Robin H. D. Corbet

A Systematic Search for Periodicities in RXTE/ASM Data

We present the results of a systematic search in 8.5 years of Rossi X-ray Timing Explorer All-Sky Monitor data for evidence of periodicities. The search was conducted by application of the Lomb-Scargle periodogram to the light curves of each of 458 actually or potentially detected sources in each of four energy bands (1.5–3 keV, 3-5 keV, 5-12 keV, and 1.5–12 keV). A whitening technique was applied to the periodograms before evaluation of the statistical significance of the powers. We discuss individual detections with focus on relatively new findings.

Ginga observations of Centaurus X-3

Attention is given to a series of observations of Centaurus X-3 made with Ginga on March 22-24, 1989, over a complete orbital cycle. The data set includes a preeclipse dip, the eclipse ingress and egress, the eclipse itself, and a phase of high steady emission after egress. The mideclipse ephemeris obtained yields an estimate of the rate of change of the orbital period of -(1.738 +/- 0.004) x 10 exp 6/yr, confirming the rate of change reported by Kelley et al. (1983) and improving the precision by a factor of 20. The pulse profiles obtained with Ginga during the high postegress phase are double-peaked at low energies, and show a reversal of phase of 180 deg. No pulsations are detected during the eclipse and at low energies during the preeclipse dip, indicating that X-ray emission at these phases is due to scattering by extended matter. The energy of the iron emission line varies between 6.5 and 6.7 keV during the observation, while its equivalent width varies between 0.18 and 1.5 keV.

The spectrum and pulses of 1E 2259+586 from ASCA and BBXRT observations

The 7 s X-ray pulsator 1E 2259+586 was observed for approximately 1 day in 1993 with the Advanced Satellite for Cosmology and Astrophysics (ASCA). Observations were also obtained with Broad Band X-ray Telescope (BBXRT) in 1990 a few months after Ginga had observed 1E 2259+586 to be brighter than normal and the BBXRT data show 1E 2259+586 to be at an intermediate brightness level. By contrast, the ASCA data appear to have been obtained during a more common lower luminosity state. The pulse profiles we obtain are consistent with a connection between flux and pulse shape reported from Ginga data, and the pulsator continues to spin down. We use our high spectral resolution data to search for cyclotron lines in the spectrum that were claimed from observations made with other satellites. We find that the ASCA spectra of 1E 2259+586 cannot be satisfactorily fitted with either a single power law or a combination of two power laws, and that significant residuals occur around 1.5 and 5 keV. However, a combination of a power law and blackbody gives a good fit over the entire ASCA energy band with no evidence of spectral features. We have reanalyzed a Ginga LAC spectrum and find that this is also significantly better fitted by this two-component spectrum than a single power law. A possible explanation for such a two-component spectrum is that the blackbody emission comes from a neutron star and that the power-law component comes, at least in part, from a surrounding nebula. As there has, so far, no direct evidence that 1E 2259+586 is a binary system we consider whether there are other plausible mechanisms that might power the observed X-ray emission.

LMC X-4 : GINGA observations and search for orbital period changes

The results of a 2.4 day observation of the binary X-ray pulsar LMC X-4 with the Ginga satellite are reported. A high-quality X-ray spectrum which includes an Fe emission line with an equivalent width of 0.18 keV is presented. Timing analyses of the 13.5 s X-ray pulsations yield a Doppler delay curve, which, in turn, provides the most accurate determination of the LMC X-4 orbital parameters available to date. The phase of the 1.4 day orbit is determined with an accuracy of ∼60 s. This phase is combined with four previous determinations of the orbital phase to yield an estimate of the rate of change in the orbital period of (1.1 ± 0.8)×10 −6 1/yr

Long-Term Behavior of X-Ray Pulsars in the Small Magellanic Cloud

Results of a 4 yr X-ray monitoring campaign of the Small Magellanic Cloud using the Rossi X-ray Timing Explorer (RXTE) are presented. This large data set makes possible detailed investigation of a significant sample of SMC X-ray binaries. Eight new X-ray pulsars were discovered, and a total of 20 different systems were detected. Spectral and timing parameters were obtained for 18. In the case of 10 pulsars, repeated outbursts were observed, allowing determination of candidate orbital periods for these systems. We also discuss the spatial and pulse-period distributions of the SMC pulsars.

