Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userOn the Radio-to-X-ray light curves of SN 1998bw and GRB 980425
261
0
0.0
(
0
)
Authors
K.Iwamoto
Ask ChatGPT about the research
No Arabic abstract
We calculate radio-to-X-ray light curves for afterglows caused by non-thermal emission from a highly relativistic blast wave, which is inferred from the gamma-ray flux detected in GRB 980425 and from the very bright radio emission detected in SN 1998bw. We find that the observed gamma-ray and radio light curves are roughly reproduced by the synchrotron emission from a relativistic fireball. The optical flux predicted for the non-thermal emission is well below that of the thermal emission observed for SN 1998bw so that it will not be seen at least for a few years. The model predicts the X-ray flux just above the detection limit of BeppoSAX for the epoch when it was pointed to the field of GRB980425. Therefore, the nondetection of X-ray and optical afterglows is consistent with the model. The models presented here are consistent with the physical association between SN 1998bw and GRB980425, and lend further support to the idea that this object might correspond to an event similar to the ``hypernova or ``collapsar -- events in which the collapse of a massive star forms a rotating black hole surrounded by a disk of the remnant stellar mantle.
rate research
Read More
We present the first spatially resolved study of molecular gas in the vicinity of a Gamma Ray Burst, using CO(2-1) emission line observations with the Atacama Large Millimetre Array (ALMA) at ~50 pc scales. The host galaxy of GRB 980425 contains a ring of high column density HI gas which is likely to have formed due to a collision between the GRB host and its companion galaxy, within which the GRB is located. We detect eleven molecular gas clumps in the galaxy, seven of which are within the gas ring. The clump closest to the GRB position is at a projected separation of ~280 pc. Although it is plausible that the GRB progenitor was ejected from clusters formed in this clump, we argue that the in situ formation of the GRB progenitor is the most likely scenario. We measure the molecular gas masses of the clumps and find them to be sufficient for forming massive star clusters. The molecular gas depletion times of the clumps show a variation of ~2 dex, comparable with the large variation in depletion times found in starburst galaxies in the nearby Universe. This demonstrates the presence of starburst modes of star formation on local scales in the galaxy, even while the galaxy as a whole cannot be categorised as a starburst based on its global properties. Our findings suggest that the progenitor of GRB 9802425 was originated in a young massive star cluster formed in the starburst mode of star formation.
In spite of the complicated behavior in the time domain, long GRBs show a simpler behavior in the Fourier domain of frequencies, represented by power density spectra, PDS. Recently, there are some relations found between GRBs properties and PDS parameters, modeled by power-laws. Among them, the correlation between peak energy $E_{peak}$ and PDS slope $alpha$ shows a clear evidence. In this work we try to understand the origin of this correlation, making use of synthetic pulses. We find some preliminary evidences that $E_{peak}-alpha$ relation can be seen as a new confirmation of the empiric relations $E_{peak}-L$ and $t_{p}-L$ for GRBs.
We have collected all of the published photometry for GRB 990123 and GRB 990510, the first two gamma-ray bursts where breaks were seen in the light curves of their optical afterglows, and determined the shapes of their light curves and the break times. These parameters were used to investigate the physical mechanisms responsible for the breaks and the nature of the ambient medium that the bursts occurred in. The light curve for GRB 990123 is best fit by a broken power law with a break 1.68 +/- 0.19 days after the burst, a slope of alpha1 = -1.12 +/- 0.08 before the break, and a slope of alpha2 = -1.69 +/- 0.06 after the break. This is consistent with a collimated outflow with a fixed opening angle (theta0) of approximately five degrees. In this case the break in the light curve is due to the relativistic fireball slowing to a gamma factor of approximately 1 / theta0. The light curve for GRB 990510 is best fit by a continuous function with an early-time slope of alpha1 = -0.54 +/- 0.14, a late-time slope of alpha2 = -1.98 +/- 0.19, and a slow transition between the two regimes approximately one day after the burst. This is consistent with a collimated outflow with an opening angle of approximately five degrees that is initially radiative, but undergoes a sideways expansion that begins approximately one day after the burst. This sideways expansion is responsible for the slow break in the light curve.
We have reconstructed the spectrum of the afterglow of GRB 970508, on May 21.0 UT (12.1 days after the GRB), on the basis of observations spanning the X-ray to radio range. The low-frequency power law index of the spectrum, $alpha=0.44pm0.07$ ($F_ upropto u^alpha$), is in agreement with the expected value $alpha=1/3$ for optically thin synchrotron radiation. The 1.4 GHz emission is self-absorbed. We infer constraints on the break frequecies $ u_{c}$ and $ u_{m}$ on May 21.0 UT from a spectral transition from $F_{ u} sim u^{-0.6}$ to $F_{ u} sim u^{-1.1}$ in the optical passband around 1.4 days. A model of an adiabatically expanding blast wave, emitting synchrotron radiation, in which a significant fraction of the electrons cool rapidly provides a successful and consistent description of the afterglow observations over nine decades in frequency, ranging in time from trigger until several months later.
Over the six years since the discovery of the gamma-ray burst GRB 980425, associated with the nearby (distance, ~40 Mpc) supernova 1998bw, astronomers have fiercely debated the nature of this event. Relative to bursts located at cosmological distances, (redshift, z~1), GRB 980425 was under-luminous in gamma-rays by three orders of magnitude. Radio calorimetry showed the explosion was sub-energetic by a factor of 10. Here, we report observations of the radio and X-ray afterglow of the recent z=0.105 GRB 031203 and demonstrate that it too is sub-energetic. Our result, when taken together with the low gamma-ray luminosity, suggest that GRB 031203 is the first cosmic analogue to GRB 980425. We find no evidence that this event was a highly collimated explosion viewed off-axis. Like GRB 980425, GRB 031203 appears to be an intrinsically sub-energetic gamma-ray burst. Such sub-energetic events have faint afterglows. Intensive follow-up of faint bursts with smooth gamma-ray light curves (common to both GRBs 031203 and 980425) may enable us to reveal their expected large population.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
