Gamma-ray Bursts (GRBs) are the most explosive phenomena in the universe after the big bang. A large fraction of GRB lightcurves (LCs) shows X-ray plateaus. We perform the most comprehensive analysis of all GRBs (with known and unknown redshifts) with plateau emission observed by The Neil Gehrels Swift Observatory from its launch until 2019 August. We fit 455 LCs showing a plateau and explore whether these LCs follow closure relations, relations between the temporal and spectral indices of the afterglow, corresponding to two distinct astrophysical environments and cooling regimes within the external forward shock (ES) model, and find that the ES model works for the majority of cases. The most favored environments are a constant-density interstellar or wind medium with slow cooling. We also confirm the existence of the fundamental plane relation between the restframe time and luminosity at the end of the plateau emission and the peak prompt luminosity for this enlarged sample, and test this relation on groups corresponding to the astrophysical environments of our known redshift sample. The plane becomes a crucial discriminant corresponding to these environments in terms of the best-fitting parameters and dispersions. Most GRBs for which the closure relations are fulfilled with respect to astrophysical environments have an intrinsic scatter σ compatible within 1σ of that of the "Gold" GRBs, a subset of long GRBs with relatively flat plateaus. We also find that GRBs satisfying closure relations indicating a fast cooling regime have a lower σ than ever previously found in literature.

The nova rate in the Milky Way remains largely uncertain, despite its vital importance in constraining models of Galactic chemical evolution as well as understanding progenitor channels for Type Ia supernovae. The rate has been previously estimated to be in the range of ≈10-300 yr −1 , either based on extrapolations from a handful of very bright optical novae or the nova rates in nearby galaxies; both methods are subject to debatable assumptions. The total discovery rate of optical novae remains much smaller (≈5-10 yr −1 ) than these estimates, even with the advent of all-sky optical time-domain surveys. Here, we present a systematic sample of 12 spectroscopically confirmed Galactic novae detected in the first 17 months of Palomar Gattini-IR (PGIR), a wide-field near-infrared time-domain survey. Operating in the J band (≈1.2 μm), which is significantly less affected by dust extinction compared to optical bands, the extinction distribution of the PGIR sample is highly skewed to a large extinction values (>50% of events obscured by A V  5 mag). Using recent estimates for the distribution of Galactic mass and dust, we show that the extinction distribution of the PGIR sample is commensurate with dust models. The PGIR extinction distribution is inconsistent with that reported in previous optical searches (null-hypothesis probability <0.01%), suggesting that a large population of highly obscured novae have been systematically missed in previous optical searches. We perform the first quantitative simulation of a 3π time-domain survey to estimate the Galactic nova rate using PGIR, and derive a rate of » -+ 43.7 8.719.5 yr −1 . Our results suggest that all-sky near-infrared timedomain surveys are well poised to uncover the Galactic nova population.

The Neil Gehrels Swift observatory observes gamma-ray burst (GRB) plateaus in X-rays. We test the reliability of the closure relations through the fireball model when dealing with GRB plateau emissions. We analyze 455 X-ray light curves collected by Swift from 2005 January until 2019 August for which the redshifts are either known or unknown using the phenomenological Willingale 2007 model. Using these fits, we analyze the emission mechanisms and astrophysical environments of these GRBs through the closure relations within the time interval of the plateau emission. Finally, we test the three-dimensional fundamental plane relation (Dainotti relation) which connects the prompt peak luminosity, the time at the end of the plateau (rest frame), and the luminosity at that time, for the GRBs with redshift, concerning groups determined by the closure relations. This allows us to check if the intrinsic scatter σint of any of these groups is reduced compared to previous literature. The most fulfilled environments for the electron spectral distribution, p &gt; 2, are wind slow cooling (SC) and interstellar material (ISM) slow cooling for cases in which the parameter q, which indicates the flatness of the plateau emission and accounts for the energy injection, is 0 and 0.5, respectively, in cases with both known and unknown redshifts. We also find that for short GRBs all ISM environments with q = 0 have the smallest σint = 0.04 ± 0.15 in terms of the fundamental plane relation holding a probability of occurring by chance of p = 0.005. We have shown that the majority of GRBs presenting plateau emission fulfill the closure relations, including the energy injection, with a particular preference for the wind SC environment. The subsample of GRBs that fulfill the given relations can be used as possible standard candles and can suggest a way to reduce the intrinsic scatter of these studied relationships.

Long-duration gamma-ray bursts (GRBs) are powerful cosmic explosions, signaling the death of massive stars. Among them, GRB 221009A is by far the brightest burst ever observed. Because of its enormous energy ( E iso ≈ 10 55 erg) and proximity ( z ≈ 0.15), GRB 221009A is an exceptionally rare event that pushes the limits of our theories. We present multiwavelength observations covering the first 3 months of its afterglow evolution. The x-ray brightness decays as a power law with slope ≈ t −1.66 , which is not consistent with standard predictions for jetted emission. We attribute this behavior to a shallow energy profile of the relativistic jet. A similar trend is observed in other energetic GRBs, suggesting that the most extreme explosions may be powered by structured jets launched by a common central engine.

The Galactic magnetar SGR 1935+2154 has been reported to produce the first example of a bright millisecondduration radio burst (FRB 200428) similar to the cosmological population of fast radio bursts (FRBs). The detection of a coincident bright X-ray burst represents the first observed multiwavelength counterpart of an FRB. However, the search for similar emission at optical wavelengths has been hampered by the high inferred extinction on the line of sight. Here, we present results from the first search for second-timescale emission from the source at near-infrared (NIR) wavelengths using the Palomar Gattini-IR observing system in the J band, enabled by a novel detector readout mode that allows short exposure times of ≈0.84 s with 99.9% observing efficiency. With a total observing time of ≈12 hr (≈47,728 images) during its 2020 outburst, we place median 3σ limits on the secondtimescale NIR fluence of 18Jy ms (13.1 AB mag). The corresponding extinction-corrected limit is 125Jy ms for an estimated extinction of A J =2.0 mag. Our observations were sensitive enough to easily detect an NIR counterpart of FRB 200428 if the NIR emission falls on the same power law as observed across its radio to X-ray spectrum. We report nondetection limits from epochs of four simultaneous X-ray bursts detected by the Insight-HXMT and NuSTAR telescopes during our observations. These limits provide the most stringent constraints to date on fluence of flares at ∼10 14 Hz, and constrain the fluence ratio of the NIR emission to coincident X-ray bursts to R NIR 0.025 (fluence index 0.35).