Extended Emission from Short Gamma-Ray Bursts Detected with SPI-ACS/INTEGRAL

The short duration (T90 < 2 s) gamma-ray bursts (GRBs) detected in the SPI-ACS experiment onboard the INTEGRAL observatory are investigated. Averaged light curves have been constructed for various groups of events, including short GRBs and unidentified short events. Extended emission has been found in the averaged light curves of both short GRBs and unidentified short events. It is shown that the fraction of the short GRBs in the total number of SPI-ACS GRBs can range from 30 to 45%, which is considerably larger than has been thought previously.Comment: 27 pages, 10 figure

Observations of giant outbursts from Cygnus X-1

We present interplanetary network localization, spectral, and time history information for 7 episodes of exceptionally intense gamma-ray emission from Cygnus X-1. The outbursts occurred between 1995 and 2003, with durations up to \~28000 seconds. The observed 15 - 300 keV peak fluxes and fluences reached 3E-7 erg /cm2 s, and 8E-4 erg / cm2 respectively. By combining the triangulations of these outbursts we derive an ~1700 square arcminute (3 sigma) error ellipse which contains Cygnus X-1 and no other known high energy sources. The outbursts reported here occurred both when Cyg X-1 was in the hard state as well as in the soft one, and at various orbital phases. The spectral data indicate that these outbursts display the same parameters as those of the underlying hard and soft states, suggesting that they represent another manifestation of these states.Comment: 27 pages, 9 figures. Revised version. Accepted for publication in the Astrophysical Journal, tentatively scheduled for October 20, 2003 Part

Reactivation and Precise IPN Localization of the Soft Gamma Repeater SGR1900+14

In 1998 May, the soft gamma repeater SGR1900+14 emerged from several years of quiescence and emitted a series of intense bursts, one with a time history unlike any previously observed from this source. Triangulation using Ulysses, BATSE, and KONUS data give a 1.6 square arcminute error box near the galactic supernova remnant G42.8+0.6. This error box contains a quiescent soft X-ray source which is probably a neutron star associated with the soft repeater.Comment: Accepted for publication in the Astrophysical Journal Letter

The Current Performance of the Third Interplanetary Network

The 3rd Interplanetary Network (IPN) has been operating since April 2001 with two distant spacecraft, Ulysses and Mars Odyssey, and numerous near-Earth spacecraft, such as BeppoSAX, Wind, and HETE-II. Mars Odyssey is presently in orbit about Mars, and the network has detected approximately 30 cosmic, SGR, and solar bursts. We discuss the results obtained to date and use them to predict the future performance of the network.Comment: To appear in the proceedings of the conference on Gamma-Ray Bursts and Afterglow Astronomy 2001: A Workshop Celebrating the First Year of the HETE Mission, to be published by AI

Study of the Fast X-Ray Transient XTE J1901+014 Based on INTEGRAL, RXTE and ROSAT Data

The source XTE J1901+014 discovered by the RXTE observatory during an intense outburst of hard radiation and classified as a fast X-ray transient is studied. The source's spectral characteristics in the quiescent state have been investigated for the first time both in the soft X-ray energy range (0.6-20 keV) based on ROSAT and RXTE data and in the hard energy range (>20 keV) based on INTEGRAL data. A timing analysis of the source's properties has revealed weak nonperiodic bursts of activity on time scales of several tens of seconds and two intense ($\sim$0.5-1 Crab) outbursts more than several hundred seconds in duration. Certain assumptions about the nature of the object under study are made.Comment: 19 pages, 7 figure

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.

