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Register a new userA Study of the Gamma-Ray Burst Fundamental Plane
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Added by
Christian Gilbertson
Publication date
2016
fields
Physics
and research's language is
English
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A class of long gamma-ray bursts (GRBs) with a plateau phase in their X-ray afterglows obeys a three-dimensional (3D) relation (Dainotti et al. 2016), between the rest-frame time at the end of the plateau, Ta, its corresponding X-ray luminosity, La, and the peak luminosity in the prompt emission, Lpeak. We extended the original analysis with X-ray data from July 2014 to July 2016 achieving a total sample of 183 Swift GRBs with afterglow plateaus and known redshifts. We added the most recent GRBs to the previous gold sample (now including 45 GRBs) and obtained a relation plane with intrinsic scatter compatible within one sigma with the previous result. We compared several GRB categories, such as short with extended emission, X-ray Flashes, GRBs associated with SNe, long-duration GRBs, and the gold sample, composed only by GRBs with light curves with good data coverage and relatively flat plateaus and evaluated their relation planes. We found that they are not statistically different from the fundamental plane derived from the gold sample and that the fundamental plane still has the smallest scatter. We compared the jet opening angles tabulated in literature with the angles derived using the Eiso-Egamma relation of the method in Pescalli et al. (2015) and calculated the relation plane for a sample of long GRBs accounting for the different jet opening angles. We observed that this correction does not significantly reduce the scatter. In an extended analysis, we found that the fundamental plane is independent from several prompt and afterglow parameters.
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The relative strength between forward and reverse shock emission in early gamma-ray burst afterglow reflects that of magnetic energy densities in the two shock regions. We numerically show that with the current standard treatment, the fireball magnetization is underestimated by up to two orders of magnitude. This discrepancy is especially large in the sub-relativistic reverse shock regime (i.e. the thin shell and intermediate regime) where most optical flashes were detected. We provide new analytic estimates of the reverse shock emission based on a better shock approximation, which well describe numerical results in the intermediate regime. We show that the reverse shock temperature at the onset of afterglow is constant, $(bar{Gamma}_d-1)sim 8times10^{-2}$, when the dimensionless parameter $xi_{0}$ is more than several. Our approach is applied to case studies of GRB 990123 and 090102, and we find that magnetic fields in the fireballs are even stronger than previously believed.
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The emission processes active in the highly relativistic jets of gamma-ray bursts (GRBs) remain unknown. In this paper we propose a new measure to describe spectra: the width of the $EF_E$ spectrum, a quantity dependent only on finding a good fit to the data. We apply this to the full sample of GRBs observed by Fermi/GBM and CGRO/BATSE. The results from the two instruments are fully consistent. We find that the median widths of spectra from long and short GRBs are significantly different (chance probability $<10^{-6}$). The width does not correlate with either duration or hardness, and this is thus a new, independent distinction between the two classes. Comparing the measured spectra with widths of spectra from fundamental emission processes -- synchrotron and blackbody radiation -- the results indicate that a large fraction of GRB spectra are too narrow to be explained by synchrotron radiation from a distribution of electron energies: for example, 78% of long GRBs and 85% of short GRBs are incompatible with the minimum width of standard slow cooling synchrotron emission from a Maxwellian distribution of electrons, with fast cooling spectra predicting even wider spectra. Photospheric emission can explain the spectra if mechanisms are invoked to give a spectrum much broader than a blackbody.
A preponderance of evidence links long-duration, soft-spectrum gamma-ray bursts (GRBs) with the death of massive stars. The observations of the GRB-supernova (SN) connection present the most direct evidence of this physical link. We summarize 30 GRB-SN associations and focus on five ironclad cases, highlighting the subsequent insight into the progenitors enabled by detailed observations. We also address the SN association (or lack thereof) with several sub-classes of GRBs, finding that the X-ray Flash (XRF) population is likely associated with massive stellar death whereas short-duration events likely arise from an older population not readily capable of producing a SN concurrent with a GRB. Interestingly, a minority population of seemingly long-duration, soft-spectrum GRBs show no evidence for SN-like activity; this may be a natural consequence of the range of Ni-56 production expected in stellar deaths.
Regardless of their different types of progenitors and central engines, gamma-ray bursts (GRBs) were always assumed to be standalone systems after they formed. Little attention has been paid to the possibility that a stellar companion can still accompany a GRB itself. This paper investigates such a GRB-involved binary system and studies the effects of the stellar companion on the observed GRB emission when it is located inside the jet opening angle. Assuming a typical emission radius of $sim10^{15},$cm, we show that the blockage by a companion star with a radius of $R_mathrm{c}sim67,mathrm{R_odot}$ becomes non-negligible when it is located within a typical GRB jet opening angle (e.g., $sim10$ degrees) and beyond the GRB emission site. In such a case, an on-axis observer will see a GRB with a similar temporal behavior but 25% dimmer. On the other hand, an off-axis observer outside the jet opening angle (hence missed the original GRB) can see a delayed reflected GRB, which is much fainter in brightness, much wider in the temporal profile and slightly softer in energy. Our study can naturally explain the origin of some low-luminosity GRBs. Moreover, we also point out that the companion star may be shocked if it is located inside the GRB emission site, which can give rise to an X-ray transient or a GRB followed by a delayed X-ray bump on top of X-ray afterglows.
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