Long Term Radiative Behavior of SGR 1900+14

The prolific magnetar SGR 1900+14 showed two outbursts in the last decade and has been closely monitored in the X-rays to track the changes in its radiative properties. We use archival Chandra and XMM-Newton observations of SGR 1900+14 to construct a history of its spectrum and persistent X-ray flux spanning a period of about seven years. We show that the decline of its X-ray flux in these two outburst episodes follows the same trend. The flux begins to decline promptly and rapidly subsequent to the flares, then decreases gradually for about 600 days, at which point it resumes a more rapid decline. Utilizing the high quality spectral data in each epoch, we also study the spectral coevolution of the source with its flux. We find that neither the magnetic field strength nor the magnetospheric properties change over the period spanned by the observations, while the surface temperature as well as the inferred emitting area both decline with time following both outbursts. We also show that the source reached the same minimum flux level in its decline from these two subsequent outbursts, suggesting that this flux level may be its steady quiescent flux.Comment: Accepted for publication in the Ap

Characteristic Variability Time Scales of Long Gamma-Ray Bursts

We determined the characteristic variability time scales (\Delta t_p) of 410 bright and long GRBs, by locating the peaks of their Power Density Spectra, defined and calculated in the time domain. We found that the averaged variability time scale decreases with the peak flux. This is consistent with the time-dilation effect expected for the cosmological origin of GRBs. We also found that the occurrence distribution of the characteristic variability time scale shows bimodality, which might be interpreted as that the long GRB sample is composed of two sub-classes with different variability time scales. However, we found no difference for some other characteristics of these two sub-classes.Comment: 10 pages, 5 figures, corrected some typos and syntaxes, enlarged the label fonts in fig.3 and fig.

Identification of the infrared counterpart of SGR 1935+2154 with the Hubble Space Telescope

We present deep Hubble Space Telescope observations of a new magnetar source, the soft gamma-repeater SGR 1935+2154, discovered by Swift. We obtained three epochs of observations: while the source was active in March 2015, during a quiescent period in August 2015, and during a further active phase in May 2016. Close to the center of the X-ray error region identified by Chandra we find a faint (F140W(AB)=25.3) source, which fades by a factor of ~2 over the course of 5 months between the first two epochs of observations, before rebrightening during the second active period. If this source is indeed the counterpart to SGR 1935+2154 then it is amongst the faintest yet located for a magnetar. Our observations are spaced over 1.3 years and enable us to place limits on the source velocity of $\mu = (60 \pm 40)$ km s$^{-1}$ kpc$^{-1}$; observations on timescales of a decade can hence probe proper motion limits smaller than the velocities observed for the majority of pulsars. The comparison of the optical/IR and X-ray lightcurves of the source suggests that emission in the two regimes is associated but not directly correlated, offering support for a magnetospheric versus a fallback disc origin.Comment: 7 pages, 3 figures, accepted for publication in Ap

The Successful Synergy of Swift and Fermi/GBM in Magnetars

The magnetar rate of discovery has increased dramatically in the last decade. Five sources were discovered in the last three years alone as a result of the very efficient synergy among three X- and .gamma-ray instruments on NASA satellites: the Swift/Burst Alert Telescope (BAT), the Fermi/Gamma ray Burst Monitor (GBM), and the Rossi X-Ray Timing Explorer; RXTE/Proportional Counter Array (PCA). To date, there are approx. 25 magnetar candidates, of which two are (one each) in the Large and Small Magellanic Cloud and the rest reside on the Galactic plane of our Milky Way. I will discuss here the main properties of the Magnetar Population and the common projects that can be achieved with the synergy of Swift and GBM

Probes of Diffusive Shock Acceleration using Gamma-Ray Burst Prompt Emission

The principal paradigm for gamma-ray bursts (GRBs) suggests that the prompt transient gamma-ray signal arises from multiple shocks internal to the relativistic expansion. This paper explores how GRB prompt emission spectra can constrain electron (or ion) acceleration properties at the relativistic shocks that pertain to GRB models. The array of possible high-energy power-law indices in accelerated populations is highlighted, focusing on how spectra above 1 MeV can probe the field obliquity in GRB internal shocks, and the character of hydromagnetic turbulence in their environs. When encompassing the MeV-band spectral break, fits to BATSE/EGRET burst data indicate that the preponderance of electrons responsible for the prompt emission reside in an intrinsically non-thermal population. This differs markedly from typical populations generated in acceleration simulations; potential resolutions of this conflict such as the action of self-absorption are mentioned. Spectral modeling also suggests that the synchrotron mechanism is favored over synchrotron self-Compton scenarios due to the latter's typically broad curvature near the peak. Such diagnostics will be enhanced by the broadband spectral coverage of bursts by the Fermi Gamma-Ray Space Telescope; the GBM will provide key information on the lower energy portions of the non-thermal particle population, while the LAT will constrain the power-law regime of particle acceleration.Comment: 6 pages, 1 embedded figure, to appear in Proc. of the 6th Huntsville Gamma-Ray Burst Symposium, eds. C. A. Meegan, N. Gehrels, and C. Kouveliotou (AIP Conf. Proc., New York

Probing the very high redshift Universe with Gamma-ray Bursts: prospects for observations with future X-ray instruments

Gamma-Ray Bursts (GRBs) are the most violent explosions in the Universe. Long duration GRBs are associated with the collapse of massive stars, rivaling their host galaxies in luminosity. The discovery of the most distant spectroscopically confirmed object in the Universe, GRB090423, opened a new window on the high redshift Universe, making it possible to study the cosmic 're-ionization' epoch and the preceding dark ages, as well as the generation of the first stars (Population III) using GRBs. Obviously this enables a wealth of new studies using the near infrared (nIR) characteristics of GRB afterglows. Here we explore a different path, focusing on the next generation of X-ray missions with large area focusing telescopes and fast repointing capabilities. We found that X-ray data can complement nIR observations and for the brightest GRBs can provide an accurate and independent redshift determination. Metallicity studies can also be carried out profitably once the redshift is known. Finally we discuss observational signatures of GRBs arising from Population III stars in the X-ray band.Comment: Accepted for publication on MNRAS (1 figure, 3 tables

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 1627$-$41 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

Optical observational biases in the GRB redshift

The measured redshifts of gamma-ray bursts (GRBs), which were first detected by the Swift satellite, seem to be bigger on average than the redshifts of GRBs detected by other satellites. We analyzed the redshift distribution of GRBs triggered and observed by different satellites (Swift, HETE2, BeppoSax, Ulyssses). After considering the possible biases {significant difference was found at the p=95.70% level in the redshift distributions of GRBs measured by HETE and the Swift.Comment: 3 pages, 1 table, 2 figures, to appear in the proceedings of the Sixth Huntsville Gamma-Ray Burst Symposium, edited by C.A. Meegan, N. Gehrels, and C. Kouvelioto

Crashing Neutron Stars Can Make Gamma-Ray Burst Jets

No abstract availabl