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Register a new userDependence of the optical brightness on the gamma and X-ray properties of GRBs
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The Swift satellite made a real break through with measuring simultaneously the gamma X-ray and optical data of GRBs, effectively. Although, the satellite measures the gamma, X-ray and optical properties almost in the same time a significant fractions of GRBs remain undetected in the optical domain. In a large number of cases only an upper bound is obtained. Survival analysis is a tool for studying samples where a part of the cases has only an upper (lower) limit. The obtained survival function may depend on some other variables. The Cox regression is a way to study these dependencies. We studied the dependence of the optical brightness (obtained by the UVOT) on the gamma and X-ray properties, measured by the BAT and XRT on board of the Swift satellite. We showed that the gamma peak flux has the greatest impact on the afterglows optical brightness while the gamma photon index and the X-ray flux do not. This effect probably originates in the energetics of the jet launched from the central engine of the GRB which triggers the afterglow.
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We study an extensive sample of 87 GRBs for which there are well sampled and simultaneous optical and X-ray light-curves. We extract the cleanest possible signal of the afterglow component, and compare the temporal behaviors of the X-ray light-curve, observed by Swift XRT, and optical data, observed by UVOT and ground-based telescopes for each individual burst. Overall we find 62% GRBs that are consistent with the standard afterglow model. When more advanced modeling is invoked, up to 91% of the bursts in our sample may be consistent with the external shock model. A large fraction of these bursts are consistent with occurring in a constant interstellar density medium (ISM) (61%) while only 39% of them occur in a wind-like medium. Only 9 cases have afterglow light-curves that exactly match the standard fireball model prediction, having a single power law decay in both energy bands which are observed during their entire duration. In particular, for the bursts with chromatic behavior additional model assumptions must be made over limited segments of the light-curves in order for these bursts to fully agree with the external shock model. Interestingly, for 54% of the X-ray and 40% of the optical band observations the end of the shallow decay ($t^{sim-0.5}$) period coincides with the jet break ($t^{sim-p}$) time, causing an abrupt change in decay slope. The fraction of the burst that consistent with the external shock model is independent of the observational epochs in the rest frame of GRBs. Moreover, no cases can be explained by the cooling frequency crossing the X-ray or optical band.
We study the X-ray absorption of a complete sample of 99 bright Swift gamma-ray bursts. Over the last few years, a strong correlation between the intrinsic X-ray absorbing column density (N_H(z)) and the redshift was found. This absorption excess in high-z GRBs is now thought to be due to the overlooked contribution of the absorption along the intergalactic medium, by means of both intervening objects and the diffuse warm-hot intergalactic medium along the line of sight. In this work we neglect the absorption along the IGM, because our purpose is to study the eventual effect of a radical change in the Galactic absorption model on the N_H(z) distribution. Therefore, we derive the intrinsic absorbing column densities using two different Galactic absorption models, the Leiden Argentine Bonn HI survey and the more recent model including molecular hydrogen. We find that, if on the one hand the new Galactic model considerably affects the single column density values, on the other hand there is no drastic change in the distribution as a whole. It becomes clear that the contribution of Galactic column densities alone, no matter how improved, is not sufficient to change the observed general trend and it has to be considered as a second-order correction. The cosmological increase of N_H(z) as a function of redshift persists and, in order to explain the observed distribution, it is necessary to include the contribution of both the diffuse intergalactic medium and the intervening systems along the line of sight of the GRBs.
We show that Compton scattering by electrons of the hot intergalactic gas in galaxy clusters should lead to peculiar distortions of the cosmic background X-ray and soft gamma-ray radiation - an increase in its brightness at E<60-100 keV and a drop at higher energies. The distortions allow the most important cluster parameters to be measured. The spectral shape of the distortions and its dependence on the gas temperature, optical depth, and surface density distribution law have been studied using Monte Carlo computations and confirmed by analytical estimations. In the cluster frame the maximum of the background decrease due to the recoil effect occurs at ~500-600 keV. The photoionization of H- and He-like iron and nickel ions leads to additional distortions in the background spectrum - a strong absorption line with the threshold at ~9 keV (and also to an absorption jump at ~2 keV for cold clusters). The absorption of intrinsic thermal radiation from the cluster gas by these ions also leads to such lines. In nearby (z<1) clusters the line at ~2 keV is noticeably enhanced by absorption in the colder (~10^6 K) plasma of their peripheral (~3 Mpc) regions; moreover, the absorption line at ~1.3 keV splits off from it. The redshift of distant clusters shifts the absorption lines in the background spectrum (at ~2, ~9, and ~500 keV) to lower energies. Thus, in contrast to the microwave background scattering effect, this effect depends on the cluster redshift z, but in a very peculiar way. When observing clusters at z>1, the effect allows one to determine how the X-ray background evolved and how it was gathered with z. To detect the effect, the accuracy of measurements should reach ~0.1%. We consider the most promising clusters for observing the effect and discuss the techniques whereby the influence of the thermal gas radiation hindering the detection of background distortions should be minimal.
The morphology of the extended $gamma$-ray source is governed by the propagation process of parent relativistic particles. In this paper, we investigate the surface brightness radial profile of extended $gamma$-ray sources illuminated by cosmic ray protons and electrons, considering the radiation mechanisms, projection effects, and the response of instruments. We found that the parent particle species and the propagation process can cause considerable differences in the observed radial profiles. Thus, the surface brightness profile can be used as a unique tool to identify the radiation mechanism and the propagation process of the parent particles. In addition, We also discuss the possible implications regarding the latest discoveries %results from very/ultra-high energy $gamma$-ray instruments like LHAASO and HAWC.
The X-ray afterglow plateau emission observed in many Gamma-ray Bursts (GRBs) has been interpreted as either being fueled by fallback onto a newly formed black hole, or by the spin-down luminosity of an ultra-magnetized millisecond neutron star. If the latter model is assumed, GRB X-ray afterglow light curves can be analytically reproduced. We fit a sample of GRB X-ray plateaus, interestingly yielding a distribution in the magnetic field versus spin period (B-P) diagram consistent with $Bpropto P^{7/6}$. This is expected from the well-established physics of the spin-up line minimum period for Galactic millisecond pulsars. The normalisation of the relation we obtain perfectly matches spin-up line predictions for the expected masses ($sim 1 M_{odot}$) and radii ($sim 10 {rm ~km}$) of newly born magnetars, and mass accretion rates consistent with GRB expectations of $10^{-4} M_{odot}/{rm s} <dot{M}< 10^{-1} M_{odot}/{rm s}$. Short GRBs with extended emission (SEE) appear towards the high period end of the distribution, while the long GRBs (LGRBs) towards the short period end. This result is consistent with spin-up limit expectations where the total accreted mass determines the position of the neutron star in the B-P diagram. The P-B distribution for LGRBs and SEE are statistically different, further supporting the idea that the fundamental plane relation citep{dainotti16c,Dainotti2017} is a powerful discriminant among those populations. Our conclusions are robust against suppositions regarding the GRB collimation angle and magnetar breaking index, which shifts the resulting magnetar properties parallel to the spin-up line, and strongly support a magnetar origin for GRBs presenting X-ray plateaus.
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