Chryssa Kouveliotou

Evidence for a canonical GRB afterglow light curve in the Swift/XRT data

We present new observations of the early X-ray afterglows of the first 27 gamma-ray bursts (GRBs) detected with the Swift X-ray Telescope (XRT). The early X-ray afterglows show a canonical behavior, where the light curve broadly consists of three distinct power law segments. These power law segments are separated by two corresponding break times. On top of this canonical behavior of the early X-ray light curve, many events have superimposed X-ray flares, which are most likely caused by internal shocks due to long lasting sporadx activity of the central engine, up to several hours after the GRB. We find that the initial steep decay is consistent with it being the tail of the prompt emission: from photons that are radiated at large angles relative to our line of sight. The first break in the light curve takes place when the forward shock emission becomes dominant, with the intermediate shallow flux decay likely caused by the continuous energy injection into the external shock. When this energy injection stops, a second break is then observed in the light curve. This energy injection increases the energy of the afterglow shock by at least a factor of f greater than or approx. equal to 4, and augments the already severe requirements for the efficiency of the prompt gamma-ray emission.

The unusual γ-ray burst GRB 101225A from a helium star/neutron star merger at redshift 0.33

C. C. Thöne1,2,∗, A. de Ugarte Postigo, C. L. Fryer, K. L. Page, J. Gorosabel, M. A. Aloy, D. A. Perley, C. Kouveliotou, H. T. Janka, P. Mimica, J. L. Racusin, H. Krimm, J. Cummings, S. R. Oates, S. T. Holland, M. H. Siegel, M. De Pasquale, E. Sonbas, M. Im, W.-K. Park, D. A. Kann, S. Guziy, L. Hernández Garcı́a, A. Llorente, K. Bundy, C. Choi, H. Jeong, H. Korhonen, P. Kubanek, J. Lim, A. Moskvitin, T. Muñoz-Darias, S. Pak, I. Parrish 1 IAA CSIC, Glorieta de la Astronomı́a s/n, 18008 Granada, Spain 2 Niels Bohr International Academy, Niels Bohr Institute, Blegdamsvej 17, 2100 Copenhagen, Denmark 3 Dark Cosmology Centre, Niels Bohr Institute, Univ. of Copenhagen,Long γ-ray bursts (GRBs) are the most dramatic examples of massive stellar deaths, often associated with supernovae. They release ultra-relativistic jets, which produce non-thermal emission through synchrotron radiation as they interact with the surrounding medium. Here we report observations of the unusual GRB 101225A. Its γ-ray emission was exceptionally long-lived and was followed by a bright X-ray transient with a hot thermal component and an unusual optical counterpart. During the first 10 days, the optical emission evolved as an expanding, cooling black body, after which an additional component, consistent with a faint supernova, emerged. We estimate its redshift to be z = 0.33 by fitting the spectral-energy distribution and light curve of the optical emission with a GRB-supernova template. Deep optical observations may have revealed a faint, unresolved host galaxy. Our proposed progenitor is a merger of a helium star with a neutron star that underwent a common envelope phase, expelling its hydrogen envelope. The resulting explosion created a GRB-like jet which became thermalized by interacting with the dense, previously ejected material, thus creating the observed black body, until finally the emission from the supernova dominated. An alternative explanation is a minor body falling onto a neutron star in the Galaxy.

Evolution of the polarization of the optical afterglow of the γ-ray burst GRB030329

The association of a supernova with GRB030329 strongly supports the ‘collapsar’ model of γ-ray bursts, where a relativistic jet forms after the progenitor star collapses. Such jets cannot be spatially resolved because γ-ray bursts lie at cosmological distances; their existence is instead inferred from ‘breaks’ in the light curves of the afterglows, and from the theoretical desire to reduce the estimated total energy of the burst by proposing that most of it comes out in narrow beams. Temporal evolution of the polarization of the afterglows may provide independent evidence for the jet structure of the relativistic outflow. Small-level polarization (∼1–3 per cent) has been reported for a few bursts, but its temporal evolution has yet to be established. Here we report polarimetric observations of the afterglow of GRB030329. We establish the polarization light curve, detect sustained polarization at the per cent level, and find significant variability. The data imply that the afterglow magnetic field has a small coherence length and is mostly random, probably generated by turbulence, in contrast with the picture arising from the high polarization detected in the prompt γ-rays from GRB021206 (ref. 18).

