arXiv subjects

Nial R. Tanvir

Publications and source records attributed to Nial R. Tanvir.

At least 37 records · Page 2Linked to original sources

arXiv2025

The kangaroo's first hop: the early fast cooling phase of EP250108a/SN 2025kg

Fast X-ray transients (FXTs) are a rare and poorly understood population of events. Previously difficult to detect in real time, the launch of the Einstein Probe with its wide field X-ray telescope has led to a rapid expansion in the sample and allowed the exploration of these transients across the electromagnetic spectrum. EP250108a is a recently detected example linked to an optical counterpart, SN 2025kg, or 'the kangaroo'. Together with a companion paper (Rastinejad et al. 2025), we present our observing campaign and analysis of this event. In this letter, we focus on the early evolution of the optical counterpart over the first six days, including our measurement of the redshift of $z=0.17641$. We find that the source is well-modelled by a rapidly expanding cooling blackbody. We show the observed X-ray and radio properties are consistent with a collapsar-powered jet that is low energy ($\lesssim10^{51}$ erg) and/or fails to break out of the dense material surrounding it. While we examine the possibility that the optical emission emerges from the shock produced as the supernova ejecta expand into a dense shell of circumstellar material, due to our X-ray and radio inferences, we favour a model where it arises from a shocked cocoon resulting from the trapped jet. This makes SN 2025kg one of the few examples of this currently observationally rare event.

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arXiv2025

Accreted Globular Clusters and Horizontal Branch Morphology in the Outer Halo of M31

M31 hosts a rich population of outer halo ($R_{\rm{proj}} > 25$ kpc) globular clusters (GCs), many of which show strong evidence for spatial and/or kinematical associations with large-scale tidal debris features. We present deep Hubble Space Telescope photometry of 48 halo GCs, including 18 with clear ties to stellar streams and 13 with potential associations. Using the colour-magnitude diagrams (CMDs), we quantify the horizontal branch (HB) morphologies and employ new empirical relationships, calibrated on Milky Way (MW) GCs, to consistently derive metallicities and line-of-sight extinctions. We find a remarkable correlation between HB morphology and the presence of substructure: GCs with very red HBs are almost exclusively associated with substructure, while non-substructure GCs have extended blue HBs. This provides the first direct evidence that red HB halo clusters originate from satellite accretion, a notion introduced nearly 50 years ago from MW studies which has remained unconfirmed until now. In addition to a more metal-rich tail, the substructure GC sample also contains a few clusters with very low metallicities and red HBs, unlike any objects known in the MW. We suggest these are recently accreted young clusters, supporting the growing evidence that M31 has experienced a more prolonged accretion history than the MW.

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arXiv2024

The Einstein Probe transient EP240414a: Linking Fast X-ray Transients, Gamma-ray Bursts and Luminous Fast Blue Optical Transients

Detections of fast X-ray transients (FXTs) have been accrued over the last few decades. However, their origin has remained mysterious. There is now rapid progress thanks to timely discoveries and localisations with the Einstein Probe mission. Early results indicate that FXTs may frequently, but not always, be associated with gamma-ray bursts (GRBs). Here, we report on the multi-wavelength counterpart of FXT EP240414a, which has no reported gamma-ray counterpart. The transient is located 25.7~kpc in projection from a massive galaxy at $z=0.40$. We perform comprehensive photometric and spectroscopic follow-up. The optical light curve shows at least three distinct emission episodes with timescales of $\sim 1, 4$ and 15 days and peak absolute magnitudes of $M_R \sim -20$, $-21$, and $-19.5$, respectively. The optical spectrum at early times is extremely blue, inconsistent with afterglow emission. It may arise from the interaction of both jet and supernova shock waves with the stellar envelope and a dense circumstellar medium, as has been suggested for some Fast Blue Optical Transients (LFBOTs). At late times, the spectrum evolves to a broad-lined~Type~Ic supernova, similar to those seen in collapsar long-GRBs. This implies that the progenitor of EP240414a is a massive star creating a jet-forming supernova inside a dense envelope, resulting in an X-ray outburst with a luminosity of $\sim 10^{48}$ erg s$^{-1}$, and the complex observed optical/IR light curves. If correct, this argues for a causal link between the progenitors of long-GRBs, FXTs and LFBOTs.

