435 research outputs found
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
The Dynamic Behavior of Soft Gamma Repeaters
Soft Gamma Repeaters (SGRs) undergo changes in their pulse properties and
persistent emission during episodes of intense burst activity. Both SGR 1900+14
and SGR 1806-20 have shown significant changes in their spin-down rates during
the last several years, yet the bulk of this variability is not correlated with
burst activity. SGR 1900+14 has undergone large changes in flux and a dramatic
pulse profile change following burst activity in 1998. The flux level of SGR
162741 has been decreasing since its only recorded burst activity. Here, we
review the global properties of SGRs as well as the observed dynamics of the
pulsed and persistent emission properties of SGR 1900+14, SGR 1806-20 and SGR
1627-41 during and following burst active episodes and discuss what
implications these results have for the burst emission mechanism, the magnetic
field dynamics of magnetars, the nature of the torque variability, and SGRs in
general.Comment: Invited review to appear in "High Energy Studies of Supernova
Remnants and Neutron Stars" (COSPAR 2002). 12 pages, 7 figure
Burst Tails from SGR J1550-5418 Observed with Rossi X-ray Timing Explorer
We present the results of our extensive search using the Bayesian block
method for long tails following short bursts from a magnetar, SGR J1550-5418,
over all RXTE observations of the source. We identified four bursts with
extended tails, most of which occurred during its 2009 burst active episode.
The durations of tails range between ~13 s and over 3 ks, which are much longer
than the typical duration of bursts. We performed detailed spectral and
temporal analysis of the burst tails. We find that the spectra of three tails
show a thermal nature with a trend of cooling throughout the tail. We compare
the results of our investigations with the properties of four other extended
tails detected from SGR 1900+14 and SGR 1806-20 and suggest a scenario for the
origin of the tail in the framework of the magnetar model.Comment: 10 pages, 7 figures, 4 tables, accepted for publication in Ap
The Effects of Burst Activity on Soft Gamma Repeater Pulse Properties and Persistent Emission
Soft Gamma Repeaters (SGRs) undergo changes in their pulse properties and
persistent emission during episodes of intense burst activity. SGR 1900+14 has
undergone large flux increases following recent burst activity. Both SGR
1900+14 and SGR 1806-20 have shown significant changes in their pulse profile
and spin-down rates during the last several years. The pulse profile changes
are linked with the burst activity whereas the torque variations are not
directly correlated with the bursts. Here, we review the observed dynamics of
the pulsed and persistent emission of SGR 1900+14 and SGR 1806-20 during burst
active episodes and discuss what implications these results have for the burst
emission mechanism, the magnetic field dynamics of magnetars, the nature of the
torque variability, and SGRs in general.Comment: 9 pages, Woods Hole 2001 GRB and SGR Conferenc
Electrodynamics of Magnetars: Implications for the Persistent X-ray Emission and Spindown of the Soft Gamma Repeaters and Anomalous X-ray Pulsars
(ABBREVIATED) We consider the structure of neutron star magnetospheres
threaded by large-scale electrical currents, and the effect of resonant Compton
scattering by the charge carriers (both electrons and ions) on the emergent
X-ray spectra and pulse profiles. In the magnetar model for the SGRs and AXPs,
these currents are maintained by magnetic stresses acting deep inside the star.
We construct self-similar, force-free equilibria of the current-carrying
magnetosphere with a power-law dependence of magnetic field on radius, B ~
r^(-2-p), and show that a large-scale twist softens the radial dependence to p
< 1. The spindown torque acting on the star is thereby increased in comparison
with a vacuum dipole. We comment on the strength of the surface magnetic field
in the SGR and AXP sources, and the implications of this model for the narrow
measured distribution of spin periods. A magnetosphere with a strong twist,
B_\phi/B_\theta = O(1) at the equator, has an optical depth ~ 1 to resonant
cyclotron scattering, independent of frequency (radius), surface magnetic field
strength, or charge/mass ratio of the scattering charge. When electrons and
ions supply the current, the stellar surface is also heated by the impacting
charges at a rate comparable to the observed X-ray output of the SGR and AXP
sources, if B_{dipole} ~ 10^{14} G. Redistribution of the emerging X-ray flux
at the ion and electron cyclotron resonances will significantly modify the
emerging pulse profile and, through the Doppler effect, generate a non-thermal
tail to the X-ray spectrum. The sudden change in the pulse profile of SGR
1900+14 after the 27 August 1998 giant flare is related to an enhanced optical
depth to electron cyclotron scattering, resulting from a sudden twist imparted
to the external magnetic field.Comment: 31 January 2002, minor revisions, new section 5.4.
