction. The data have spatial resolution of 0.23 and 0.17 arcsec full-width at half maximum (FWHM), where at least ∼40 per cent of flux comes within 0.2, showing that high quality LGS AO observations of these objects are possible. The achieved signal-to-noise ratio (S/N ∼ 50) is sufficiently high to reliably determine the shape of the line-of-sight velocity distribution. We construct axisymmetric three-integral dynamical models which are constrained with both the NIFS and SAURON data. The best-fitting models yield M• = (8.3+2.7-1.3) × 108 M⊙ and (M/L)I = 5.8 ± 0.4 for NGC 524 and M• = (1.4 +0.2-1.3) × 107 M⊙ and (M/L)R = 4.7 ± 0.2 for NGC 2549 (all errors are at the 3σ level). We demonstrate that the wide-field SAURON data play a crucial role in the M/L determination increasing the accuracy of M/L by a factor of at least 5, and constraining the upper limits on black hole masses. The NIFS data are crucial in constraining the lower limits of M• and in combination with the large-scale data reducing the uncertainty by a factor of 2 or more. We find that the orbital structure of NGC 524 shows significant tangential anisotropy, while at larger radii both galaxies are consistent with having almost perfectly oblate velocity ellipsoids. Tangential anisotropy in NGC 524 coincides with the size of SMBH sphere of influence and the core region in the light profile. This agrees with predictions from numerical simulations where core profiles are the result of SMBH binaries evacuating the centre nuclear regions following a galaxy merger. However, being a disc dominated fast rotating galaxy, NGC 524 has probably undergone through a more complex evolution. We test the accuracy to which M• can be measured using seeings obtained from typical LGS AO observations, and conclude that for a typical conditions and M• the expected uncertainty is of the order of 50 per cent. © 2009 RAS.

Abstract:

We report the discovery of an unusual object near the spiral galaxy IC 2497, discovered by visual inspection of the Sloan Digital Sky Survey (SDSS) as part of the Galaxy Zoo project. The object, known as Hanny's Voorwerp, is bright in the SDSS g band due to unusually strong [O iii]4959, 5007 emission lines. We present the results of the first targeted observations of the object in the optical, ultraviolet and X-ray, which show that the object contains highly ionized gas. Although the line ratios are similar to extended emission-line regions near luminous active galactic nucleus (AGN), the source of this ionization is not apparent. The emission-line properties, and lack of X-ray emission from IC 2497, suggest either a highly obscured AGN with a novel geometry arranged to allow photoionization of the object but not the galaxy's own circumnuclear gas, or, as we argue, the first detection of a quasar light echo. In this case, either the luminosity of the central source has decreased dramatically or else the obscuration in the system has increased within 10 5 yr. This object may thus represent the first direct probe of quasar history on these time-scales. © 2009 RAS.

Abstract:

In this paper we study the relation of radio emission to X-ray spectral and variability properties for a large sample of black hole X-ray binary systems. This is done to test, refine and extend - notably into the timing properties - the previously published 'unified model' for the coupling of accretion and ejection in such sources. In 14 outbursts from 11 different sources we find that in every case the peak radio flux, on occasion directly resolved into discrete relativistic ejections, is associated with the bright hard to soft state transition near the peak of the outburst. We also note the association of the radio flaring with periods of X-ray flaring during this transition in most, but not all, of the systems. In the soft state, radio emission is in nearly all cases either undetectable or optically thin, consistent with the suppression of the core jet in these states and 'relic' radio emission from interactions of previously ejected material and the ambient medium. However, these data cannot rule out an intermittent, optically thin, jet in the soft state. In attempting to associate X-ray timing properties with the ejection events we find a close, but not exact, correspondence between phases of very low integrated X-ray variability and such ejections. In fact the data suggest that there is not a perfect one-to-one correspondence between the radio, X-ray spectral or X-ray timing properties, suggesting that they may be linked simply as symptoms of the underlying state change and not causally to one another. We further study the sparse data on the reactivation of the jet during the transition back to the hard state in decay phase of outbursts, and find marginal evidence for this in one case only. In summary we find no strong evidence against the originally proposed model, confirming and extending some aspects of it with a much larger sample, but note that several aspects remain poorly tested. © 2009 RAS.