Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userDeciphering the properties of the central engine in GRB collapsars
109
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
The central engine in long gamma-ray bursts (GRBs) is thought to be a compact object produced by the core collapse of massive stars, but its exact nature (black hole or millisecond magnetar) is still debatable. Although the central engine of GRB collapsars is hidden to direct observation, its properties may be imprinted on the accompanying electromagnetic signals. We aim to decipher the generic properties of central engines that are consistent with prompt observations of long GRBs detected by the Burst Alert Telescope (BAT) on board the Neil Gehrels Swift Observatory. Adopting a generic model for the central engine, in which the engine power and activity timescale are independent of each other, we perform Monte Carlo simulations of long GRBs produced by jets that successfully breakout from the star. Our simulations consider the dependence of the jet breakout timescale on the engine luminosity and the effects of the detectors flux threshold. The two-dimensional (2D) distribution of simulated detectable bursts in the gamma-ray luminosity versus gamma-ray duration plane is consistent with the observed one for a range of parameter values describing the central engine. The intrinsic 2D distribution of simulated collapsar GRBs peaks at lower gamma-ray luminosities and longer durations than the observed one, a prediction that can be tested in the future with more sensitive detectors. Black-hole accretors, whose power and activity time are set by the large-scale magnetic flux through the progenitor star and stellar structure, respectively, are compatible with the properties of the central engine inferred by our model.
rate research
Read More
We present optical photometry of the afterglow of the long GRB 180205A with the COATLI telescope from 217 seconds to about 5 days after the {itshape Swift}/BAT trigger. We analyse this photometry in the conjunction with the X-ray light curve from {itshape Swift}/XRT. The late-time light curves and spectra are consistent with the standard forward-shock scenario. However, the early-time optical and X-ray light curves show non-typical behavior; the optical light curve exhibits a flat plateau while the X-ray light curve shows a flare. We explore several scenarios and conclude that the most likely explanation for the early behavior is late activity of the central engine.
The double burst, GRB 110709B, triggered Swift/BAT twice at 21:32:39 UT and 21:43:45 UT, respectively, on 9 July 2011. This is the first time we observed a GRB with two BAT triggers. In this paper, we present simultaneous Swift and Konus-WIND observations of this unusual GRB and its afterglow. If the two events originated from the same physical progenitor, their different time-dependent spectral evolution suggests they must belong to different episodes of the central engine, which may be a magnetar-to-BH accretion system.
Gamma-ray bursts (GRBs) are the most luminous explosions in the universe, yet the nature and physical properties of their energy sources are far from understood. Very important clues, however, can be inferred by studying the afterglows of these events. We present optical and X-ray observations of GRB 130831A obtained by Swift, Chandra, Skynet, RATIR, Maidanak, ISON, NOT, LT and GTC. This burst shows a steep drop in the X-ray light-curve at $simeq 10^5$ s after the trigger, with a power-law decay index of $alpha sim 6$. Such a rare behaviour cannot be explained by the standard forward shock (FS) model and indicates that the emission, up to the fast decay at $10^5$ s, must be of internal origin, produced by a dissipation process within an ultrarelativistic outflow. We propose that the source of such an outflow, which must produce the X-ray flux for $simeq 1$ day in the cosmological rest frame, is a newly born magnetar or black hole. After the drop, the faint X-ray afterglow continues with a much shallower decay. The optical emission, on the other hand, shows no break across the X-ray steep decrease, and the late-time decays of both the X-ray and optical are consistent. Using both the X-ray and optical data, we show that the emission after $simeq 10^5$ s can be explained well by the FS model. We model our data to derive the kinetic energy of the ejecta and thus measure the efficiency of the central engine of a GRB with emission of internal origin visible for a long time. Furthermore, we break down the energy budget of this GRB into the prompt emission, the late internal dissipation, the kinetic energy of the relativistic ejecta, and compare it with the energy of the associated supernova, SN 2013fu.
GRB 200219A is a short gamma-ray burst (GRB) with an extended emission (EE) lasting $sim 90$s. By analyzing data observed with the {em Swift}/BAT and {em Fermi}/GBM, we find that a cutoff power-law model can adequately fit the spectra of the initial short pulse with $rm E_{p}=1387^{+232}_{-134}$ keV. More interestingly, together with the EE component and early X-ray data, it exhibits plateau emission smoothly connected with a $sim t^{-1}$ segment and followed by an extremely steep decay. The short GRB composed of those three segments is unique in the {em Swift} era and is very difficult to explain with the standard internal/external shock model of a black hole central engine, but could be consistent with the prediction of a magnetar central engine from the merger of an NS binary. We suggest that the plateau emission followed by a $sim t^{-1}$ decay phase is powered by the spin-down of a millisecond magnetar, which loses its rotation energy via GW quadrupole radiation. Then, the abrupt drop decay is caused by the magnetar collapsing into a black hole before switching to EM-dominated emission. This is the first short GRB for which the X-ray emission has such an intriguing feature powered by a magnetar via GW-dominated radiation. If this is the case, one can estimate the physical parameters of a magnetar, the GW signal powered by a magnetar and the merger-nova emission are also discussed.
The jet compositions, central engines, and progenitors of gamma-ray bursts (GRBs) remain open questions in GRB physics. Applying broadband observations, including GRB prompt emission and afterglow properties derived from {em Fermi} and {em Swift} data, as well as from Keck host-galaxy observations, we address these questions for the peculiar, bright GRB 110731A. By using the pair-opacity method, we derive $Gamma_{0}>190$ during the prompt emission phase. Alternatively, we derive $Gamma_{0} approx 580$ and $Gamma_{0} approx 154$ by invoking the early-afterglow phase within the homogeneous density and wind cases, respectively. On the other hand, nondetection of a thermal component in the spectra suggests that the prompt emission is likely powered by dissipation of a Poynting-flux-dominated jet leading to synchrotron radiation in an optically thin region. The nondetection of a jet break in the X-ray and optical bands allows us to place a lower limit on the jet opening angle $theta_{rm j}>5.5^{circ}$. Within a millisecond magnetar central engine scenario, we derive the period $P_{0}$ and polar magnetic field strength $B_{rm p}$, which have extreme (but still allowed) values. The moderately short observed duration (7.3,s) and relatively large redshift ($z=2.83$) places the burst as a rest-frame short GRB, so the progenitor of the burst is subject to debate. Its relatively large $f_{{rm eff}, z}$ parameter (ratio of the 1,s peak flux of a pseudo-GRB and the background flux) and a large physical offset from a potential host galaxy suggest that the progenitor of GRB 110731A may be a compact-star merger.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
