8,339 research outputs found
Physical Mechanisms for the Variable Spin-down of SGR 1900+14
We consider the physical implications of the rapid spindown of Soft Gamma
Repeater 1900+14, and of the apparent "braking glitch", \Delta P/P = l x 10^-4,
that was concurrent with the Aug. 27th giant flare. A radiation-hydrodynamical
outflow associated with the flare could impart the required torque, but only if
the dipole magnetic field is stronger than ~ 10^14 G and the outflow lasts
longer and/or is more energetic than the observed X-ray flare. A positive
period increment is also a natural consequence of a gradual, plastic
deformation of the neutron star crust by an intense magnetic field, which
forces the neutron superfluid to rotate more slowly than the crust. Sudden
unpinning of the neutron vortex lines during the August 27th event would then
induce a glitch opposite in sign to those observed in young pulsars, but of a
much larger magnitude as a result of the slower rotation.
The change in the persistent X-ray lightcurve following the August 27 event
is ascribed to continued particle heating in the active region of that
outburst. The enhanced X-ray output can be powered by a steady current flowing
through the magnetosphere, induced by the twisting motion of the crust. The
long term rate of spindown appears to be accelerated with respect to a simple
magnetic dipole torque. Accelerated spindown of a seismically-active magnetar
will occur when its persistent output of Alfven waves and particles exceeds its
spindown luminosity. We suggest that SGRs experience some episodes of relative
inactivity, with diminished spindown rates, and that such inactive magnetars
are observed as Anomalous X-ray Pulsars (AXPs). The rapid reappearence of
persistent X-ray emission following August 27 flare gives evidence against
accretion-powered models.Comment: 24 pages, no figure
Discovery of Pulsed X-ray Emission from the SMC Transient RX J0117.6-7330
We report on the detection of pulsed, broad-band, X-ray emission from the
transient source RX J0117.6-7330. The pulse period of 22 seconds is detected by
the ROSAT/PSPC instrument in a 1992 Sep 30 - Oct 2 observation and by the
CGRO/BATSE instrument during the same epoch. Hard X-ray pulsations are
detectable by BATSE for approximately 100 days surrounding the ROSAT
observation (1992 Aug 28 - Dec 8). The total directly measured X-ray luminosity
during the ROSAT observation is 1.0E38 (d/60 kpc)^2 ergs s-1. The pulse
frequency increases rapidly during the outburst, with a peak spin-up rate of
1.2E-10 Hz s-1 and a total frequency change 1.8%. The pulsed percentage is
11.3% from 0.1-2.5 keV, increasing to at least 78% in the 20-70 keV band. These
results establish RX J0117.6-7330 as a transient Be binary system.Comment: 17 pages, Latex, aasms, accepted for publication in ApJ Letter
XTE J1946+274 = GRO J1944+26: An Enigmatic Be/X-ray Binary
XTE J1946+274 = GRO J1944+26 is a 15.8 s Be/X-ray pulsar discovered
simultaneously in 1998 September with the Burst and Transient Source Experiment
(BATSE) on the Compton Gamma Ray Observatory (CGRO) and the All-Sky Monitor
(ASM) on the Rossi X-ray Timing Explorer (RXTE). Here we present new results
from BATSE and {\em RXTE} including a pulse timing analysis, spectral analysis,
and evidence for an accretion disk. Our pulse timing analysis yielded an
orbital period of 169.2 days, a moderate eccentricity of 0.33, and implied a
mass function of 9.7 M_sun. We observed evidence for an accretion disk, a
correlation between measured spin-up rate and flux, which was fitted to obtain
a distance estimate of 9.5 +/- 2.9 kpc. XTE J1946+274 remained active from 1998
September - 2001 July, undergoing 13 outbursts that were not locked in orbital
phase. Comparing RXTE PCA observations from the initial bright outburst in 1998
and the last pair of outbursts in 2001, we found energy and intensity dependent
pulse profile variations in both outbursts and hardening spectra with
increasing intensity during the fainter 2001 outbursts. In 2001 July, optical
Halpha observations indicate a density perturbation appeared in the Be disk as
the X-ray outbursts ceased. We propose that the equatorial plane of the Be star
is inclined with respect to the orbital plane in this system and that this
inclination may be a factor in the unusual outburst behavior of the system.Comment: 18 pages, 15 figures, To appear in ApJ v584, Feb 20, 2003 issu
Long-term source monitoring with BATSE
The uncollimated Burst and Transient Source Experiment (BATSE) large area detectors (LADs) are well suited to nearly continuous monitoring of the stronger hard x-ray sources, and time series analysis for pulsars. An overview of the analysis techniques presently being applied to the data are discussed, including representative observations of the Crab Nebula, Crab pulsar, and summaries of the sources detected to data. Results of a search for variability in the Crab Pulsar pulse profile are presented
Occultation analysis of BATSE data: Operational aspects
The Burst and Transient Source Experiment (BATSE) large area detectors are being used to monitor hard x-ray/gamma ray sources on a daily basis for evidence of transient behavior. Flux measurements are performed using a simple earth occultation technique. Daily searches are also being performed to detect occultation steps of sources which are not being routinely monitored. Topics concerning the operational aspects of the occultation measurements are presented. Preliminary spectral results are also presented for several of the brighter sources