GRB 030329: 3 years of radio afterglow monitoring

Radio observations of gamma-ray burst (GRB) afterglows are essential for our understanding of the physics of relativistic blast waves, as they enable us to follow the evolution of GRB explosions much longer than the afterglows in any other wave band. We have performed a 3-year monitoring campaign of GRB 030329 with the Westerbork Synthesis Radio Telescopes and the Giant Metrewave Radio Telescope. Our observations, combined with observations at other wavelengths, have allowed us to determine the GRB blast wave physical parameters, such as the total burst energy and the ambient medium density, as well as to investigate the jet nature of the relativistic outflow. Further, by modelling the late-time radio light curve of GRB 030329, we predict that the Low-Frequency Array (30–240 MHz) will be able to observe afterglows of similar GRBs, and constrain the physics of the blast wave during its non-relativistic phase.

BATSE observations of the second outburst of GRO J1744-28

On 2 December 1996, exactly one year after its discovery, hard X-ray bursts were again detected with BATSE from the Bursting Pulsar, GRO J1744-28. Similar to the first outburst, a flurry of bursts were observed on the first day, the rate of which dropped dramatically to a near constant level for the next several months. From 2 December 1996 to 10 April 1997, BATSE observed over 2400 bursts. After April 10th, burst intensities dropped below the instrument’s threshold. We present preliminary results of spectral analysis performed on 1663 bursts together with a comparison of these results to the burst properties of the first outburst.

RXTE observations of GRB afterglows

Abstract Variable X-ray sources that appear to be the afterglows of the strong gamma-ray bursts GRB 970616 and GRB 970828 have been discovered with the RXTE PCA. First seen less than 4 hours after the burst, the flux from the sources decreased with time. Although near the sensitivity limit of the PCA, the sources are the brightest afterglows yet seen in X-rays. Similar observations of two other bursts did not detect any afterglows. These results are part of a continuing collaboration between RXTE, BATSE, and IPN scientists to rapidly detect X-ray afterglows of bright gamma-ray bursts.

GRO J1744−28: the Bursting Pulsar

During more than 25 years of studies of X-ray binaries several dozens of X-ray pulsars and X-ray bursters were discovered; not a single one of these showed both pulsations and bursts. Here we describe the properties of the Bursting Pulsar, a unique X-ray source, discovered in December 1995 with the Burst And Transient Source Experiment (BATSE), which seems to be the first to defy the dictum ‘pulsars don’t burst, and bursters don’t pulse.’

All-Sky Monitoring of Variable Sources with Fermi GBM

Using the Gamma ray Burst Monitor (GBM) on Fermi, we monitor the transient hard X-ray/soft gamma ray sky. The twelve GBM NaI detectors span 8 keV to 1 MeV, while the two BGO detectors span 150 keV to 40 MeV. We use the Earth occultation technique to monitor a number of sources, including X-ray binaries, AGN, and solar flaring activity. Our monitoring reveals predictable and unpredictable phenomena such as transient outbursts and state changes. With GBM we also track the pulsed flux and spin frequency of accretion powered pulsars using epoch-folding techniques. Searches for quasi-periodic oscillations and X-ray bursts are also possible with GBM all-sky monitoring. Highlights from the Earth Occultation and Pulsar projects will be presented including our recent surprising discovery of variations in the total flux from the Crab. Inclusion of an all-sky monitor is crucial for a successful future X-ray timing mission.

Constraint on the early expansion of XRF080109, related to supernova SN2008D

Richard Strom Astron, Dwingeloo, Netherlands University of Amsterdam, The Netherlands E-mail: strom@astron.nl We present results from two epochs global VLBI observations of XRF 080109/SN 2008D. With these sensitive global observations it is possible to give reasonable estimates to the speed of the radio ejecta. These early epoch data are crucial for the understanding of ejecta from type Ib/c SNe. The 9th European VLBI Network Symposium on The role of VLBI in the Golden Age for Radio Astronomy and EVN Users Meeting September 23-26, 2008 Bologna, Italy

Multiwavelength observations and speculations on SGR 1900+14

We present optical and IR imaging and photometry of the field of SGR 1900+14 and identify a possible stellar binary counterpart of this source. Two IR‐bright stars were discovered at a location consistent with the ROSAT source discussed in the companion paper by Hurley et al. Spectroscopy of the two counterpart candidates suggests that they are evolved stars of spectral class M5. The relationship of these stars to the X‐ray source and the nearby IPN position is not clear at present. However, if these stars are indeed associated with the SGR, the association with a fast (∼1,000 km s−1) neutron star ejected in the event that created the nearby supernova remnant G42.8+0.6 is not feasible. If one retains the fast pulsar hypothesis for SGRs, the IR stars must be considered a coincidence, although this appears to be unlikely. We discuss a scenario in which the X‐ray emission is due to sporadic energy deposition into a nebula by particle bursts from the SGR. The data suggest that SGR 1900+14 is located at a heli...