The low luminosity behaviour of the 4U 0115+63 Be/X-ray transient

The Be/X-ray transient 4U 0115+63 exhibited a giant, type-II outburst in October 2015. The source did not decay to its quiescent state but settled in a meta-stable plateau state (a factor ~10 brighter than quiescence) in which its luminosity slowly decayed. We used XMM-Newton to observe the system during this phase and we found that its spectrum can be well described using a black-body model with a small emitting radius. This suggests emission from hot spots on the surface, which is confirmed by the detection of pulsations. In addition, we obtained a relatively long (~7.9 ksec) Swift/XRT observation ~35 days after our XMM-Newton one. We found that the source luminosity was significantly higher and, although the spectrum could be fitted with a black-body model the temperature was higher and the emitting radius smaller. Several weeks later the system started a sequence of type-I accretion outbursts. In between those outbursts, the source was marginally detected with a luminosity consistent with its quiescent level. We discuss our results in the context of the three proposed scenarios (accretion down to the magnestospheric boundary, direct accretion onto neutron star magnetic poles or cooling of the neutron star crust) to explain the plateau phase.Comment: 8 pages, 4 figures, 2 tables, accepted for publication in MNRA

Dramatic spectral transition of X-ray pulsar GX 304-1 in low luminous state

We report on the discovery of a dramatic change in the energy spectrum of the X-ray pulsar GX 304-1 appearing at low luminosity. Particularly, we found that the cutoff power-law spectrum typical for accreting pulsars, including GX 304-1 at higher luminosities of $L_{\rm X}\sim 10^{36} - 10^{37}$ erg s$^{-1}$, transformed at lower luminosity of $L_{\rm X}\sim 10^{34}$ erg s$^{-1}$ to a two-component spectrum peaking around 5 and 40 keV. We suggest that the observed transition corresponds to a change of the dominant mechanism responsible for the deceleration of the accretion flow. We argue that the accretion flow energy at low accretion rates is released in the atmosphere of the neutron star, and the low-energy component in the source spectrum corresponds to the thermal emission of the optically thick, heated atmospheric layers. The most plausible explanations for the high-energy component are either the cyclotron emission reprocessed by the magnetic Compton scattering or the thermal radiation of deep atmospheric layers partly Comptonized in the overheated upper layers. Alternative scenarios are also discussed.Comment: 5 pages, 2 figures, accepted by MNRAS Letter

The distant, galaxy cluster environment of the short GRB 161104A at z∼0.8z\sim 0.8 and a comparison to the short GRB host population

We present optical observations of the Swift short-duration gamma-ray burst (GRB) GRB 161104A and its host galaxy at $z=0.793 \pm 0.003$. We model the multiband photometry and spectroscopy with the stellar population inference code Prospector, and explore the posterior using nested sampling. We find that the mass-weighted age $t_m = 2.12^{+0.23}_{-0.21}$~Gyr, stellar mass $\log{(M/M_\odot)} = 10.21 \pm 0.04$, metallicity $\log{(Z/Z_\odot)} = 0.08^{+0.05}_{-0.06}$, dust extinction $A_V = 0.08^{+0.08}_{-0.05}$ mag, and the star formation rate $\text{SFR} = 9.9 \times 10^{-2} M_\odot$~yr$^{-1}$. These properties, along with a prominent 4000 Angstrom break and optical absorption lines classify this host as an early-type, quiescent galaxy. Using Dark Energy Survey galaxy catalogues, we demonstrate that the host of GRB 161104A resides on the outskirts of a galaxy cluster at $z\approx 0.8$, situated $\approx 1$ Mpc from the likely brightest cluster galaxy. We also present new modeling for 20 additional short GRB hosts ($\approx33\%$ of which are early-type galaxies), finding population medians of $\log(M/M_\odot) = 9.94^{+0.88}_{-0.98}$ and $t_m = 1.07^{+1.98}_{-0.67}$~Gyr ($68\%$ confidence). We further find that the host of GRB 161104A is more distant, less massive, and younger than the four other short GRB hosts known to be associated with galaxy clusters. Cluster short GRBs have faint afterglows, in the lower $\approx 11\%$ ($\approx 30\%$) of observed X-ray (optical) luminosities. We place a lower limit on the fraction of short GRBs in galaxy clusters versus those in the field of $\approx 5-13\%$, consistent with the fraction of stellar mass $\approx 10-20\%$ in galaxy clusters at redshifts $0.1 \leq z \leq 0.8$.Comment: 20 pages, 9 figures, ApJ: Vol. 904, No.

