Supernova 2002ap: the first month

Supernova (SN) 2002ap in M74 was discovered on 2002 January 29. Being one of the nearest (10 Mpc) SN events in the last decades, and spectroscopically similar to the so-called 'hypernovae' 1997ef and 1998bw, both possibly associated with gamma-ray bursts (GRBs), it is of great interest. Shortly after its discovery, we launched an intensive photometric and spectroscopic monitoring campaign of this event, and here we report the results of the first month of observations. We use our UBVRI photometry to estimate the magnitudes at, and dates of, peak brightness. Our data suggest that this object reached its peak B -band luminosity on February     . Based on its similarity to SN 1998bw, we estimate the range of possible dates for a GRB that may have been associated with SN 2002ap. We find that it may include dates outside the time frame for which all available gamma-ray data have been intensively scanned, according to recent reports. The absolute magnitude at peak brightness of SN 2002ap  ( M _B =-16.9)  shows that it was significantly fainter than SN 1998bw, or normal Type Ia SNe, but similar to SN 1997ef. Our spectroscopic observations confirm that SN 2002ap is strikingly similar to SNe 1998bw and 1997ef. We briefly describe the spectral evolution of this object. To assist other observers and to stimulate theoretical models, we make our entire data set publicly available in digital form (http://wise-obs.tau.ac.il/∼avishay/local.html).     

X-ray emission lines in the early afterglows of gamma-ray bursts

We computed the thermal time-dependent X-ray spectrum of a hot plasma in the vicinity of a gamma-ray burst (GRB). An allowance for time-dependent processes in a hot rarefied plasma is shown to strongly affect the observed spectrum. These computations can give an alternative explanation for the observed X-ray emission lines in the early afterglows of GRBs (e.g., GRB 011211). Our technique allows the GRB collimation angle and the environment clumpiness parameters to be independently constrained.     

Extinction of Beamed Gamma-ray Burst Afterglows in a Dense Circumstellar Cloud

Broadband afterglow observations provide a probe of the density structure of the circumburst medium. In the spreading jet model, prompt and intense X-ray/UV radiation from the reverse shock may destroy and clear the dust in the circumburst cloud out to about 30 pc within the initial solid angle of the jet. Asthe jet expands significantly, optical radiation from the high-latitude part of the jet may suffer extinction by dust outside the initial solid angle, while radiation from the part within the initial solid angle can be observed without extinction. In previous studies, it is usually assumed that the extinction is complete. We calculate the extinction effect by taking the optical depth into account. Our numerical results showthat a break appears in the light curve of optical afterglow but it extends over a factor of ～ 80 in time rather than a factor of ～ 10 in time for the case of strong dust extinction and a factor of ～ 60 in time for the case without dust extinction. These results may provide a way to judge how large the number density of the circumburst cloud is. Finally, we carry out a detailed modeling for the afterglow of GRB 000926. Our model can provide a good fit to the multi-color observations of this event.