arXiv subjects

Hale Bradt

Publications and source records attributed to Hale Bradt.

8 recordsLinked to original sources

arXiv2001

Observations of Gamma-ray Bursts with the Rossi X-ray Timing Explorer

The role of the Rossi X-ray Timing Explorer (RXTE) in the study of Gamma-ray Bursts (GRBs) is reviewed. Through April 2001, the All-Sky Monitor (ASM) and the Proportional Counter Array (PCA) instruments have detected 30 GRBs. In 16 cases, an early celestial position was released to the community, sometimes in conjunction with IPN results. The subsequent optical and radio searches led to the detection of 5 x-ray afterglows, to at least 6 optical or radio afterglows, to 3 of the 17 secure redshifts known at this writing, and to 2 other likely redshifts. The decay curves of early x-ray afterglows have been measured. The rapid determination of the location of GRB 970828 and the absence of optical afterglow at that position gave one of the first indications that GRBs occur in star-forming regions (Groot et al. 1998, ApJ 493, L27). The location of GRB 000301C led to the determination of a break in the optical decay rate (Rhoads and Fruchter 2001, ApJ 546, 117) which is evidence for a jet, and to variability in the optical light curve that could represent gravitational lensing (Garnavich, Loeb, and Stanek 2000, ApJ 544, L11). X-ray light curves of GRB from the ASM in conjunction with gamma-ray light curves exhibit striking differences in different bands and may reveal the commencement of the x-ray afterglow (Smith et al. 2001, ApJ submitted, astroph 0103357).

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arXiv2001

An All-sky Advanced X-ray Monitor (AXM) Mission

A concept for an all-sky monitor mission substantially more powerful than the RXTE ASM is presented in brief. It makes use of 31 Gas Electron Multiplier (GEM) proportional counters to provide continuous monitoring of the entire x-ray sky, except for earth occultations and a small region around the sun. This coverage would allow the study of a wide variety of short duration cosmic outbursts. At time scales longer than a few minutes, the sensitivity for the monitoring of x-ray sources (1.5 - 15 keV) would be greatly improved over the RXTE ASM, a factor of 6 in one day. This mission concept, the Advanced X-ray Monitor (AXM), is described in more detail in Remillard et al. (2000).

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arXiv2000

X-ray Transients Monitored by the All-Sky Monitor on RXTE: A Tabulation

We present a tabulation of 46 transient x-ray sources monitored with the All-Sky Monitor (ASM) on the Rossi X-ray Timing Explorer (RXTE). They fall into four broad categories: short (~d), intermediate, and long (>500 d) duration of outbursts, and long period binary systems that flare up at periastron (e.g., Be systems). The mixture of outburst/quiescent cycles and low-level persistent emission in a few systems could indicate conditions are near the limit for stable mass flow in the accretion disk. The two short-time-scale systems, CI Cam and V4641 Sgr, are within 1 kpc of the sun, and hence many more such systems may await discovery.

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arXiv2000

Transient X--Ray Sources Observed with the Rxte All-Sky Monitor after 3.5 Years

We present light curves of a sample of "transient" sources observed with the All-Sky Monitor (ASM) of the Rossi X-ray Timing Explorer (RXTE). The light curves extend over 3.5 years. They are presented in three groups: six neutron-star systems, eight black-hole-candidate systems, and an additional diverse set of six objects that are either transient sources (in the sense of usually being undetectable) or persistent sources showing transient behavior. The outburst profiles of these sources show reproducible characteristics within one source and from source to source, as well as large variations. These profiles together with the profiles of the hardness ratios from the ASM are a valuable resource for the understanding of accretion instabilities. We summarize briefly some recent work by observers on these somewhat arbitrarily selected sources.

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arXiv1998

Observations of Circinus X-1 with RXTE

Data accumulated with RXTE during the active state of Cir X-1 as well as during an unusually long transition from the active state to the quiescent state are reported. The long decline from the active state allowed the source characteristics to be studied systematically as a function of intensity. The following results are presented: (1) spectral fits during entry into a dip clearly show absorption with partial covering as previously reported (Brandt et al. 1996), and (2) correlations between position in the hardness-intensity plane and the character of the power density spectrum as the source entered the quiescent state are suggestive of Z source behavior seen in LMXB sources.

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arXiv1997

Some Early Results from the Rossi X-ray Timing Explorer (RXTE)

The Rossi X-ray Timing Explorer (RXTE) was successfully launched on 1995 December 30 and has been operational since that time. Its three instruments are probing regions close to compact objects, degenerate dwarfs, neutron stars, stellar black holes and the central engines of AGN. Temporal studies with the ASM and PCA have already yielded rich results which pertain to the environs, evolution, and nature of the compact objects in galactic systems. Here I review some selected results from these instruments, as obtained by various RXTE observers. The ASM is providing detailed light curves of about 60 detected sources and has uncovered new temporal/spectral states of galactic binary systems. The bizarre behavior of the possibly very young binary system, Cir X-1, is being revealed in detail by both the ASM and the PCA. A rare high/soft state of the black-hole candidate, Cyg X-1 provides new insight into the nature of the low/high transitions in black-hole binaries. The PCA has made possible the discovery of oscillations near 1 kHz in ten low-mass X-ray binary systems. These are most probably direct indicators of the neutron-star spin in some cases and probably indirect indicators in others. Studies of microquasars have unveiled a host of new temporal phenomena which may provide links between accretion processes and the radio jets in these systems.

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arXiv1996

RXTE Discovery of Coherent Millisecond Pulsations during an X-ray Burst from KS 1731-260

A highly coherent 523.92+-0.05 Hz periodic X-ray signal has been observed during a type I X-ray burst from the low-mass X-ray binary system KS 1731-260 with the PCA on RXTE. The spectral evolution of the burst indicates photospheric-radius expansion and contraction. The 524 Hz signal occurred at the end of the contraction phase, lasted for ~2 s, was highly coherent (Q >~ 900), and had a pulse fraction (ratio of sinusoidal amplitude to mean count rate) of 6.2+-0.6%. KS 1731-260 is one of only three systems that have exhibited high-coherence millisecond oscillations during X-ray bursts and the first reported where the pulsations are associated with photospheric contraction. These coherent signals may be interpreted as a direct indication of the neutron star spin.

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arXiv1996

First Results from the All-Sky Monitor on the Rossi X-ray Timing Explorer

The All-Sky Monitor on the Rossi X-ray Timing Explorer has been monitoring the sky in the 1.5 - 12 keV band since late February. The instrument consists of three coded-aperture cameras which can be rotated to view different regions by a motorized drive assembly. Intensities of ~100 known sources are obtained via least-square fits of shadow patterns to the data and compiled to form x-ray light curves. Six orbital periodicities and four long-term periodicities, all previously known, have been detected in these light curves. Searches for additional sources have also been conducted. X-ray light curves for the Crab Nebula, Cyg X-1, 4U 1705-44, GRO J1655-40, and SMC X-1 are reported. They illustrate the quality of the results and the range of observed phenomena.

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