Short GRB Host Galaxies. II. A Legacy Sample of Redshifts, Stellar Population Properties, and Implications for their Neutron Star Merger Origins

We present the stellar population properties of 69 short gamma-ray burst (GRB) host galaxies, representing the largest uniformly-modeled sample to-date. Using the Prospector stellar population inference code, we jointly fit photometry and/or spectroscopy of each host galaxy. We find a population median redshift of $z=0.64^{+0.83}_{-0.32}$ ($68\%$ confidence), including 10 new or revised photometric redshifts at $z\gtrsim1$. We further find a median mass-weighted age of $t_m=0.8^{+2.71}_{-0.53}$Gyr, stellar mass of $\log(M_*/M_\odot)=9.69^{+0.75}_{-0.65}$, star formation rate of SFR=$1.44^{+9.37}_{-1.35}M_\odot$yr$^{-1}$, stellar metallicity of $\log(Z_*/Z_\odot)=-0.38^{+0.44}_{-0.42}$, and dust attenuation of $A_V=0.43^{+0.85}_{-0.36}$~mag (68\% confidence). Overall, the majority of short GRB hosts are star-forming ($\approx84\%$), with small fractions that are either transitioning ($\approx6\%$) or quiescent ($\approx10\%$); however, we observe a much larger fraction ($\approx40\%$) of quiescent and transitioning hosts at $z\lesssim0.25$, commensurate with galaxy evolution. We find that short GRB hosts populate the star-forming main sequence of normal field galaxies, but do not include as many high-mass galaxies, implying that their binary neutron star (BNS) merger progenitors are dependent on a combination of host star formation and stellar mass. The distribution of ages and redshifts implies a broad delay-time distribution, with a fast-merging channel at $z>1$ and a decreased BNS formation efficiency at lower redshifts. If short GRB hosts are representative of BNS merger hosts within the horizon of current gravitational wave detectors, these results can inform future searches for electromagnetic counterparts. All of the data and modeling products are available on the BRIGHT website.Comment: 32 pages, 15 figures, 3 tables, accepted to Ap

Mapping Obscured Star Formation in the Host Galaxy of FRB 20201124A

We present high-resolution 1.5--6 GHz Karl G. Jansky Very Large Array (VLA) and $\textit{Hubble Space Telescope}$ ($\textit{HST}$) optical and infrared observations of the extremely active repeating fast radio burst (FRB) FRB$\,$20201124A and its barred spiral host galaxy. We constrain the location and morphology of star formation in the host and search for a persistent radio source (PRS) coincident with FRB$\,$20201124A. We resolve the morphology of the radio emission across all frequency bands and measure a star formation rate SFR $\approx 8.9\,M_{\odot}$ yr$^{-1}$, a factor of $\approx 4-6$ larger than optically-inferred SFRs, demonstrating dust-obscured star formation throughout the host. Compared to a sample of all known FRB hosts with radio emission, the host of FRB$\,$20201124A has the most significant obscured star formation. While ${\it HST}$ observations show the FRB to be offset from the bar or spiral arms, the radio emission extends to the FRB location. We propose that the FRB progenitor could have formed $\textit{in situ}$ (e.g., a magnetar central engine born from the explosion of a massive star). It is still plausible, although less likely, that the progenitor of FRB$\,$20201124A migrated from the central bar of the host, e.g., via a runaway massive star. We further place a limit on the luminosity of a putative PRS at the FRB position of $L_{\rm 6.0 \ GHz}$$\lesssim$2.6$\times$$10^{27}$erg s$^{-1}$Hz$^{-1}$, two orders of magnitude below any PRS known to date. However, this limit is still broadly consistent with both magnetar nebulae and hypernebulae models assuming a constant energy injection rate of the magnetar and an age of$\gtrsim 10^{5}$ yr in each model, respectively.Comment: 21 pages, 6 figures, 3 tables, Submitte