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arXiv2024

A massive, neutral gas reservoir permeating a galaxy proto-cluster after the reionization era

Galaxy clusters are the most massive, gravitationally-bound structures in the Universe, emerging through hierarchical structure formation of large-scale dark matter and baryon overdensities. Early galaxy ``proto-clusters'' are believed to be important physical drivers of the overall cosmic star-formation rate density and serve as ``hotspots'' for the reionization of the intergalactic medium. Our understanding of the formation of these structures at the earliest cosmic epochs is, however, limited to sparse observations of their galaxy members, or based on phenomenological models and cosmological simulations. Here we report the detection of a massive neutral, atomic hydrogen (HI) gas reservoir permeating a galaxy proto-cluster at redshift $z=5.4$, observed one billion years after the Big Bang. The presence of this cold gas is revealed by strong damped Lyman-$α$ absorption features observed in several background galaxy spectra taken with JWST/NIRSpec in close on-sky projection. While overall the sightlines probe a large range in HI column densities, $N_{\rm HI} = 10^{21.7}-10^{23.5}$ cm$^{-2}$, they are similar across nearby sightlines, demonstrating that they probe the same dense, neutral gas. This observation of a massive, large-scale overdensity of cold neutral gas challenges current large-scale cosmological simulations and has strong implications for the reionization topology of the Universe.

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arXiv2024

The fast X-ray transient EP240315a: a z ~ 5 gamma-ray burst in a Lyman continuum leaking galaxy

The nature of the minute-to-hour long Fast X-ray Transients (FXTs) localised by telescopes such as Chandra, Swift, and XMM-Newton remains mysterious, with numerous models suggested for the events. Here, we report multi-wavelength observations of EP240315a, a 1600 s long transient detected by the Einstein Probe, showing it to have a redshift of z=4.859. We measure a low column density of neutral hydrogen, indicating that the event is embedded in a low-density environment, further supported by direct detection of leaking ionising Lyman-continuum. The observed properties are consistent with EP240315a being a long-duration gamma-ray burst, and these observations support an interpretation in which a significant fraction of the FXT population are lower-luminosity examples of similar events. Such transients are detectable at high redshifts by the Einstein Probe and, in the (near) future, out to even larger distances by SVOM, THESEUS, and Athena, providing samples of events into the epoch of reionisation.

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arXiv2023

Constraints on the $z\sim5$ Star-Forming Galaxy Luminosity Function From $\textit{Hubble Space Telescope}$ Imaging of an Unbiased and Complete Sample of Long Gamma-ray Burst Host Galaxies

We present rest-frame UV \textit{Hubble Space Telescope} imaging of the largest and most complete sample of 23 long duration gamma-ray burst (GRB) host galaxies between redshifts 4 and 6. Of these 23, we present new WFC3/F110W imaging for 19 of the hosts, which we combine with archival WFC3/F110W and WFC3/F140W imaging for the remaining four. We use the photometry of the host galaxies from this sample to characterize both the rest-frame UV luminosity function (LF) and the size-luminosity relation of the sample. We find that when assuming the standard Schechter-function parameterization for the UV LF, the GRB host sample is best fit with $α= -1.30^{+0.30}_{-0.25}$ and $M_* = -20.33^{+0.44}_{-0.54}$ mag, which is consistent with results based on $z\sim5$ Lyman-break galaxies. We find that $\sim68\%$ of our size-luminosity measurements fall within or below the same relation for Lyman-break galaxies at $z\sim4$. This study observationally confirms expectations that at $z\sim5$ Lyman-break and GRB host galaxies should trace the same population and demonstrates the utility of GRBs as probes of hidden star-formation in the high-redshift universe. Under the assumption that GRBs unbiasedly trace star formation at this redshift, our non-detection fraction of 7/23 is consistent at the $95\%$-confidence level with $13 - 53\%$ of star formation at redshift $z\sim5$ occurring in galaxies fainter than our detection limit of $M_{1600 A} \sim -18.3$ mag.