A Study of the Populations of X-Ray Sources in the Small Magellanic Cloud with ASCA

The Advanced Satellite for Cosmology and Astrophysics (ASCA) has made multiple observations of the Small Magellanic Cloud (SMC). X-ray mosaic images in the soft (0.7-2.0 keV) and hard (2.0-7.0 keV) bands are separately constructed, and the latter provides the first hard X-ray view of the SMC. We extract 39 sources from the two-band images with a criterion of S/N > 5 and conduct timing and spectral analyses for all of these sources. Coherent pulsations are detected from 12 X-ray sources, five of which are new discoveries. Most of the 12 X-ray pulsars are found to exhibit long-term flux variabilities; hence they are likely to be X-ray binary pulsars (XBPs). On the other hand, we classify four supernova remnants (SNRs) as thermal SNRs, because their spectra exhibit emission lines from highly ionized atoms. We find that XBPs and thermal SNRs in the SMC can be clearly separated by their hardness ratio (the ratio of the count rate between the hard and soft bands). Using this empirical grouping, we find many XBP candidates in the SMC, although no pulsations have yet been detected from these sources. Possible implications on the star formation history and evolution of the SMC are presented by a comparison of the source populations in the SMC and our Galaxy.

Transient X-Ray Sources, Luminosity Gaps, and Neutron Star Densities

A simple model of accretion onto magnetized neutron stars predicts a large luminosity change during a transition from magnetospheric accretion to neutron star accretion. It is pointed out that, for this model, the ratio of the minimum neutron star accretion luminosity and maximum magnetospheric accretion luminosity depends on just the neutron star density and rotation period. Hence, the detection of such a luminosity gap would, at least in principle, give direct insight into the mass-radius relationship of neutron stars. Some potential problems that may affect this measurement are also discussed and some observational results are briefly reviewed.

Evidence for a Very Slow X-Ray Pulsar in 2S 0114+650 from Rossi X-Ray Timing Explorer All-Sky Monitor Observations

Rossi X-Ray Timing Explorer (RXTE) all-sky monitor (ASM) observations of the X-ray binary 2S 0114+650 show modulations at periods close to both the optically derived orbital period (11.591 days) and proposed pulse period (~2.7 hr). The pulse period shows frequency and intensity variability during the more than 2 years of ASM observations analyzed. The pulse properties are consistent with this arising from accretion onto a rotating neutron star, and this would be the slowest such period known. The shape of the orbital light curve shows modulation over the course of the entire orbit, and a comparison is made with the orbital light curve of Vela X-1. However, the expected phase of eclipse, based on an extrapolation of the optical ephemeris, does not correspond with the observed orbital minimum. The orbital period derived from the ASM light curve is also slightly longer than the optical period.

The X-Ray Properties of M101 ULX-1 = CXOKM101 J140332.74+542102

We report our analysis of X-ray data on M101 ULX-1, concentrating on high-state Chandra and XMM-Newton observations. We find that the high state of M101 ULX-1 may have a preferred recurrence timescale. If so, the underlying clock may have periods around 160 or 190 days, or possibly around 45 days. Its short-term variations resemble those of X-ray binaries at high accretion rates. If this analogy is correct, we infer that the accretor is a 20-40 M☉ object. This is consistent with our spectral analysis of the high-state spectra of M101 ULX-1, from which we find no evidence for an extreme (>1040 ergs s-1) luminosity. We present our interpretation in the framework of a high-mass X-ray binary system consisting of a B supergiant mass donor and a large stellar-mass black hole.

Ginga observations of the dipping low-mass X-ray binaries XB 1916-053 and EXO 1748-676

Ginga observations of XB 1916-053 and EXO 0748-676, low-mass X-ray binaries with pronounced dips of variable depth and duration in their X-ray light curves, are reported. Periodicity studies of XB-1916-053 show results consistent with those from previous Ginga observations, specifically, that the derived X-ray period is shorter than the optical period by about 1 percent. This problem is discussed, and a more refined model for the system is described. Three bursts were detected during the observation of XB 1916-053, the second of which occurred during a dip. Spectral analysis suggests that the second burst caused an almost instantaneous ionization of the absorbing medium in the line of sight. This material then returned to its equilibrium state on the same time scale as the burst decay. Loose limits are placed upon the possible clumpiness of the material responsible for the accretion disk structure in XB 1916-053 and EXO 0748-676.