200 research outputs found
Sidebands Due to Quasi-periodic Oscillations in 4U 1626-67
The low-mass X-ray binary pulsar 4U 1626-67 shows 0.048 Hz quasi-periodic
oscillations (QPOs) and red noise variability as well as coherent pulsations at
the 0.130 Hz neutron star spin frequency. Power density spectra of observations
made with the Rossi X-ray Timing Explorer show significant sidebands separated
from the pulsar spin frequency (and its harmonics) by the QPO frequency. These
show that the instantaneous amplitude of the coherent pulsations is modulated
by the amplitude of the QPOs. This phenomenon is expected in models such as the
magnetospheric beat frequency model where the QPOs originate near the polar
caps of the neutron star. In the 4--8 keV energy range, however, the
lower-frequency sidebands are significantly stronger than their
higher-frequency complements; this is inconsistent with the magnetospheric beat
frequency model. We suggest that the 0.048 Hz QPOs are instead produced by a
structure orbiting the neutron star at the QPO frequency. This structure
crosses the line of sight once per orbit and attenuates the pulsar beam,
producing the symmetric (amplitude modulation) sidebands. It also reprocesses
the pulsar beam at the beat frequencies between the neutron star spin frequency
and the QPOs, producing the excess variability observed in the lower-frequency
sidebands. Quite independently, we find no evidence that the red noise
variability modulates the amplitude of the coherent pulsations. This is also in
contrast to the expectations of the magnetospheric beat frequency model and
differs from the behavior in some high-mass X-ray binary pulsars.Comment: 8 pages, 3 figures, AAS macros v4.0. To appear in ApJ Letter
On the nature of XTE J0421+560/CI Cam
We present the results of the analysis of RXTE, BATSE and optical/infrared
data of the 1998 outburst of the X-ray transient system XTE J0421+560 (CI Cam).
The X-ray outburst shows a very fast decay (initial e-folding time ~0.5 days,
slowing down to ~2.3 days). The X-ray spectrum in the 2-25 keV band is complex,
softening considerably during decay and with strongly variable intrinsic
absorption. A strong iron emission line is observed. No fast time variability
is detected (<0.5 % rms in the 1-4096 Hz band at the outburst peak). The
analysis of the optical/IR data suggests that the secondary is a B[e] star
surrounded by cool dust and places the system at a distance of >~ 2 kpc. At
this distance the peak 2-25 keV luminosity is ~4 x 10^37 erg/s. We compare the
properties of this peculiar system with those of the Be/NS LMC transient A
0538-66 and suggest that CI Cam is of similar nature. The presence of strong
radio emission during outburst indicates that the compact object is likely to
be a black hole or a weakly magnetized neutron star.Comment: Accepted for publication on The Astrophysical Journal, July 199
Report of the Implementation of Work Package 6 "Implementation of Methodology" in the Framework of the IRNet Project
This article, prepared by an international team of researchers from different
scientific areas connected with ICT, e-learning, pedagogy, and other related
disciplines, focuses on the objectives and some results of the international project
IRNet (www.irnet.us.edu.pl). In particular, the article describes research tools,
methods, and a procedure of the Work Package 6 âImplementation of Methodology,â
that is, objectives, tasks, deliverables, publications, and implementation of research
trips in the context of the next stages and Work Packages of IRNet project â
International Research Network
On the Association of -Ray Bursts with Supernovae
The recent discovery of a supernova (SN 1998bw) seemingly associated with GRB~980425 adds a new twist to the decades-old debate over the origin of gamma-ray bursts. To investigate the possibility that some (or all) bursts are associated with supernovae, we performed a systematic search for temporal/angular correlations using catalogs of BATSE and BATSE/{\it Ulysses} burst locations. We find no associations with any of the precise BATSE/ Ulysses locations, which allows us to conclude that the fraction of high-fluence gamma-ray bursts associated with known supernovae is small (0.2%). For the more numerous weaker bursts, the corresponding limiting fraction of 2.5% is far less constraining due to the imprecise locations of these events. This fraction (2.5%) of bursts corresponds to 30% of the recent supernovae used as a comparison data set. Thus, although we find no significant evidence to support a burst/supernova association, the possibility cannot be excluded for weak bursts
Limits on the cosmological abundance of supermassive compact objects from a millilensing search in gamma-ray burst data
A new search for the gravitational lens effects of a significant cosmological
density of supermassive compact objects (SCOs) on gamma-ray bursts has yielded
a null result. We inspected the timing data of 774 BATSE-triggered GRBs for
evidence of millilensing: repeated peaks similar in light-curve shape and
spectra. Our null detection leads us to conclude that, in all candidate
universes simulated, is favored for , while in some universes and mass ranges the density
limits are as much as 10 times lower. Therefore, a cosmologically significant
population of SCOs near globular cluster mass neither came out of the
primordial universe, nor condensed at recombination.Comment: 14 pages including 3 figures, appeared 2001 January 2
Radio and X-ray Signatures of Merging Neutron Stars
We describe the possible electromagnetic signals expected from the
magnetospheric interactions of a neutron star binary prior to merger. We find
that both radio and X-ray signals of detectable strength are possible. We
discuss possible links with the phenomenon of gamma-ray bursts and describe the
prospects for direct detection of these signals in searches for radio and X-ray
transients.Comment: LaTeX, 9 pages plus 2 postscript figures. submitted to M.N.R.A.
Chandra and RXTE Spectroscopy of the Galactic Microquasar XTE J1550-564 in Outburst
On two occasions, we obtained nearly simultaneous ~ 4 kilosecond snapshot
observations of the Galactic black hole and microquasar XTE J1550-564 with
Chandra and RXTE near the peak of its May, 2000 outburst. The low-energy
sensitivity of Chandra and the resolution of the High Energy Transmission
Grating Spectrometer (HETGS), coupled with the broad energy range and large
collecting area of RXTE, have allowed us to place constraints on the outburst
accretion flow geometry of this source in the ``intermediate'' X-ray state. The
0.65-25.0 keV continuum spectra are well-described by relatively hot (kT ~ 0.8
keV) accretion disk and hard (Gamma ~ 2.3) coronal power-law components. Broad,
relatively strong Fe K-alpha emission line (EW ~170 eV) and smeared absorption
edge components consistent with Fe XXV are strongly required in joint spectral
fits. The resolution of the Chandra/HETGS reveals that the broad Fe K-alpha
emission lines seen clearly in the individual RXTE spectra are not due to an
intrinsically narrow line.Comment: 11 pages, 4 figures (2 in color). MNRAS, accepted. Significant
updates from the initial versio
The Interplanetary Network Supplement to the BeppoSAX Gamma-Ray Burst Catalogs
Between 1996 July and 2002 April, one or more spacecraft of the
interplanetary network detected 787 cosmic gamma-ray bursts that were also
detected by the Gamma-Ray Burst Monitor and/or Wide-Field X-Ray Camera
experiments aboard the BeppoSAX spacecraft. During this period, the network
consisted of up to six spacecraft, and using triangulation, the localizations
of 475 bursts were obtained. We present the localization data for these events.Comment: 89 pages, 3 figures. Submitted to the Astrophysical Journal
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