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Register a new userLow-efficiency long gamma-ray bursts: A case study with AT2020blt
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The Zwicky Transient Facility recently announced the detection of an optical transient AT2020blt at redshift $z=2.9$, consistent with the afterglow of a gamma-ray burst. No prompt emission was observed. We analyse AT2020blt with detailed models, showing the data are best explained as the afterglow of an on-axis long gamma-ray burst, ruling out other hypotheses such as a cocoon and a low-Lorentz factor jet. We search textit{Fermi} data for prompt emission, setting deeper upper limits on the prompt emission than in the original detection paper. Together with konus{} observations, we show that the gamma-ray efficiency of AT2020blt is $lesssim 2.8%$, lower than $98.4%$ of observed gamma-ray bursts. We speculate that AT2020blt and AT2021any belong to the low-efficiency tail of long gamma-ray burst distributions that are beginning to be readily observed due to the capabilities of new observatories like the Zwicky Transient Facility.
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The discovery of a number of gamma-ray bursts with duration exceeding 1,000 seconds, in particular the exceptional case of GRB 111209A with a duration of about 25,000 seconds, has opened the question on whether these bursts form a new class of sources, the so called {em ultra-long} GRBs, or if they are rather the tail of the distribution of the standard long GRB duration. In this Letter, using the long GRB sample detected by {em Swift}, we investigate on the statistical properties of ultra-long GRBs and compare them with the overall long burst population. We discuss also on the differences observed in their spectral properties. We find that ultra-long GRBs are statistically different from the standard long GRBs with typical burst duration less than 100-500 seconds, for which a Wolf Rayet star progenitor is usually invoked. We interpret this result as an indication that an alternative scenario has to be found in order to explain the ultra-long GRB extreme energetics, as well as the mass reservoir and its size that can feed the central engine for such a long time.
Long gamma-ray bursts (GRBs) have been linked to extreme core-collapse supernovae from massive stars. Gravitational waves (GW) offer a probe of the physics behind long GRBs. We investigate models of long-lived (~10-1000s) GW emission associated with the accretion disk of a collapsed star or with its protoneutron star remnant. Using data from LIGOs fifth science run, and GRB triggers from the swift experiment, we perform a search for unmodeled long-lived GW transients. Finding no evidence of GW emission, we place 90% confidence level upper limits on the GW fluence at Earth from long GRBs for three waveforms inspired by a model of GWs from accretion disk instabilities. These limits range from F<3.5 ergs cm^-2 to $F<1200 ergs cm^-2, depending on the GRB and on the model, allowing us to probe optimistic scenarios of GW production out to distances as far as ~33 Mpc. Advanced detectors are expected to achieve strain sensitivities 10x better than initial LIGO, potentially allowing us to probe the engines of the nearest long GRBs.
Gamma-ray bursts (GRBs) are the most luminous explosions and can be detectable out to the edge of Universe. It has long been thought they can extend the Hubble diagram to very high redshifts. Several correlations between temporal or spectral properties and GRB luminosities have been proposed to make GRBs cosmological tools. However, those correlations cannot be properly standardized. In this paper, we select a long GRB sample with X-ray plateau phases produced by electromagnetic dipole emissions from central new-born magnetars. A tight correlation is found between the plateau luminosity and the end time of the plateau in X-ray afterglows out to the redshift $z=5.91$. We standardize these long GRBs X-ray light curves to a universal behavior by this correlation for the first time, with a luminosity dispersion of 0.5 dex. The derived distance-redshift relation of GRBs is in agreement with the standard $Lambda$CDM model both at low and high redshifts. The evidence of accelerating universe from this GRB sample is $3sigma$, which is the highest statistical significance from GRBs to date.
The current event rate estimates of long gamma-ray bursts based on distinct methods or samples especially at lower redshift are largely debated, which motivates us to re-study the dependence of luminosity function and event rates for different burst samples on the criteria of sample selection and threshold effect in this letter. To ensure the sample completeness as possible, we have chosen two samples including 88 and 118 long bright bursts with known redshift and peak flux over 2.6 ph cm$^{-2}$ s$^{-1}$. It is found that the evolution of luminosity with redshift can be expressed by $Lpropto(1+z)^k$ with a diverse $k$ relied more on the sample selection. Interestingly, the cumulative distributions of either non-evolving luminosities or redshifts are found to be also determined by the sample selection rather the instrumental sensitivity. Nevertheless, the non-evolving luminosities of our samples are similarly distributed with a comparable break luminosity of $L_0sim10^{51}$ erg s$^{-1}$. Importantly, we verify with a K-S test that three cases of event rates for the two burst samples evolve with redshift similarly except a small discrepancy due to sampling differences at low-redshift of $z<1$, in which all event rates show an excess of gaussian profile instead of monotonous decline. Most importantly, it is found that the low-redshift burst event rates violate the star formation rates, while both of them are good in agreement with each other in the higher-redshift regions as many authors discovered previously. Consequently, we predict that two types of long gamma-ray bursts should be expected on the basis of whether they match the star formation or not.
Long-duration gamma-ray bursts (LGRBs) are the signatures of extraordinarily high-energy events occurring in our universe. Since their discovery, we have determined that these events are produced during the core-collapse deaths of rare young massive stars. The host galaxies of LGRBs are an excellent means of probing the environments and populations that produce their unusual progenitors. In addition, these same young stellar progenitors makes LGRBs and their host galaxies valuable potentially powerful tracers of star formation and metallicity at high redshifts. However, properly utilizing LGRBs as probes of the early universe requires a thorough understanding of their formation and the host environments that they sample. This review looks back at some of the recent work on LGRB host galaxies that has advanced our understanding of these events and their cosmological applications, and considers the many new questions that we are poised to pursue in the coming years.
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