arXiv subjects

Alan M. Levine

Publications and source records attributed to Alan M. Levine.

67 records · Page 4Linked to original sources

arXiv2000

Complete RXTE Spectral Observations of the Black Hole X-ray Nova XTE J1550-564

We report on the X-ray spectral behavior of the exceptionally bright X-ray nova XTE J1550-564 during its 1998-99 outburst. Our study is based on 209 pointed observations using the PCA and HEXTE instruments onboard the Rossi X-ray Timing Explorer spanning 250 days and covering the entire double-peaked eruption that occurred from 1998 September until 1999 May. The spectra are fit to a model including multicolor blackbody disk and power-law components. The source is observed in the very high and high/soft outburst states of black hole X-ray novae. During the very high state, when the power-law component dominated the spectrum, the inner disk radius is observed to vary by more than an order of magnitude; the radius decreased by a factor of 16 in one day during a 6.8 Crab flare. If the larger of these observed radii is taken to be the last stable orbit, then the smaller observed radius would imply that the inner edge of the disk is inside the event horizon! However, we conclude that the apparent variations of the inner disk radius observed during periods of increased power-law emission are probably caused by the failure of the multicolor disk/power-law model; the actual physical radius of the inner disk may remain fairly constant. This interpretation is supported by the fact that the observed inner disk radius remains approximately constant over 120 days in the high state, when the power-law component is weak, even though the disk flux and total flux vary by an order of magnitude. The mass of the black hole inferred by equating the approximately constant inner disk radius observed in the high/soft state with the last stable orbit for a Schwarzschild black hole is M_BH = 7.4 M_sun (D/6 kpc) (cos i)^{-1/2}.

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arXiv2000

Orbital Decay in LMC X-4

We report on the results of observations of the binary X-ray pulsar LMC X-4 with the Rossi X-ray Timing Explorer. Our analysis of the Doppler delays of the 13.5-s X-ray pulsations yields the most accurate determination of the LMC X-4 orbital parameters available to date. The epoch of orbital phase zero for the 1.4 day orbit is determined with an uncertainty of about 20 s, and is combined with 5 earlier determinations of the epoch of phase zero to obtain the first high significance measurement of the rate of change in the orbital period. We find that the orbital decay timescale is close to 1,000,000 years. We present data on one of three strong X-ray flares as well as energy-dependent pulse profiles for both non-flaring and flaring time intervals. The pulse profiles during the non-flaring time intervals are typically strikingly different from the flare profiles, but at other times can be similar. We reconsider the orbital decay of LMC X-4 in the context of tidal evolution. We find that, while the orbital decay is most likely driven by tidal interactions, the asynchronism between the orbit and the rotation of the companion star is probably maintained by the evolutionary expansion of the companion star, and that the evidence favors the companion star being in a late stage of its life on the main sequence.

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arXiv2000

The Orbit of X Per and Its Neutron Star Companion

We have observed the Be/X-ray pulsar binary system X Per/4U 0352+30 on 61 occasions spanning an interval of 600 days with the PCA instrument on RXTE. Pulse timing analyses of the 837-s pulsations yield strong evidence for the presence of orbital Doppler delays. We confirm the Doppler delays by using measurements made with the RXTE All-Sky Monitor. We obtain an orbital period of 250 days, a projected semimajor axis of the neutron star of 454 lt-s, a mass function of 1.61 solar masses, and a modest eccentricity of 0.11. We discuss the formation of the system in the context of the standard evolutionary scenario for Be/X-ray binaries with consideration of the possibility that the birth of the neutron star was accompanied by a kick of the type often inferred from the velocity distribution of isolated radio pulsars. The orbital eccentricity just after the supernova explosion was almost certainly virtually the same as at present, because the Be star is much smaller than the orbital separation. We find that the system most likely formed from a pair of massive progenitor stars, and probably involved a quasi-stable and nearly conservative transfer of mass from the primary to the secondary. We find that the He star remnant of the primary most likely had a mass less than 6 solar masses after mass transfer. Finally, we speculate that there may be a substantial population of neutron stars formed with little or no kick.