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arXiv2023

Late time HST UV and optical observations of AT~2018cow: extracting a cow from its background

The bright, blue, rapidly evolving AT2018cow is a well-studied peculiar extragalactic transient. Despite an abundance of multi-wavelength data, there still is no consensus on the nature of the event. We present our analysis of three epochs of Hubble Space Telescope (HST) observations spanning the period from 713-1474 days post burst, paying particular attention to uncertainties of the transient photometry introduced by the complex background in which AT2018cow resides. Photometric measurements show evident fading in the UV and more subtle but significant fading in the optical. During the last HST observation, the transient's optical/UV colours were still bluer than those of the substantial population of compact, young, star-forming regions in the host of AT2018cow, suggesting some continued transient contribution to the light. However, a compact source underlying the transient would substantially modify the resulting spectral energy distribution, depending on its contribution in the various bands. In particular, in the optical filters, the complex, diffuse background poses a problem for precise photometry. An underlying cluster is expected for a supernova occurring within a young stellar environment or a tidal-disruption event (TDE) within a dense older one. While many recent works have focused on the supernova interpretation, we note the substantial similarity in UV light-curve morphology between AT2018cow and several tidal disruption events around supermassive black holes. Assuming AT2018cow arises from a TDE-like event, we fit the late-time emission with a disc model and find $M_{BH} = 10^{3.2{\pm}0.8}$ M$_{\odot}$. Further observations are necessary to determine the late-time evolution of the transient and its immediate environment.

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arXiv2023

Extreme damped Lyman-$α$ absorption in young star-forming galaxies at $z=9-11$

The onset of galaxy formation is thought to be initiated by the infall of neutral, pristine gas onto the first protogalactic halos. However, direct constraints on the abundance of neutral atomic hydrogen (HI) in galaxies have been difficult to obtain at early cosmic times. Here we present spectroscopic observations with JWST of three galaxies at redshifts $z=8.8 - 11.4$, about $400-600$ Myr after the Big Bang, that show strong damped Lyman-$α$ absorption ($N_{\rm HI} > 10^{22}$ cm$^{-2}$) from HI in their local surroundings, an order of magnitude in excess of the Lyman-$α$ absorption caused by the neutral intergalactic medium at these redshifts. Consequently, these early galaxies cannot be contributing significantly to reionization, at least at their current evolutionary stages. Simulations of galaxy formation show that such massive gas reservoirs surrounding young galaxies so early in the history of the universe is a signature of galaxy formation in progress.

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arXiv2023

A long-duration gamma-ray burst of dynamical origin from the nucleus of an ancient galaxy

The majority of long duration ($>2$ s) gamma-ray bursts (GRBs) are believed to arise from the collapse of massive stars \cite{Hjorth+03}, with a small proportion created from the merger of compact objects. Most of these systems are likely formed via standard stellar evolution pathways. However, it has long been thought that a fraction of GRBs may instead be an outcome of dynamical interactions in dense environments, channels which could also contribute significantly to the samples of compact object mergers detected as gravitational wave sources. Here we report the case of GRB 191019A, a long GRB (T_90 = 64.4 +/- 4.5 s) which we pinpoint close (<100 pc projected) to the nucleus of an ancient (>1~Gyr old) host galaxy at z=0.248. The lack of evidence for star formation and deep limits on any supernova emission make a massive star origin difficult to reconcile with observations, while the timescales of the emission rule out a direct interaction with the supermassive black hole in the nucleus of the galaxy, We suggest that the most likely route for progenitor formation is via dynamical interactions in the dense nucleus of the host, consistent with the centres of such galaxies exhibiting interaction rates up to two orders of magnitude larger than typical field galaxies. The burst properties could naturally be explained via compact object mergers involving white dwarfs (WD), neutron stars (NS) or black holes (BH). These may form dynamically in dense stellar clusters, or originate in a gaseous disc around the supermassive black hole. Future electromagnetic and gravitational-wave observations in tandem thus offer a route to probe the dynamical fraction and the details of dynamical interactions in galactic nuclei and other high density stellar systems.