The pre-outburst flare of the A 0535+26 August/September 2005 outburst
We study the spectral and temporal behavior of the High Mass X-ray Binary A
0535+26 during a `pre-outburst flare' which took place ~5 d before the peak of
a normal (type I) outburst in August/September 2005. We compare the studied
behavior with that observed during the outburst. We analyse RXTE observations
that monitored A 0535+26 during the outburst. We complete spectral and timing
analyses of the data. We study the evolution of the pulse period, present
energy-dependent pulse profiles both at the initial pre-outburst flare and
close to outburst maximum, and measure how the cyclotron resonance-scattering
feature (hereafter CRSF) evolves. We present three main results: a constant
period P=103.3960(5)s is measured until periastron passage, followed by a
spin-up with a decreasing period derivative of Pdot=(-1.69+/-0.04)x10^(-8)s/s
at MJD 53618, and P remains constant again at the end of the main outburst. The
spin-up provides evidence for the existence of an accretion disk during the
normal outburst. We measure a CRSF energy of Ecyc~50kev during the pre-outburst
flare, and Ecyc~46kev during the main outburst. The pulse shape, which varies
significantly during both pre-outburst flare and main outburst, evolves
strongly with photon energy.Comment: 4 pages, 4 figures, accepted for publication in A&A Letters. To be
published in parallel to Postnov et al. 200
INTEGRAL observations of the Small Magellanic Cloud
The first INTEGRAL observations of the Small Magellanic Cloud (carried out in
2003) are reported in which two sources are clearly detected. The first source,
SMC X-1, shows a hard X-ray eclipse and measurements of its pulse period
indicate a continuation of the long-term spin-up now covering ~30 years. The
second source is likely to be a high mass X-ray binary, and shows a potential
periodicity of 6.8s in the IBIS lightcurve. An exact X-ray or optical
counterpart cannot be designated, but a number of proposed counterparts are
discussed. One of these possible counterparts shows a strong coherent optical
modulation at ~2.7d, which, together with the measured hard X-ray pulse period,
would lead to this INTEGRAL source being classified as the fourth known high
mass Roche lobe overflow system.Comment: Accepted for publication in MNRA
Timing Noise in SGR 1806-20
We have phase connected a sequence of RXTE PCA observations of SGR 1806-20
covering 178 days. We find a simple secular spin-down model does not adequately
fit the data. The period derivative varies gradually during the observations
between 8.1 and 11.7 * 10^-11 s/s (at its highest, ~40% larger than the long
term trend), while the average burst rate as seen with BATSE drops throughout
the time interval. The phase residuals give no compelling evidence for
periodicity, but more closely resemble timing noise as seen in radio pulsars.
The magnitude of the timing noise, however, is large relative to the noise
level typically found in radio pulsars. Combining these results with the noise
levels measured for some AXPs, we find all magnetar candidates have \Delta_8
values larger than those expected from a simple extrapolation of the
correlation found in radio pulsars. We find that the timing noise in SGR
1806-20 is greater than or equal to the levels found in some accreting systems
(e.g., Vela X-1, 4U 1538-52 and 4U 1626-67), but the spin-down of SGR 1806-20
has thus far maintained coherence over 6 years. Alternatively, an orbital model
with a period P_orb = 733 days provides a statistically acceptable fit to the
data. If the phase residuals are created by Doppler shifts from a
gravitationally bound companion, then the allowed parameter space for the mass
function (small) and orbital separation (large) rule out the possibility of
accretion from the companion sufficient to power the persistent emission from
the SGR.Comment: 11 pages, accepted for publication in ApJ Letter
On the nature of the ultraluminous X-ray transient in Cen~A (NGC 5128)
We combine 9 ROSAT, 9 Chandra, and 2 XMM-Newton observations of the Cen~A
galaxy to obtain the X-ray light curve of 1RXH J132519.8-430312 (=CXOU
J132519.9430317) spanning 1990 to 2003. The source reached a peak 0.1-2.4
keV flux F_X>10^{-12} ergs cm^{-2} s^{-1} during a 10~day span in 1995 July.
The inferred peak isotropic luminosity of the source therefore exceeded 3
10^{39} ergs s^{-1}, which places the source in the class of ultra-luminous
X-ray sources. Coherent pulsations at 13.264 Hz are detected during a second
bright episode (F_X >3 times 10^{-13} ergs cm^{-2} s^{-1}) in 1999 December.
The source is detected and varies significantly within three additional
observations but is below the detection threshold in 7 observations. The X-ray
spectrum in 1999 December is best described as a cut-off power law or a
disk-blackbody (multi-colored disk). We also detect an optical source, m_F555W
~ 24.1 mag, within the Chandra error circle of 1RXH J132519.8-430312 in HST
images taken 195~days before the nearest X-ray observation. The optical
brightness of this source is consistent with a late O or early B star at the
distance of Cen A. If the optical source is the counterpart, then the X-ray and
optical behavior of 1RXH J132519.8-430312 are similar to the transient Be/X-ray
pulsar A 0538-66.Comment: 7 pages, 8 figures. ApJ (accepted
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