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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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arXiv1999

Correlations between Low-Frequency QPOs and Spectral Parameters in XTE J1550-564 and GRO J1655-40

Utilizing observations obtained with the Rossi X-ray Timing Explorer, we examine correlations between the properties of 0.08-22 Hz variable frequency QPOs and the X-ray spectral parameters for the black hole candidates XTE J1550-564 and GRO J1655-40. The spectra were fitted to a model including a multi-temperature blackbody disk and a power-law component. We find that the QPO frequency and amplitude are well correlated with the spectral parameters, although the correlations found for XTE J1550-564 are generally opposite to those for GRO J1655-40. There is one exeption: Both sources exhibit a general increase in the QPO frequency as the disk flux increases (or as the mass accretion rate through the disk increases). In addition, these QPOs are observed only when the power-law component contributes more than 20% of the 2-20 keV flux, which indicates that both the disk and the power-law components are linked to the QPO phenomenon.

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arXiv1999

X-ray Orbital Modulation of Cygnus X-1

We have analyzed over 2 years of RXTE/ASM data for Cygnus X-1. We have detected the 5.6-day orbital period in Lomb-Scargle periodograms of both light curves and hardness ratios when Cyg X-1 was in the hard state. This detection was made with improved sensitivity and temporal coverage compared with previous detections by other X-ray missions. The folded light curves and hardness ratios show a broad intensity dip accompanied by spectral hardening centered on superior conjunction of the X-ray source. The dip has a duration of about 27% of the orbital period and depth ranging from 8% to 23% of the non-dip intensities in three energy bands. Variability on time scales of hours is often evident within the broad dip in the unfolded data. In contrast, no feature at the orbital period is evident in the periodograms or folded light curves for the soft state. Absorption of X-rays by a stellar wind from the companion star can reproduce the observed X-ray orbital modulations in the hard state. To explain the low orbital modulation in the soft-state data, a reduction of the wind density during the soft state would be required. As an alternative, a partial covering scenario is described which could also account for the lack of the orbital modulation in the soft state.

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arXiv1999

RXTE All-Sky Monitor Localization of SGR 1627-41

The fourth unambiguously identified Soft Gamma Repeater (SGR), SGR1627-41, was discovered with the BATSE instrument on 1998 June 15 (Kouveliotou et al. 1998). Interplanetary Network (IPN) measurements and BATSE data constrained the location of this new SGR to a 6 deg segment of a narrow (19") annulus (Hurley et al. 1999; Woods et al. 1998). We present two bursts from this source observed by the All-Sky Monitor (ASM) on RXTE. We use the ASM data to further constrain the source location to a 5' long segment of the BATSE/IPN error box. The ASM/IPN error box lies within 0.3' of the supernova remnant (SNR) G337.0-0.1. The probability that a SNR would fall so close to the error box purely by chance is ~5%.

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arXiv1999

Scattering and Iron Fluorescence Revealed During Absorption Dips in Circinus X-1

We show that dramatic spectral evolution associated with dips occurring near phase zero in RXTE observations of Cir X-1 is well-fit by variable and at times heavy absorption (N_H > 10^24 cm^-2) of a bright component, plus an underlying faint component which is not attenuated by the variable column and whose flux is ~10% of that of the unabsorbed bright component. A prominent Fe emission line at ~6.5 keV is evident during the dips. The absolute line flux outside the dips is similar to that during the dips, indicating that the line is associated with the faint component. These results are consistent with a model in which the bright component is radiation received directly from a compact source while the faint component may be attributed to scattered radiation. Our results are also generally consistent with those of Brandt et al., who found that a partial- covering model could explain ASCA spectra of a low-to-high transition in Cir X-1. The relative brightness of the two components in our model requires a column density of ~2*10^23 cm^-2 if the faint component is due to Thomson scattering in material that mostly surrounds the source. We find that illumination of such a scattering cloud by the observed direct component would produce an Fe K-alpha fluorescence flux that is in rough agreement with the flux of the observed emission line. We also conclude that if the scattering medium is not highly ionized, our line of sight to the compact source does not pass through it. Finally, we discuss simple pictures of the absorbers responsible for the dips themselves.

