LOFAR Observations of Swift J1644+57 and Implications for Short-Duration Transients

Aims: We have analyzed low frequency radio data of tidal disruption event (TDE) Swift J1644+57 to search for a counterpart. We consider how brief transient signals (on the order of seconds or minutes) originating from this location would appear in our data. We also consider how automatic radio frequency interference (RFI) flagging at radio telescope observatories might affect these and other transient observations in the future, particularly with brief transients of a few seconds duration. Methods: We observed the field in the low-frequency regime at 149 MHz with data obtained over several months with the Low Frequency Array (LOFAR). We also present simulations where a brief transient is injected into the data in order to see how it would appear in our measurement sets, and how it would be affected by RFI flagging. Finally, both based on simulation work and the weighted average of the observed background over the course of the individual observations, we present the possibility of brief radio transients in the data. Results: Our observations of Swift J1644+57 yielded no detection of the source and a peak flux density at this position of 24.7 $pm$ 8.9 mJy. Our upper limit on the transient rate of the snapshot surface density in this field at sensitivities < 0.5 Jy is $rho < 2.2 times10^{-2}$ deg$^{-2}$. We also conclude that we did not observe any brief transient signals originating specifically from the Swift J1644+57 source itself, and searches for such transients are severely limited by automatic RFI flagging algorithms which flag transients of less than 2 minutes duration. As such, careful consideration of RFI flagging techniques must occur when searching for transient signals.

Low frequency observations of the radio nebula produced by the giant flare from SGR 1806-20

The 2004 December 27 giant flare from SGR 1806-20 produced a radio nebula that was detectable for weeks. It was observed at a wide range of radio frequencies. We made a total of 19 WSRT observations. Most of these were performed quasi simultaneously at either two or three frequencies, starting 2005 January 4 and ending 2005 January 29. We reobserved the field in 2005 April/May, which facilitated an accurate subtraction of background sources. At 350 MHz, we find that the total intensity of the source is lower than expected from the GMRT 240 MHz and 610 MHz measurements and inconsistent with spectral indices published previously. Our 850 MHz flux densities, however, are consistent with earlier results. There is no compelling evidence for significant depolarization at any frequency. We do, however, find that polarization angles differ substantially from those at higher frequencies. Low frequency polarimetry and total intensity measurements provide a number of clues with regard to substructure in the radio nebula associated with SGR 1806-20. In general, for a more complete understanding of similar events, low frequency observations can provide new insights into the physics of the radio source.

Gamma-Ray Burst afterglow scaling coefficients for general density profile

Gamma-ray burst (GRB) afterglows are well described by synchrotron emission originating from the interaction between a relativistic blast wave and the external medium surrounding the GRB progenitor. We introduce a code to reconstruct spectra and light curves from arbitrary fluid configurations, making it especially suited to study the effects of fluid flows beyond those that can be described using analytical approximations. As a check and first application of our code we use it to fit the scaling coefficients of theoretical models of afterglow spectra. We extend earlier results of other authors to general circumburst density profiles. We rederive the physical parameters of GRB 970508 and compare with other authors

Robust photometric redshift determinations of gamma-ray burst afterglows at z > 2

Theory suggests that about 10% of Swift-detected gamma-ray bursts (GRBs) will originate at redshifts greater than 5 yet a number of high redshift candidates may be left unconfirmed due to the lack of measured redshifts. Here we introduce our code, GRBz, a method of simultaneous multi-parameter fitting of GRB afterglow optical and near infrared, spectral energy distributions. It allows for early determinations of the photometric redshift, spectral index and host extinction to be made. We assume that GRB afterglow spectra are well represented by a power-law decay and model the effects of absorption due to the Lyman forest and host extinction. We use a genetic algorithm-based routine to simultaneously fit the parameters of interest, and a Monte Carlo error analysis. We use GRBs of previously determined spectroscopic redshifts to prove our method, while also introducing new near infrared data of GRB 990510 which further constrains the value of the host extinction. Our method is effective in estimating the photometric redshift of GRBs, relatively unbiased by assumptions of the afterglow spectral index or the host galaxy extinction. Monte Carlo error analysis is required as the method of error estimate based on the optimum population of the genetic algorithm underestimates errors significantly.

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 three-year monitoring campaign of GRB 030329 with the Westerbork Synthesis Radio Telescopes (WSRT) and the Giant Metrewave Radio Telescope (GMRT). 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 investigate the jet nature of the relativistic outflow. Further, by modeling the late-time radio light curve of GRB 030329, we predict that the Low-Frequency Array (LOFAR, 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.

GRB 030329: three 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 three-year monitoring campaign of GRB 030329 with the Westerbork Synthesis Radio Telescopes (WSRT) and the Giant Metrewave Radio Telescope (GMRT). 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 investigate the jet nature of the relativistic outflow. Further, by modeling the late-time radio light curve of GRB 030329, we predict that the Low Frequency Array (LOFAR, 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.