arXiv subjects

J. Swank

Publications and source records attributed to J. Swank.

26 records · Page 2Linked to original sources

arXiv2000

Discovery of the Orbit of the Transient X ray Pulsar SAX J2103.5+4545

Using X-ray data from the Rossi X-Ray Timing Explorer (RXTE), we carried out pulse timing analysis of the transient X-ray pulsar SAX J2103.5+4545. An outburst was detected by All Sky Monitor (ASM) October 25 1999 and reached a peak X-ray brightness of 27 mCrab October 28. Between November 19 and December 27, the RXTE/PCA carried out pointed observations which provided us with pulse arrival times. These yield an eccentric orbit (e= 0.4 \pm 0.2) with an orbital period of 12.68 \pm 0.25 days and light travel time across the projected semimajor axis of 72 \pm 6 sec. The pulse period was measured to be 358.62171 \pm 0.00088 s and the spin-up rate (2.50 \pm 0.15) \times 10^{-13} Hz s^{-1}. The ASM data for the February to September 1997 outburst in which BeppoSAX discovered SAX J2103.5+4545 (Hulleman, in't Zand and Heise 1998) are modulated at time scales close to the orbital period. Folded light curves of the 1997 ASM data and the 1999 PCA data are similar and show that the intensity increases at periastron passages.

astro-ph↗

arXiv1999

The Timing Evolution of 4U 1630-47 During its 1998 Outburst

We report on the evolution of the timing of 4U1630-47 during its 1998 outburst using data obtained with the Rossi X-ray Timing Explorer (RXTE). The count rate and position in hardness-intensity, color-color diagrams and simple spectral fits are used to track the concurrent spectral changes. The source showed seven distinct types of timing behavior, most of which show differences with the canonical black hole spectral/timing states. In marked contrast to previous outbursts, we find quasi-periodic oscillation (QPO) signals during nearly all stages of the outburst with frequencies between 0.06Hz and 14Hz and a remarkable variety of other characteristics. In particular we find large (up to 23% rms) amplitude QPO on the early rise. Later, slow 0.1Hz semi- regular short (~ 5 sec), 9 to 16% deep dips dominate the light curve. At this time there are two QPOs, one stable near 13.5Hz and the other whose frequency drops from 6--8Hz to ~4.5 Hz during the dips. BeppoSAX observations during the very late declining phase show 4U1630-47 in a low state.

astro-ph↗

arXiv1999

Long-Term X-ray Monitoring of 1E 1740.7-2942 and GRS 1758-258

We report on long-term observations of the Galactic-bulge black hole candidates 1E 1740.7-2942 and GRS 1758-258 with the Rossi X-Ray Timing Explorer. 1E 1740.7-2942 has been observed 77 times and GRS 1758-258 has been observed 82 times over the past 1000 days. The flux of each object has varied by no more than a factor of 2.5 during this period, and the indices of the energy spectra have varied by no more than 0.4. The power spectra are similar to other black-hole candidates: flat-topped noise, breaking to a power law. Each object has exhibited a brightening that lasted for several months, and we have a found a time lag between the photon power-law index and the count rate. In both sources, the spectrum is softest during the decline from the brightening. This behavior can be understood in the context of thin-disk and advection-dominated accretion flows coexisting over a wide range of radii, with the implication that both sources have low-mass companions and accrete via Roche-lobe overflow.

astro-ph↗

arXiv1998

Discovery of High Frequency Quasi-Periodic Oscillations in 4u1915-05

The type I X-ray burster and dipper 4U1915-05 (also known as XB1916-053) was monitored by the Rossi X-ray Timing Explorer between February and October, 1996. The source was observed in various spectral states; the highest luminosity state (LX ~ 1.5 10^37 ergs/s, 10 kpc, 1-20 keV) is associated with a soft spectrum, whereas for the lower luminosity state (down to ~5 10^36 ergs/s) the spectrum is significantly harder. Using the high time resolution data provided by the Proportional Counter Array (PCA), we have discovered High-Frequency Quasi-Periodic Oscillations (HFQPOs) in the persistent X-ray emission of 4U1915-05 while its luminosity was ~8 10^36 ergs/s. The QPO frequency ranges from 600 Hz up to ~1000 Hz, with typical Full Width at Half Maximum (FWHM) ~50-100 Hz and Root Mean Squared (RMS) values 15%. In addition, by using the ``shift and add'' technique, we have detected a twin HFQPO (5.5 sigma level) separated from the main peak by ~355 Hz. 4U1915-05 is the eighth Atoll source displaying simultaneous twin HFQPOs. Based on current knowledge of HFQPO sources, our observations suggest that 4U1915-05 might contain a 2.8 (or 5.6) millisecond rotating neutron star.

