10 research outputs found
X-Ray and Infrared Enhancement of Anomalous X-ray Pulsar 1E 2259+58
The long term (~1.5 years) X-ray enhancement and the accompanying infrared
enhancement light curves of the anomalous X-ray pulsar 1E 2259+58 following the
major bursting epoch can be accounted for by the relaxation of a fall back disk
that has been pushed back by a gamma-ray flare. The required burst energy
estimated from the results of our model fits is low enough for such a burst to
have remained below the detection limits. We find that an irradiated disk model
with a low irradiation efficiency is in good agreement with both X-ray and
infrared data. Non-irradiated disk models also give a good fit to the X-ray
light curve, but are not consistent with the infrared data for the first week
of the enhancement.Comment: 17 pages, 3 figures, accepted for publication in Ap
An Extended Burst Tail from SGR 1900+14 with a Thermal X-ray Spectrum
The Soft Gamma Repeater, SGR 1900+14, entered a new phase of activity in
April 2001 initiated by the intermediate flare recorded on April 18. Ten days
following this flare, we discovered an abrupt increase in the source flux
between consecutive RXTE orbits. This X-ray flux excess decayed over the next
several minutes and was subsequently linked to a high fluence burst from SGR
1900+14 recorded by other spacecraft (Ulysses and KONUS) while the SGR was
Earth-occulted for RXTE. We present here spectral and temporal analysis of both
the burst of 28 April and the long X-ray tail following it. We find strong
evidence of an exclusively thermal X-ray tail in this event and bring this
evidence to bear on other bursts and flares from SGR 1900+14 which have shown
extended X-ray excesses (e.g. 1998 August 29). We include in this comparison a
discussion of the physical origins of SGR bursts and extended X-ray tails.Comment: 27 pages, 13 figures, ApJ submissio
Large Torque Variations in Two Soft Gamma Repeaters
We have monitored the pulse frequencies of the two soft gamma repeaters SGR
1806-20 and SGR 1900+14 through the beginning of year 2001 using primarily
Rossi X-ray Timing Explorer Proportional Counter Array observations. In both
sources, we observe large changes in the spin-down torque up to a factor of ~4,
which persist for several months. Using long baseline phase-connected timing
solutions as well as the overall frequency histories, we construct torque noise
power spectra for each SGR. The power spectrum of each source is very red
(power-law slope ~-3.5). The torque noise power levels are consistent with some
accreting systems on time scales of ~1 year, yet the full power spectrum is
much steeper in frequency than any known accreting source. To the best of our
knowledge, torque noise power spectra with a comparably steep frequency
dependence have only been seen in young, glitching radio pulsars (e.g. Vela).
The observed changes in spin-down rate do not correlate with burst activity,
therefore, the physical mechanisms behind each phenomenon are also likely
unrelated. Within the context of the magnetar model, seismic activity cannot
account for both the bursts and the long-term torque changes unless the
seismically active regions are decoupled from one another.Comment: 26 pages, 11 figures included, accepted for publication in ApJ,
analysis of torque noise power density spectra is revised from previous
version and minor text changes were mad
Evidence for a Sudden Magnetic Field Reconfiguration in SGR 1900+14
We report the detection of large flux changes in the persistent X-ray flux of
SGR 1900+14 during its burst active episode in 1998. Most notably, we find a
factor ~700 increase in the non-burst X-ray flux following the August 27th
flare, which decayed in time as a power-law. Our measurements indicate that the
pulse fraction remains constant throughout this decay. This suggests a global
flux enhancement as a consequence of the August 27th flare rather than
localized heating. While the persistent flux has since recovered to the
pre-outburst level, the pulse profile has not. The pulse shape changed to a
near sinusoidal profile within the tail of the August 27th flare (in
gamma-rays) and this effect has persisted for more than 1.5 years (in X-rays).
The results presented here suggest the magnetic field of the neutron star in
SGR 1900+14 was significantly altered (perhaps globally) during the giant flare
of August 27.Comment: 17 pages, accepted for publication in Ap
Temporal and Spectral Characteristics of Short Bursts from the Soft Gamma Repeaters 1806-20 and 1900+14
We study the temporal and coarse spectral properties of 268 bursts from SGR
1806-20 and 679 bursts from SGR 1900+14, all observed with the Rossi X-Ray
Timing Explorer/Proportional Counter Array. Hardness ratios and temporal
parameters, such as T_90 durations and tau_90 emission times are determined for
these bursts. We find a lognormal distribution of burst durations, ranging over
more than two orders of magnitude: T_90 ~ 10^-2 to >~ 1 s, with a peak at ~ 0.1
s. The burst light curves tend to be asymmetrical, with more than half of all
events showing rise times t_r < 0.3 T_90. We find that there exists a
correlation between the duration and fluence of bursts from both sources. We
also find a significant anti-correlation between hardness ratio and fluence for
SGR 1806-20 bursts and a marginal anti-correlation for SGR 1900+14 events.
Finally, we discuss possible physical implications of these results within the
framework of the magnetar model.Comment: 24 pages, 11 figures, accepted for publication in Ap
The 2001 April Burst Activation of SGR 1900+14: Pulse Properties and Torque
We report on observations of SGR 1900+14 made with the Rossi X-ray Timing
Explorer (RXTE) and BeppoSAX during the April 2001 burst activation of the
source. Using these data, we measure the spindown torque on the star and
confirm earlier findings that the torque and burst activity are not directly
correlated. We compare the X-ray pulse profile to the gamma-ray profile during
the April 18 intermediate flare and show that (i) their shapes are similar and
(ii) the gamma-ray profile aligns closely in phase with the X-ray pulsations.
The good phase alignment of the gamma-ray and X-ray profiles suggests that
there was no rapid spindown following this flare of the magnitude inferred for
the August 27 giant flare. We discuss how these observations further constrain
magnetic field reconfiguration models for the large flares of SGRs.Comment: 13 pages, 3 figures, accepted for publication in Ap
Chandra Observations of the Anomalous X-ray Pulsar 4U 0142+61
We present X-ray imaging, timing, and phase resolved spectroscopy of the
anomalous X-ray pulsar 4U 0142+61 using the Chandra X-ray Observatory. The
spectrum is well described by a power law plus blackbody model with power law
index = 3.35(2), kT_BB=0.458(3) keV, and N_H=0.91(2) x 10^{22} cm^{-2}$; we
find no significant evidence for spectral features (0.5-7.0 keV). Time resolved
X-ray spectroscopy shows evidence for evolution in phase in either index, or
KT_BB, or some combination thereof as a function of pulse phase. We derive a
precise X-ray position for the source and determine its spin period,
P=8.68866(30) s. We have detected emission beyond 4 arcsec from the central
source and extending beyond 100 arcsec, likely due to dust scattering in the
interstellar medium.Comment: 15 pages, accepted for publication in Ap