Short GRB Host Galaxies I: Photometric and Spectroscopic Catalogs, Host Associations, and Galactocentric Offsets

We present a comprehensive optical and near-infrared census of the fields of 90 short gamma-ray bursts (GRBs) discovered in 2005-2021, constituting all short GRBs for which host galaxy associations are feasible ($\approx$ 60% of the total Swift short GRB population). We contribute 245 new multi-band imaging observations across 49 distinct GRBs and 25 spectra of their host galaxies. Supplemented by literature and archival survey data, the catalog contains 335 photometric and 40 spectroscopic data sets. The photometric catalog reaches $3\sigma$ depths of $\gtrsim 24-27$ mag and $\gtrsim 23-26$ mag for the optical and near-infrared bands, respectively. We identify host galaxies for 84 bursts, in which the most robust associations make up 54% (49/90) of events, while only a small fraction, 6.7%, have inconclusive host associations. Based on new spectroscopy, we determine 17 host spectroscopic redshifts with a range of $z\approx 0.15-1.6$ and find that $\approx$ 25-44% of Swift short GRBs originate from $z>1$. We also present the galactocentric offset catalog for 83 short GRBs. Taking into account the large range of individual measurement uncertainties, we find a median of projected offset of $\approx 7.9$ kpc, for which the bursts with the most robust associations have a smaller median of $\approx 4.9$ kpc. Our catalog captures more high-redshift and low-luminosity hosts, and more highly-offset bursts than previously found, thereby diversifying the population of known short GRB hosts and properties. In terms of locations and host luminosities, the populations of short GRBs with and without detectable extended emission are statistically indistinguishable. This suggests that they arise from the same progenitors, or from multiple progenitors which form and evolve in similar environments. All of the data products are available on the BRIGHT website.Comment: 53 pages, 9 figures, 6 tables, submitte

A Radio Flare in the Long-Lived Afterglow of the Distant Short GRB 210726A: Energy Injection or a Reverse Shock from Shell Collisions?

We present the discovery of the radio afterglow of the short $\gamma$-ray burst (GRB) 210726A, localized to a galaxy at a photometric redshift of $z\sim 2.4$. While radio observations commenced $\lesssim 1~$day after the burst, no radio emission was detected until $\sim11$~days. The radio afterglow subsequently brightened by a factor of $\sim 3$ in the span of a week, followed by a rapid decay (a ``radio flare''). We find that a forward shock afterglow model cannot self-consistently describe the multi-wavelength X-ray and radio data, and underpredicts the flux of the radio flare by a factor of $\approx 5$. We find that the addition of substantial energy injection, which increases the isotropic kinetic energy of the burst by a factor of $\approx 4$, or a reverse shock from a shell collision are viable solutions to match the broad-band behavior. At $z\sim 2.4$, GRB\,210726A is among the highest redshift short GRBs discovered to date as well as the most luminous in radio and X-rays. Combining and comparing all previous radio afterglow observations of short GRBs, we find that the majority of published radio searches conclude by $\lesssim 10~$days after the burst, potentially missing these late rising, luminous radio afterglows.Comment: 28 pages, 10 figures, submitted to Ap

Heavy element production in a compact object merger observed by JWST

The mergers of binary compact objects such as neutron stars and black holes are of central interest to several areas of astrophysics, including as the progenitors of gamma-ray bursts (GRBs) 1, sources of high-frequency gravitational waves (GWs) 2 and likely production sites for heavy-element nucleosynthesis by means of rapid neutron capture (the r-process) 3. Here we present observations of the exceptionally bright GRB 230307A. We show that GRB 230307A belongs to the class of long-duration GRBs associated with compact object mergers 4–6 and contains a kilonova similar to AT2017gfo, associated with the GW merger GW170817 (refs. 7–12). We obtained James Webb Space Telescope (JWST) mid-infrared imaging and spectroscopy 29 and 61 days after the burst. The spectroscopy shows an emission line at 2.15 microns, which we interpret as tellurium (atomic mass A = 130) and a very red source, emitting most of its light in the mid-infrared owing to the production of lanthanides. These observations demonstrate that nucleosynthesis in GRBs can create r-process elements across a broad atomic mass range and play a central role in heavy-element nucleosynthesis across the Universe