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arXiv2021

GRB jet structure and the jet break

We investigate the shape of the jet break in within-beam gamma-ray burst (GRB) optical afterglows for various lateral jet structure profiles. We consider cases with and without lateral spreading and a range of inclinations within the jet core half-opening angle, $θ_c$. We fit model and observed afterglow lightcurves with a smoothly-broken power-law function with a free-parameter $κ$ that describes the sharpness of the break. We find that the jet break is sharper ($κ$ is greater) when lateral spreading is included than in the absence of lateral spreading. For profiles with a sharp-edged core, the sharpness parameter has a broad range of $0.1\lesssimκ\lesssim4.6$, whereas profiles with a smooth-edged core have a narrower range of $0.1\lesssimκ\lesssim2.2$ when models both with and without lateral spreading are included. For sharp-edged jets, the jet break sharpness depends strongly on the inclination of the system within $θ_c$, whereas for smooth-edged jets, $κ$ is more strongly dependent on the size of $θ_c$. Using a sample of 20 GRBs we find nine candidate smooth-edged jet structures and eight candidate sharp-edged jet structures, while the remaining three are consistent with either. The shape of the jet break, as measured by the sharpness parameter $κ$, can be used as an initial check for the presence of lateral structure in within-beam GRBs where the afterglow is well-sampled at and around the jet-break time.

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arXiv2021

GRB 180418A: A possibly-short GRB with a wide-angle outflow in a faint host galaxy

We present X-ray and multi-band optical observations of the afterglow and host galaxy of GRB 180418A, discovered by ${\it Swift}$/BAT and ${\it Fermi}$/GBM. We present a reanalysis of the GBM and BAT data deriving durations of the prompt emission of $T_{90}\approx$2.56s and $\approx$1.90s, respectively. Modeling the ${\it Fermi}$/GBM catalog of 1405 bursts (2008-2014) in the Hardness-$T_{90}$ plane, we obtain a probability of $\approx$60% that GRB 180418A is a short-hard burst. From a combination of ${\it Swift}$/XRT and ${\it Chandra}$ observations, the X-ray afterglow is detected to $\approx$38.5 days after the burst, and exhibits a single power-law decline with $F_{\rm X} \propto t^{-0.98}$. Late-time Gemini observations reveal a faint r$\approx$25.69 mag host galaxy at an angular offset of $\approx$0.16''. At the likely redshift range of z$\approx$1-2.25, we find that the X-ray afterglow luminosity of GRB 180418A is intermediate between short and long GRBs at all epochs during which there is contemporaneous data, and that GRB 180418A lies closer to the $E_{γ,{\rm peak}}-E_{γ,{\rm iso}}$ correlation for short GRBs. Modeling the multi-wavelength afterglow with the standard synchrotron model, we derive the burst explosion properties and find a jet opening angle of $θ_{\rm j} \gtrsim 9-14^{\circ}$. If GRB 180418A is a short GRB that originated from a neutron star merger, it has one of the brightest and longest-lived afterglows along with an extremely faint host galaxy. If instead the event is a long GRB that originated from a massive star collapse, it has among the lowest luminosity afterglows, and lies in a peculiar space in terms of the Hardness-$T_{90}$ and $E_{γ,{\rm peak}}-E_{γ,{\rm iso}}$ planes.

