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Register a new userHigh Energy Observations of XRF 030723: Evidence for an Off-axis Gamma-Ray Burst?
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We report High Energy Transient Explorer 2 (HETE-2) Wide Field X-ray Monitor/French Gamma Telescope observations of XRF030723 along with observations of the XRF afterglow made using the 6.5m Magellan Clay telescope and the Chandra X-ray Observatory. The observed peak energy E_pk_obs of the nu F_nu burst spectrum is found to lie within (or below) the WXM 2-25 keV passband at 98.5% confidence, and no counts are detected above 30 keV. Our best fit value is E_pk_obs=8.4+3.5/-3.4 keV. The ratio of X-ray to Gamma-ray flux for the burst follows a correlation found for GRBs observed with HETE-2, and the duration of the burst is similar to that typical of long-duration GRBs. If we require that the burst isotropic equivalent energy E_iso and E_pk_rest satisfy the relation discovered by Amati et al. (2002), a redshift of z=0.38+0.36/-0.18 can be determined, in agreement with constraints determined from optical observations. We are able to fit the X-ray afterglow spectrum and to measure its temporal fade. Although the best-fit fade is shallower than the concurrent fade in the optical, the spectral similarity between the two bands indicates that the X-ray fade may actually trace the optical fade. If this is the case, the late time rebrightening observed in the optical cannot be due to a supernova bump. We interpret the prompt and afterglow X-ray emission as arising from a jetted GRB observed off-axis and possibly viewed through a complex circumburst medium due to a progenitor wind.
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Optical afterglows from two-component jets under various configurations are investigated numerically. Generally, the light curve is characterized by a rapid rebrightening when the observer is off-axis with respect to the narrow component, with the amplitude and peak time depending on detailed parameters. We further show that the optical afterglow of XRF 030723, especially its notable and rapid rebrightening, can be well explained by a typical two-component jet. This X-ray flash, together with GRB 030329, strongly hints the two-component jet model as a unified picture for X-ray flashes and gamma-ray bursts. With a narrow but ultra-relativistic inner outflow and a wide but less energetic outer ejecta, a two-component jet will be observed as a typical gamma-ray burst if our line of sight is within the angular scope of the narrow outflow. Otherwise, if the line of sight is within or slightly beyond the cone of the wide component, an X-ray flash will be detected.
We investigated the radio properties of the host galaxy of X-ray flash, XRF020903, which is the best example for investigating of the off-axis origin of gamma-ray bursts(GRBs). Dust continuum at 233 GHz and CO are observed using the Atacama Large millimeter/submillimeter array. The molecular gas mass derived by applying the metalicity-dependent CO-to-H$_{2}$ conversion factor matches the global trend along the redshift and stellar mass of the GRB host galaxies. The estimated gas depletion timescale (pertaining to the potential critical characteristics of GRB host galaxies) is equivalent to those of GRBs and super-luminous supernova hosts in the same redshift range. These properties of the XRF020903 host galaxy observed in radio resemble those of GRB host galaxies, thereby supporting the identical origin of XRF020903 and GRBs.
We present results for a large number of gamma-ray burst (GRB) afterglow light curve calculations, done by combining high resolution two-dimensional relativistic hydrodynamics simulations using RAM with a synchrotron radiation code. Results were obtained for jet energies, circumburst medium densities and jet angles typical for short and underluminous GRBs, different observer angles and observer frequencies from low radio (75 MHz) to X-ray (1.5 keV). We summarize the light curves through smooth power law fits with up to three breaks, covering jet breaks for small observer angles, the rising phase for large observer angles and the rise and decay of the counterjet. All light curve data are publicly available via http://cosmo.nyu.edu/afterglowlibrary . The data can be used for model fits to observational data and as an aid for predicting observations by future telescopes such as LOFAR or SKA and will benefit the study of neutron star mergers using different channels, such as gravitational wave observations with LIGO or Virgo. For small observer angles, we find jet break times that vary significantly between frequencies, with the break time in the radio substantially postponed. Increasing the observer angle also postpones the measured jet break time. The rising phase of the light curve for large observer angle has a complex shape that can not always be summarized by a simple power law. Except for very large observer angles, the counter jet is a distinct feature in the light curve, although in practice the signal will be exceedingly difficult to observe by then.
Long-duration gamma-ray bursts (GRBs) are almost unequivocally associated with very energetic, broad-lined supernovae (SNe) of Type Ic-BL. While the gamma-ray emission is emitted in narrow jets, the SN emits radiation isotropically. Therefore, some SN Ic-BL not associated with GRBs have been hypothesized to arise from events with inner engines such as off-axis GRBs or choked jets. Here we present observations of the nearby ($d = 120$ Mpc) SN 2020bvc (ASAS-SN 20bs) which support this scenario. textit{Swift} UVOT observations reveal an early decline (up to two days after explosion) while optical spectra classify it as a SN Ic-BL with very high expansion velocities ($approx$ 70,000 km/s), similar to that found for the jet-cocoon emission in SN 2017iuk associated with GRB 171205A. Moreover, textit{Swift} X-Ray Telescope and textit{CXO} X-ray Observatory detected X-ray emission only three days after the SN and decaying onwards, which can be ascribed to an afterglow component. Cocoon and X-ray emission are both signatures of jet-powered GRBs. In the case of SN 2020bvc, we find that the jet is off axis (by $approx$ 23 degrees), as also indicated by the lack of early ($approx 1$ day) X-ray emission which explains why no coincident GRB was detected promptly or in archival data. These observations suggest that SN 2020bvc is the first orphan GRB detected through its associated SN emission.
Gamma-ray bursts (GRBs) and their early afterglows ionise their circumburst material. Only high-energy spectroscopy therefore, allows examination of the matter close to the burst itself. Soft X-ray absorption allows an estimate to be made of the total column density in metals. The detection of the X-ray afterglow can also be used to place a limit on the total gas column along the line of sight based on the Compton scattering opacity. Such a limit would enable, for the first time, the determination of lower limits on the metallicity in the circumburst environments of GRBs. In this paper, we determine the limits that can be placed on the total gas column density in the vicinities of GRBs based on the Compton scattering. We simulate the effects of Compton scattering on a collimated beam of high energy photons passing through a shell of high column density material to determine the expected lightcurves, luminosities, and spectra. We compare these predictions to observations, and determine what limits can realistically be placed on the total gas column density. The smearing out of pulses in the lightcurve from Compton scattering is not likely to be observable, and its absence does not place strong constraints on the Compton depth for GRBs. However, the distribution of observed luminosities of bursts allows us to place statistical, model-dependent limits that are typically <~1e25 cm^{-2} for less luminous bursts, and as low as ~1e24 cm$^{-2} for the most luminous. Using the shape of the high-energy broadband spectrum, however, in some favourable cases, limits as low as ~5e24 cm^{-2} can placed on individual bursts, implying metallicity lower limits from X- and gamma-rays alone from 0 up to 0.01 Z/Zsun. At extremely high redshifts, this limit would be at least 0.02 Z/Z_sun, enough to discriminate population III from non-primordial GRBs.
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