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arXiv1999

The Complete "Z" Track of Circinus X-1

We carried out an extensive RXTE campaign, in 1997 June, to study Circinus X-1 during the active portion of its 16.55-d intensity cycle. The observations spanned 10 days, including 56% coverage for 7 d, and allowed us to find time segments which clearly demonstrate continuous evolution along the horizontal, normal, and flaring branches (HB/NB/FB) of a Z-source low-mass X-ray binary. These results confirm and extend the behavior we inferred from earlier observations. Here we study the continuous evolution of the Fourier power spectra and the energy spectra around the complete hardness-intensity track. A narrow quasi-periodic oscillation (QPO) peak, previously observed in the power spectra at 1.3-32 Hz, increases in frequency from 12 Hz to 25 Hz moving down a vertical extension of the HB in the hardness-intensity diagram. These horizontal branch QPOs (HBOs) occur near 30 Hz and fade in strength on the horizontal portion of the HB, while a broad peak in the power spectrum arises near 4 Hz. This peak becomes much more prominent along the NB and remains near 4 Hz (the normal branch QPOs, or NBOs). On the FB, neither QPO is present and the power spectrum is dominated by very low frequency noise. We also found that each branch of the spectral track is associated with a specific type of evolution of the energy spectrum. We explored various models for the energy spectrum and parameterized the evolution of the spectrum in terms of a two-component model consisting of a multi-temperature "disk blackbody" and a higher-temperature (~2 keV) blackbody. We also show that an unusual line- or edge-like feature occurs at ~10 keV in energy spectra from the FB and lower NB. This unusual feature is very similar to one seen on the FB and lower NB of the Z source GX 5-1.

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arXiv1998

Outbursts, State Transitions, and Periodicities Observed with the RXTE All-Sky Monitor

Results from the All-Sky Monitor (ASM) on the Rossi X-ray Timing Explorer are reviewed. A number of recurrent transient sources have been detected, while only a few previously unreported sources have been discovered. The ASM light curves show a wide variety of phenomena in general, and, in particular, those of transient sources show a wide range of properties. Examples are used to illustrate that the distinction between persistent and transient sources may be very unclear. The results of searches for periodicities in the ASM light curves are summarized, and other astrophysical investigations using ASM light curves are suggested. The latter include investigations of the possible causes of long-term quasiperiodic and chaotic variability, and comparative studies on the basis of the observed variability.

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arXiv1998

Quasi-periodic Oscillations Associated with Spectral Branches in RXTE Observations of Circinus X-1

We present Rossi X-ray Timing Explorer (RXTE) All-Sky Monitor observations of the X-ray binary Circinus X-1 which illustrate the variety of intensity profiles associated with the 16.55 d flaring cycle of the source. We also present eight observations of Cir X-1 made with the RXTE Proportional Counter Array over the course of a cycle wherein the average intensity of the flaring state decreased gradually over ~12 days. Fourier power density spectra for these observations show a narrow quasi-periodic oscillation (QPO) peak which shifts in frequency between 6.8 Hz and 32 Hz, as well as a broad QPO peak that remains roughly stationary at ~4 Hz. We identify these as Z-source horizontal and normal branch oscillations (HBOs/NBOs) respectively. Color-color and hardness-intensity diagrams (CDs/HIDs) show curvilinear tracks for each of the observations. The properties of the QPOs and very low frequency noise allow us to identify segments of these tracks with Z-source horizontal, normal, and flaring branches which shift location in the CDs and HIDs over the course of the 16.55 d cycle. These results contradict a previous prediction, based on the hypothesis that Cir X-1 is a high-Mdot atoll source, that HBOs should never occur in this source (Oosterbroek et al. 1995; van der Klis 1994).

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arXiv1997

The RXTE All Sky Monitor: First Year of Performance

The RXTE All Sky Monitor provides a public database that includes more than one year of X-ray monitoring observations (2-12 keV) of X-ray binaries and a few active galactic nuclei. The instrument operates with a 40% duty cycle, and the exposures yield roughly 5 celestial scans per day. There have been 109 source detections, including 16 X-ray transients, the majority of which are recurrent cases. The two sources of relativistic radio jets have exhibited particularly complex light curves and new types of emission states. Progress has been achieved in understanding the outburst mechanism via the reported detection of an optical precursor to the April 1996 X-ray outburst in GRO J1655-40. The ASM has also detected state changes in both Cyg X-1 and Cyg X-3, leading to new constraints on the accretion disk geometry associated with the ``soft/high'' state. X-ray variations are seen in extragalactic nuclei, such as NGC4151 and Mkn501, providing new opportunities for multifrequency timing or spectral studies. The ASM archive empowers observers with the opportunity for state-dependent observing programs with RXTE and other instruments. The ASM also provides a long-term context for source behavior, and this knowledge may be crucial in shaping the interpretation of brief observations with other telescopes.

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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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