astro-ph↗

arXiv1998

XTE J1739-302: An Unusual New X-ray Transient

A new x-ray transient, designated XTE J1739-302, was discovered with the Proportional Counter Array (PCA) on the Rossi X-ray Timing Explorer (RXTE) in data from 12 August 1997. Although it was the brightest source in the Galactic Center region while active (about 3.0 x 10^-9 ergs/cm2/s from 2 to 25 keV), it was only observed on that one day; it was not detectable nine days earlier or two days later. There is no known counterpart at other wavelengths, and its proximity to the Galactic Center will make such an identification difficult due to source confusion and extinction. The x-ray spectrum and intensity suggest a giant outburst of a Be/neutron star binary, although no pulsations were observed and the outburst was shorter than is usual from these systems.

astro-ph↗

arXiv1997

Accretion instabilities and jet formation in GRS 1915+105

We report simultaneous observations in the X-ray, infrared, and radio wavelengths of the galactic superluminal source GRS 1915+105. During episodes of rapid disappearance and follow up replenishment of the inner accretion disk evidenced by the X-ray oscillating flux, we observe the ejection of relativistic plasma clouds in the form of synchrotron flares at infrared and radio wavelengths. The expelled clouds contain very energetic particles with Lorentz factors of about 1000, or more. These ejections can be viewed as small-scale analogs of the more massive ejecta with relativistic bulk motions that have been previously observed in GRS 1915+105.

astro-ph↗

arXiv1997

Temporal Properties of Cygnus X-1 During the Spectral Transitions

We report the results from our timing analysis of 15 RXTE observations of Cygnus X-1 throughout its 1996 spectral transitions. The entire period can be divided into 3 distinct phases: (1) transition from the hard to soft state, (2) soft state, and (3) transition from the soft state back to the hard state. The observed X-ray properties in phases 1 and 3 are remarkably similar, suggesting that the same physical processes are likely involved in triggering such transitions. The power density spectrum (PDS) during the transition can be characterized by a red noise component, followed by a white noise component which extends to roughly 1-3 Hz where it is cut off, and a steeper power law at higher frequencies. The X-ray flux also exhibits apparent quasi-periodic oscillation (QPO) with the centroid frequency varying in the range of 4-12 Hz. The QPO shows no correlation with the source flux, but becomes more prominent at higher energies. This type of PDS bears resemblance to that of other black hole candidates often observed in a so-called very high state, although the origin of the observed QPO may be very different. The low-frequency red noise has not been observed in the hard state, thus seems to be positively correlated with the disk mass accretion rate which is presumably low in the hard state and high in the soft state; in fact, it completely dominates the PDS in the soft state. In the framework of thermalComptonization models, Cui et al. (see astro-ph/9610071 and astro-ph/9610072) speculated that the difference in the observed spectral and timing properties between the hard and soft states is due to the presence of a ``fluctuating'' Comptonizing corona during the transition. Here we present the measured hard X-ray time lags and coherence functions between various energy bands, and show that the results strongly support such a scenario.

astro-ph↗

arXiv1996

X-ray Variability of Cygnus X-1 In Its Soft State

We reported previously that for Cyg X-1 there is a settling period following the transition from hard to soft state (astro-ph/9610071). During the transiton, The low energy spectrum (below ~10 keV) varies significantly from observation to observation while the high energy portion changes little. The source reaches nominal soft-state brightness during the settling period. It can be characterized by a soft low-energy spectrum and significant low-frequency 1/f noise and white noise on the power density spectrum (PDS). The low-energy spectrum becomes even softer, and the PDS is completely dominated by the 1/f noise, when the ``true'' soft state is reached. In this paper, subsequent RXTE observations of Cyg X-1 in the soft state are examined, and the results confirm our earlier conclusions. Furthermore, we show the results from observations taken during a soft-to-hard transition. As expected, the white noise appears again, and accordingly, the 1/f noise becomes less dominant, similar to the settling period at the end of the hard-to-soft transition. The low-frequency 1/f noise has not been observed when Cyg X-1 is in the hard state. Therefore, it seems to be positively correlated with the disk mass accretion rate which is low in the hard state and high in the soft state. The difference in the observed spectral and timing properties between the hard and soft states is qualitatively consistent with a simple ``fluctuating corona'' model (astro-ph/9610071). Here we present more evidence for it.

astro-ph↗