Hintze Centre for Astrophysical Surveys

Abstract:

We report an anticorrelation between continuum luminosity and the equivalent width (EW) of the Hα emission line in X-ray binary systems. The effect is evident both in a universal monotonic increase in Hα EW with time following outbursts, as systems fade, and in a comparison between measured EWs and contemporaneous X-ray measurements. The effect is most clear for black hole binaries in the low/hard X-ray state, which is prevalent at X-ray luminosities below ∼1 per cent of the Eddington luminosity. We do not find strong evidence for significant changes in line profiles across accretion state changes, but this is hampered by a lack of good data at such times. The observed anticorrelation, highly significant for black hole binaries, is only marginally so for neutron star systems, for which there are far less data. Comparison with previously established correlations between optical and X-ray luminosity suggests that the line luminosity is falling as the X-ray and optical luminosities drop, but not as fast, approximately, as LHα ∝ L∼0.4X ∝ L∼0.7opt. We briefly discuss possible origins for such an effect, including the optical depth, form of the irradiating spectrum and geometry of the accretion flow. Further refinement of the relation in the future may allow measurements of Hα EW to be used to estimate the luminosity of, and hence the distance to, X-ray binary systems. Beyond this, further progress will require a better sample of spectrophotometric data. © 2009 RAS.

Abstract:

We present evidence for anomalous microwave emission in the RCW175 H II region. Motivated by 33 GHz 13′ resolution data from the Very Small Array (VSA), we observed RCW175 at 31 GHz with the Cosmic Background Imager (CBI) at a resolution of 4′. The region consists of two distinct components, G29.0-0.6 and G29.1-0.7, which are detected at high signal-to-noise ratio. The integrated flux density is 5.97 0.30 Jy at 31 GHz, in good agreement with the VSA. The 31 GHz flux density is 3.28 0.38 Jy (8.6σ) above the expected value from optically thin free-free emission based on lower frequency radio data and thermal dust constrained by IRAS and WMAP data. Conventional emission mechanisms such as optically thick emission from ultracompact H II regions cannot easily account for this excess. We interpret the excess as evidence for electric dipole emission from small spinning dust grains, which does provide an adequate fit to the data. © 2009. The American Astronomical Society. All rights reserved.

Abstract:

We report on X-ray and soft γ-ray observations of the black hole candidate GX 339-4 during its 2007 outburst, performed with the RXTE and INTEGRAL satellites. The hardness-intensity diagram of all RXTE/PCA data combined shows a q-shaped track similar to that observed in previous outbursts. The evolution in the diagram suggested that a transition from hard- to soft-intermediate state occurred, simultaneously with INTEGRAL observations performed in March. The transition is confirmed by the timing analysis presented in this work, which reveals that a weak type-A quasi-periodic oscillation (QPO) replaces a strong type-C QPO. At the same time, spectral analysis shows that the flux of the high-energy component shows a significant decrease in its flux. However, we observe a delay (roughly one day) between variations of the spectral parameters of the high-energy component and changes in the flux and timing properties. The changes in the high-energy component can be explained either in terms of the high-energy cut-off or in terms of variations in the reflection component. We compare our results with those from a similar transition during the 2004 outburst of GX 339-4. © 2008 RAS.

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.