Multi-Wavelength Studies of the Optically Dark Gamma-Ray Burst 001025A

We identify the fading X-ray afterglow of GRB 001025A from XMM-Newton observations obtained 1.9-2.3 days, 2 years, and 2.5 years after the burst. The non-detection of an optical counterpart to an upper limit of R=25.5, 1.20 days after the burst, makes GRB 001025A a ``dark'' burst. Based on the X-ray afterglow spectral properties of GRB 001025A, we argue that some bursts appear optically dark because their afterglow is faint and their cooling frequency is close to the X-ray band. This interpretation is applicable to several of the few other dark bursts where the X-ray spectral index has been measured. The X-ray afterglow flux of GRB 001025A is an order of magnitude lower than for typical long-duration gamma-ray bursts. The spectrum of the X-ray afterglow can be fitted with an absorbed synchrotron emission model, an absorbed thermal plasma model, or a combination thereof. For the latter, an extrapolation to optical wavelengths can be reconciled with the R-band upper limit on the afterglow, without invoking any optical circumburst absorption, provided the cooling frequency is close to the X-ray band. Alternatively, if the X-ray afterglow is due to synchrotron emission only, seven magnitudes of extinction in the observed R-band is required to meet the R-band upper limit, making GRB 001025A much more obscured than bursts with detected optical afterglows. Based on the column density of X-ray absorbing circumburst matter, an SMC gas-to-dust ratio is insufficient to produce this amount of extinction. The X-ray tail of the prompt emission enters a steep temporal decay excluding that the tail of the prompt emission is the onset of the afterglow (abridged).Comment: 32 pages, 8 figures, ApJ in pres

The Collimation and Energetics of the Brightest Swift Gamma-Ray Bursts

Long-duration gamma-ray bursts (GRBs) are widely believed to be highly-collimated explosions (opening angle theta ~ 1-10 deg). As a result of this beaming factor, the true energy release from a GRB is usually several orders of magnitude smaller than the observed isotropic value. Measuring this opening angle, typically inferred from an achromatic steepening in the afterglow light curve (a "jet" break), has proven exceedingly difficult in the Swift era. Here we undertake a study of five of the brightest (in terms of the isotropic prompt gamma-ray energy release, E(gamma, iso)) GRBs in the Swift era to search for jet breaks and hence constrain the collimation-corrected energy release. We present multi-wavelength (radio through X-ray) observations of GRBs 050820A, 060418, and 080319B, and construct afterglow models to extract the opening angle and beaming-corrected energy release for all three events. Together with results from previous analyses of GRBs 050904 and 070125, we find evidence for an achromatic jet break in all five events, strongly supporting the canonical picture of GRBs as collimated explosions. The most natural explanation for the lack of observed jet breaks from most Swift GRBs is therefore selection effects. However, the opening angles for the events in our sample are larger than would be expected if all GRBs had a canonical energy release of ~ 10e51 erg. The total energy release we measure for those "hyper-energetic" (E(total) >~ 10e52 erg) events in our sample is large enough to start challenging models with a magnetar as the compact central remnant.Comment: Submitted to ApJ, comments welcom

Constraining GRB Emission Physics with Extensive Early-Time, Multiband Follow-up

Understanding the origin and diversity of emission processes responsible for Gamma-ray Bursts (GRBs) remains a pressing challenge. While prompt and contemporaneous panchromatic observations have the potential to test predictions of the internal-external shock model, extensive multiband imaging has been conducted for only a few GRBs. We present rich, early-time, multiband datasets for two \swift\ events, GRB 110205A and GRB 110213A. The former shows optical emission since the early stages of the prompt phase, followed by the steep rising in flux up to ~1000s after the burst ($t^{-\alpha}$ with $\alpha=-6.13 \pm 0.75$). We discuss this feature in the context of the reverse-shock scenario and interpret the following single power-law decay as being forward-shock dominated. Polarization measurements, obtained with the RINGO2 instrument mounted on the Liverpool Telescope, also provide hints on the nature of the emitting ejecta. The latter event, instead, displays a very peculiar optical to near-infrared lightcurve, with two achromatic peaks. In this case, while the first peak is probably due to the onset of the afterglow, we interpret the second peak to be produced by newly injected material, signifying a late-time activity of the central engine.Comment: 48 pages,11 figures, 24 tables. Accepted to The Astrophysical Journa