Magnetars: the physics behind observations
Magnetars are the strongest magnets in the present universe and the
combination of extreme magnetic field, gravity and density makes them unique
laboratories to probe current physical theories (from quantum electrodynamics
to general relativity) in the strong field limit. Magnetars are observed as
peculiar, burst--active X-ray pulsars, the Anomalous X-ray Pulsars (AXPs) and
the Soft Gamma Repeaters (SGRs); the latter emitted also three "giant flares,"
extremely powerful events during which luminosities can reach up to 10^47 erg/s
for about one second. The last five years have witnessed an explosion in
magnetar research which has led, among other things, to the discovery of
transient, or "outbursting," and "low-field" magnetars. Substantial progress
has been made also on the theoretical side. Quite detailed models for
explaining the magnetars' persistent X-ray emission, the properties of the
bursts, the flux evolution in transient sources have been developed and
confronted with observations. New insight on neutron star asteroseismology has
been gained through improved models of magnetar oscillations. The long-debated
issue of magnetic field decay in neutron stars has been addressed, and its
importance recognized in relation to the evolution of magnetars and to the
links among magnetars and other families of isolated neutron stars. The aim of
this paper is to present a comprehensive overview in which the observational
results are discussed in the light of the most up-to-date theoretical models
and their implications. This addresses not only the particular case of magnetar
sources, but the more fundamental issue of how physics in strong magnetic
fields can be constrained by the observations of these unique sources.Comment: 81 pages, 24 figures, This is an author-created, un-copyedited
version of an article submitted to Reports on Progress in Physic
Variation of Spectral and Timing Properties in the Extended Burst Tails from the Magnetar 4U 0142+61
Extended emission episodes with intensity above the pre-burst level are
observed following magnetar bursts from a number of soft gamma repeaters (SGRs)
and anomalous X-ray pulsars (AXPs). Such extended tail emission were observed
subsequent to two events detected from AXP 4U 0142+61. We investigated in
detail the evolution of spectral and temporal properties during these two tail
segments using RXTE/PCA observations, and report distinct variations both in
the spectral and temporal behavior throughout the tails. In particular, sudden
enhancement of pulsation amplitude in conjunction with bursts, and smooth
decline of X-ray emission (cooling) during the tail were observed in both
cases. We suggest that an inefficiently radiating trapped fireball formed
during the burst, which can heat up the stellar surface, is able to explain the
tail properties and its energetics. We also present the episodic detection of
absorption and emission features during tails. One possible mechanism that has
been proposed to give rise to such spectral lines is the proton/ion cyclotron
resonance process which has been suggested to offer a valuable tool in probing
the complex magnetic field of magnetars.Comment: 14 pages, 9 figures, 3 tables, accepted for publication in Ap
An exceptionally bright flare from SGR1806-20 and the origins of short-duration gamma-ray bursts
Soft-gamma-ray repeaters (SGRs) are galactic X-ray stars that emit numerous
short-duration (about 0.1 s) bursts of hard X-rays during sporadic active
periods. They are thought to be magnetars: strongly magnetized neutron stars
with emissions powered by the dissipation of magnetic energy. Here we report
the detection of a long (380 s) giant flare from SGR 1806-20, which was much
more luminous than any previous transient event observed in our Galaxy. (In the
first 0.2 s, the flare released as much energy as the Sun radiates in a quarter
of a million years.) Its power can be explained by a catastrophic instability
involving global crust failure and magnetic reconnection on a magnetar, with
possible large-scale untwisting of magnetic field lines outside the star. From
a great distance this event would appear to be a short-duration, hard-spectrum
cosmic gamma-ray burst. At least a significant fraction of the mysterious
short-duration gamma-ray bursts therefore may come from extragalactic
magnetars.Comment: 21 pages, 5 figures. Published in Natur
The effect of X-ray dust-scattering on a bright burst from the magnetar 1E 1547.0-5408
A bright burst, followed by an X-ray tail lasting ~10 ks, was detected during
an XMM-Newton observation of the magnetar 1E 1547.0-5408 carried out on 2009
February 3. The burst, also observed by SWIFT/BAT, had a spectrum well fit by
the sum of two blackbodies with temperatures of ~4 keV and 10 keV and a fluence
in the 0.3-150 keV energy range of ~1e-5 erg/cm2. The X-ray tail had a fluence
of ~4e-8 erg/cm2. Thanks to the knowledge of the distances and relative optical
depths of three dust clouds between us and 1E 1547.0-5408, we show that most of
the X-rays in the tail can be explained by dust scattering of the burst
emission, except for the first ~20-30 s. We point out that other X-ray tails
observed after strong magnetar bursts may contain a non-negligible contribution
due to dust scattering.Comment: 8 pages, 2 tables and 10 figures; accepted to publication in MNRA
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