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arXiv2020

GRB 170817A as a Refreshed Shock Afterglow viewed off-axis

Energy injection into the external shock system that generates the afterglow to a gamma-ray burst (GRB) can result in a re-brightening of the emission. Here we investigate the off-axis view of a re-brightened refreshed shock afterglow. We find that the afterglow light-curve, when viewed from outside of the jet opening angle, could be characterised by a slow rise, or long-plateau, with a maximum flux determined by the total system energy. Using the broadband afterglow data for GRB170817A, associated with the gravitational wave detected binary neutron star merger GW170817, we show that a refreshed shock model with a simple top-hat jet can reproduce the observed afterglow features. We consider two particular refreshed shock models: a single episode of energy injection; and a period of continuous energy injection. The best fit model parameters give a jet opening angle, for our first or second model of $θ_j=5.2^{+1.1}_{-0.6}~$or$~6.3^{+1.7}_{-1.1}$ deg, an inclination to the line of sight $ι=16.0^{+3.4}_{-1.1}~$or$~17.8^{+4.5}_{-2.9}$ deg, an initial isotropic equivalent kinetic energy $E_1 = (0.3^{+3.5}_{-0.3}~$or$~0.5^{+6.7}_{-0.2})\times10^{52}$erg and a total/final, refreshed shock energy $E_{\rm total}=(0.42^{+5.6}_{-0.4}~$or$~1.26^{+18.2}_{-0.7})\times10^{53}$erg. The first model fitting prefers an initial bulk Lorentz factor $Γ_{0,1}<60$, with a comparatively low central value of $Γ_{0,1}=19.5$, indicating that, in this case, the on-axis jet could have been a `failed-GRB'. Alternatively, our second model is consistent with a bright GRB for an on-axis observer, with $Γ_{0,1}=162.2^{+219.7}_{-122.1}$. Due to the low-Lorentz factor or the jet opening angles at $θ_j\simι/3$, both models are unable to reproduce the $γ$-ray emission observed in GRB170817A, which would therefore require an alternative explanation such as cocoon shock-breakout.

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arXiv2018

The second closest gamma-ray burst: sub-luminous GRB 111005A with no supernova in a super-solar metallicity environment

We report the detection of the radio afterglow of a long gamma-ray burst (GRB) 111005A at 5-345 GHz, including the very long baseline interferometry observations with the positional error of 0.2 mas. The afterglow position is coincident with the disk of a galaxy ESO 580-49 at z= 0.01326 (~1" from its center), which makes GRB 111005A the second closest GRB known to date, after GRB 980425. The radio afterglow of GRB 111005A was an order of magnitude less luminous than those of local low-luminosity GRBs, and obviously than those of cosmological GRBs. The radio flux was approximately constant and then experienced an unusually rapid decay a month after the GRB explosion. Similarly to only two other GRBs, we did not find the associated supernovae (SN), despite deep near- and mid-infrared observations 1-9 days after the GRB explosion, reaching ~20 times fainter than other SNe associated with GRBs. Moreover, we measured twice solar metallicity for the GRB location. The low gamma-ray and radio luminosities, rapid decay, lack of a SN, and super-solar metallicity suggest that GRB 111005A represents a different rare class of GRBs than typical core-collapse events. We modelled the spectral energy distribution of the GRB 111005A host finding that it is a dwarf, moderately star-forming galaxy, similar to the host of GRB 980425. The existence of two local GRBs in such galaxies is still consistent with the hypothesis that the GRB rate is proportional to the cosmic star formation rate (SFR) density, but suggests that the GRB rate is biased towards low SFRs. Using the far-infrared detection of ESO 580-49, we conclude that the hosts of both GRBs 111005A and 980425 exhibit lower dust content than what would be expected from their stellar masses and optical colours.

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arXiv2017

The Distance to NGC 4993: The Host Galaxy of the Gravitational-wave Event GW170817

The historic detection of gravitational waves from a binary neutron star merger (GW170817) and its electromagnetic counterpart led to the first accurate (sub-arcsecond) localization of a gravitational-wave event. The transient was found to be $\sim$10" from the nucleus of the S0 galaxy NGC 4993. We report here the luminosity distance to this galaxy using two independent methods. (1) Based on our MUSE/VLT measurement of the heliocentric redshift ($z_{\rm helio}=0.009783\pm0.000023$) we infer the systemic recession velocity of the NGC 4993 group of galaxies in the cosmic microwave background (CMB) frame to be $v_{\rm CMB}=3231 \pm 53$ km s$^{-1}$. Using constrained cosmological simulations we estimate the line-of-sight peculiar velocity to be $v_{\rm pec}=307 \pm 230$ km s$^{-1}$, resulting in a cosmic velocity of $v_{\rm cosmic}=2924 \pm 236$ km s$^{-1}$ ($z_{\rm cosmic}=0.00980\pm 0.00079$) and a distance of $D_z=40.4\pm 3.4$ Mpc assuming a local Hubble constant of $H_0=73.24\pm 1.74$ km s$^{-1}$ Mpc$^{-1}$. (2) Using Hubble Space Telescope measurements of the effective radius (15.5" $\pm$ 1.5") and contained intensity and MUSE/VLT measurements of the velocity dispersion, we place NGC 4993 on the Fundamental Plane (FP) of E and S0 galaxies. Comparing to a frame of 10 clusters containing 226 galaxies, this yields a distance estimate of $D_{\rm FP}=44.0\pm 7.5$ Mpc. The combined redshift and FP distance is $D_{\rm NGC 4993}= 41.0\pm 3.1$ Mpc. This 'electromagnetic' distance estimate is consistent with the independent measurement of the distance to GW170817 as obtained from the gravitational-wave signal ($D_{\rm GW}= 43.8^{+2.9}_{-6.9}$ Mpc) and confirms that GW170817 occurred in NGC 4993.

