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Register a new userA peculiar low-luminosity short gamma-ray burst from a double neutron star merger progenitor
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Double neutron star (DNS) merger events are promosing candidates of short Gamma-ray Burst (sGRB) progenitors as well as high-frequecy gravitational wave (GW) emitters. On August 17, 2017, such a coinciding event was detected by both the LIGO-Virgo gravitational wave detector network as GW170817 and Gamma-Ray Monitor on board NASAs {it Fermi} Space Telescope as GRB 170817A. Here we show that the fluence and spectral peak energy of this sGRB fall into the lower portion of the distributions of known sGRBs. Its peak isotropic luminosity is abnormally low. The estimated event rate density above this luminosity is at least $190^{+440}_{-160} {rm Gpc^{-3} yr^{-1}}$, which is close to but still below the DNS merger event rate density. This event likely originates from a structured jet viewed from a large viewing angle. There are similar faint soft GRBs in the {it Fermi} archival data, a small fraction of which might belong to this new population of nearby, low-luminosity sGRBs.
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Short duration Gamma-Ray Bursts are thought to originate from the coalescence of neutron stars in binary systems. They are detected as a brief ($<$ 2s), intense flash of gamma-ray radiation followed by a weaker, rapidly decreasing afterglow. They are expected to be detected by Advanced LIGO and Virgo when their sensitivity will be low enough. In a recent study we identified a population of short Gamma-Ray Bursts that are intrinsically faint and nearby. Here we provide evidence in favor of the existence of this new population that can hardly be reproduced with a model of field neutron star binary coalescences. We propose that these systems may be produced dynamically in globular clusters, and may result from the merger of a black hole and a neutron star. The advanced LIGO and Virgo observation of a high rate of NSBH mergers compatible with the dynamical formation in globular clusters would be a confirmation of this hypothesis and would enable for the derivation of the mass function of black holes inside globular clusters, as well as the luminosity function of faint short GRBs.
Gamma-ray bursts (GRBs) occurring in the local Universe constitute an interesting sub-class of the GRB family, since their luminosity is on average lower than that of their cosmological analogs. We aim to contribute to the study of local bursts by reporting the case of GRB 171205A. This source was discovered by Swift Burst Alert Telescope (BAT) on 2017, December 5 and soon associated with a low redshift host galaxy (z=0.037), and an emerging SN (SN 2017iuk). We analyzed the full Swift, dataset, comprising the UV-Optical Telescope (UVOT), X-ray Telescope (XRT) and BAT data. In addition, we employed the Konus-Wind high energy data as a valuable extension at gamma-ray energies. The photometric SN signature is clearly visible in the UVOT u, b and v filters. The maximum emission is reached at ~ 13 (rest frame) days, and the whole bump resembles that of SN 2006aj, but lower in magnitude and with a shift in time of +2 d. A prebump in the v-band is also clearly visible, and this is the first time that such a feature is not observed achromatically in GRB-SNe. Its physical origin cannot be easily explained. The X-ray spectrum shows an intrinsic Hydrogen column density N_H,int = 7.4(+4.1 -3.6) X 10^20 / cm^2$, which is at the low end of the N_H,int, even considering just low redshift GRBs. The spectrum also features a thermal component, which is quite common in GRBs associated with SNe, but whose origin is still a matter of debate. Finally, the isotropic energy in the gamma-ray band, E_iso = 2.18(+0.63 -0.50) X 10^49 erg, is lower than those of cosmological GRBs. Combining this value with the peak energy in the same band, E_p=125(+141 -37) keV, implies that GRB 171205A is an outlier of the Amati relation, as are some other low redshift GRBs, and its emission mechanism should be different from that of canonical, farther away GRBs.
Gamma-ray bursts (GRBs) have been phenomenologically classified into long and short populations based on the observed bimodal distribution of duration. Multi-wavelength and multi-messenger observations in recent years have revealed that in general long GRBs originate from massive star core collapse events, whereas short GRBs originate from binary neutron star mergers. It has been known that the duration criterion is sometimes unreliable, and multi-wavelength criteria are needed to identify the physical origin of a particular GRB. Some apparently long GRBs have been suggested to have a neutron star merger origin, whereas some apparently short GRBs have been attributed to genuinely long GRBs whose short, bright emission is slightly above the detectors sensitivity threshold. Here we report the comprehensive analysis of the multi-wavelength data of a bright short GRB 200826A. Characterized by a sharp pulse, this burst shows a duration of 1 second and no evidence of an underlying longer-duration event. Its other observational properties such as its spectral behaviors, total energy, and host galaxy offset, are, however, inconsistent with those of other short GRBs believed to originate from binary neutron star mergers. Rather, these properties resemble those of long GRBs. This burst confirms the existence of short duration GRBs with stellar core-collapse origin, and presents some challenges to the existing models.
The long gamma ray bursts (GRBs) may arise from the core collapse of massive stars. However, the long GRB rate does not follow the star formation rate (SFR) at high redshifts. In this Letter, we focus on the binary merger model and consider the high spin helium stars after the merger as the progenitor of long GRBs. With this scenario, we estimate the GRB rate by the population synthesis method with the metallicity evolution. Low metallicity binaries are easier to become long GRB progenitors than those for solar metallicity due to the weak wind mass loss and the difference in the stellar evolution. In our results, the long GRB rate roughly agrees with the observed rate, and shows a similar behavior to the observed redshift evolution.
The combined detection of a gravitational-wave signal, kilonova, and short gamma-ray burst (sGRB) from GW170817 marked a scientific breakthrough in the field of multi-messenger astronomy. But even before GW170817, there have been a number of sGRBs with possible associated kilonova detections. In this work, we re-examine these historical sGRB afterglows with a combination of state-of-the-art afterglow and kilonova models. This allows us to include optical/near-infrared synchrotron emission produced by the sGRB as well as ultraviolet/optical/near-infrared emission powered by the radioactive decay of $r$-process elements (i.e., the kilonova). Fitting the lightcurves, we derive the velocity and the mass distribution as well as the composition of the ejected material. The posteriors on kilonova parameters obtained from the fit were turned into distributions for the peak magnitude of the kilonova emission in different bands and the time at which this peak occurs. From the sGRB with an associated kilonova, we found that the peak magnitude in H bands falls in the range [-16.2, -13.1] ($95%$ of confidence) and occurs within $0.8-3.6,rm days$ after the sGRB prompt emission. In g band instead we obtain a peak magnitude in range [-16.8, -12.3] occurring within the first $18,rm hr$ after the sGRB prompt. From the luminosity distributions of GW170817/AT2017gfo, kilonova candidates GRB130603B, GRB050709 and GRB060614 (with the possible inclusion of GRB150101B) and the upper limits from all the other sGRBs not associated with any kilonova detection we obtain for the first time a kilonova luminosity function in different bands.
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