arXiv subjects

L. Bildsten

Publications and source records attributed to L. Bildsten.

41 records · Page 3Linked to original sources

arXiv2000

Search for Millisecond Periodicities in Type I X-ray Bursts of the Rapid Burster

We have searched the rising portion of type I X-ray bursts observed from the Rapid Burster with the Rossi X-ray Timing Explorer for the presence of periodicities. The 95 per cent confidence upper limit on the average root-mean-square variation of near coherent pulsations with a width of <1 Hz (in 60--2048 Hz) during the first second of the bursts is <8.8 per cent. We find a possible detection (>98 per cent significance) at 306.5 Hz.

astro-ph↗

arXiv2000

Transient Low-Mass X-Ray Binaries in Quiescence

We summarize the quiescent X-ray observations of transient low-mass X-ray binaries. These observations show that, in quiescence, binaries containing black holes are fainter than those containing neutron stars. This has triggered a number of theoretical ideas about what causes the quiescent X-ray emission. For black hole binaries, the options are accretion onto the black hole or coronal emission from the rapidly rotating stellar companion. There are more possibilities for the neutron stars: accretion, thermal emission from the surface or non-thermal emission from a ``turned-on'' radio pulsar. We review recent theoretical work on these mechanisms and note where current observations can distinguish between them. We highlight the re-analysis of the quiescent neutron star emission by Rutledge and collaborators that showed thermal emission to be a predominant contributor in many of these systems. Our knowledge of these binaries is bound to dramatically improve now that the Chandra and XMM satellites are operating successfully,

astro-ph↗

arXiv1999

EXIST: A High Sensitivity Hard X-ray Imaging Sky Survey Mission for ISS

A deep all-sky imaging hard x-ray survey and wide-field monitor is needed to extend soft (ROSAT) and medium (ABRIXAS2) x-ray surveys into the 10-100 keV band (and beyond) at comparable sensitivity (~0.05 mCrab). This would enable discovery and study of >3000 obscured AGN, which probably dominate the hard x-ray background; detailed study of spectra and variability of accreting black holes and a census of BHs in the Galaxy; Gamma-ray bursts and associated massive star formation (PopIII) at very high redshift and Soft Gamma-ray Repeaters throughout the Local Group; and a full galactic survey for obscured supernova remnants. The Energetic X-ray Imaging Survey Telescope (EXIST) is a proposed array of 8 x 1m^2 coded aperture telescopes fixed on the International Space Station (ISS) with 160deg x 40deg field of view which images the full sky each 90 min orbit. EXIST has been included in the most recent NASA Strategic Plan as a candidate mission for the next decade. An overview of the science goals and mission concept is presented.

astro-ph↗

arXiv1999

The Evolution of Rapid Burster Outbursts

We describe the evolutionary progression of an outburst of the Rapid Burster. Four outbursts have been observed with the Rossi X-Ray Timing Explorer between February 1996 and May 1998, and our observations are consistent with a standard evolution over the course of each. An outburst can be divided into two distinct phases: Phase I is dominated by type I bursts, with a strong persistent emission component; it lasts for 15-20 days. Phase II is characterized by type II bursts, which occur in a variety of patterns. The light curves of time-averaged luminosity for the outbursts show some evidence for reflares, similar to those seen in soft X-ray transients. The average recurrence time for Rapid Burster outbursts during this period has been 218 days, in contrast with an average ~180 day recurrence period observed during 1976-1983.

astro-ph↗

arXiv1998

The Thermal X-ray Spectra of Cen X-4, Aql X-1, and 4U 1608-522 in Quiescence

We re-analyze the available X-ray spectral data of the type I bursting neutron star transients Aql X-1, Cen X-4, and 4U 1608-522 using realistic hydrogen atmosphere models. Previous spectral fits assumed a blackbody spectrum; because the free-free dominated photospheric opacity decreases with increasing frequency, blackbody spectral fits overestimate the effective temperature and underestimate, by as much as two orders of magnitude, the emitting area. Hydrogen atmosphere spectral models, when fit to the available observational data, imply systematically larger emission area radii, consistent with the canonical 10 km radius of a neutron star. This suggests that a substantial fraction of the quiescent luminosity is thermal emission from the surface of the neutron star. The magnitude of the equivalent hydrogen column density toward these systems, however, presents a considerable systematic uncertainty, which can only be eliminated by high signal-to-noise X-ray spectral measurements (e.g., with AXAF or XMM) which would permit simultaneous determination of the equivalent hydrogen column density, emission area, and thermal temperature.

astro-ph↗