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arXiv2017

Lanthanides or dust in kilonovae: lessons learned from GW170817

The unprecedented optical and near-infrared lightcurves of the first electromagnetic counterpart to a gravitational wave source, GW170817, a binary neutron star merger, exhibited a strong evolution from blue to near-infrared (a so-called 'kilonova' or 'macronova'). The emerging near-infrared component is widely attributed to the formation of r-process elements which provide the opacity to shift the blue light into the near infrared. An alternative scenario is that the light from the blue component gets extinguished by dust formed by the kilonova and subsequently is re-emitted at near-infrared wavelengths. We here test this hypothesis using the lightcurves of AT2017gfo, the kilonova accompanying GW170817. We find that of order 10$^{-5}$ $M_\odot$ of carbon is required to reproduce the optical/near-infrared lightcurves as the kilonova fades. This putative dust cools from $\sim$ 2000 K at $\sim$ 4 d after the event to $\sim$ 1500 K over the course of the following week, thus requiring dust with a high condensation temperature, such as carbon. We contrast this with the nucleosynthetic yields predicted by a range of kilonova wind models. These suggest that at most 10$^{-9}$ $M_\odot$ of carbon is formed. Moreover, the decay in the inferred dust temperature is slower than that expected in kilonova models. We therefore conclude that in current models of the blue component of the kilonova, the near-infrared component in the kilonova accompanying GW170817 is unlikely to be due to dust.

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arXiv2017

Dust reddening and extinction curves towards gamma-ray bursts at z > 4

Dust is known to be produced in the envelopes of AGB stars, the expanded shells of supernova (SN) remnants, and in situ grain growth in the ISM, although the corresponding efficiency of each of these dust formation mechanisms at different redshifts remains a topic of debate. During the first Gyr after the Big Bang, it is widely believed that there was not enough time to form AGB stars in high numbers, so that the dust at this epoch is expected to be purely from SNe, or subsequent grain growth in the ISM. The time period corresponding to z ~5-6 is thus expected to display the transition from SN-only dust to a mixture of both formation channels as we know it today. Here we aim to use afterglow observations of GRBs at redshifts larger than $z > 4$ in order to derive host galaxy dust column densities along their line-of-sight and to test if a SN-type dust extinction curve is required for some of the bursts. GRB afterglow observations were performed with the 7-channel GROND Detector at the 2.2m MPI telescope in La Silla, Chile and combined with data gathered with XRT. We increase the number of measured $A_V$ values for GRBs at z > 4 by a factor of ~2-3 and find that, in contrast to samples at mostly lower redshift, all of the GRB afterglows have a visual extinction of $A_V$ < 0.5 mag. Analysis of the GROND detection thresholds and results from a Monte-Carlo simulation show that, although we partly suffer from an observational bias against highly extinguished sight-lines, GRB host galaxies at 4 < z < 6 seem to contain on average less dust than at z ~ 2. Additionally, we find that all of the GRBs can be modeled with locally measured extinction curves and that the SN-like dust extinction curve provides a better fit for only two of the afterglow SEDs. For the first time we also report a photometric redshift of $z = 7.88$ for GRB 100905A, making it one of the most distant GRBs known to date.

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arXiv2016

Late-Time VLA Reobservations Rule Out ULIRG-Like Host Galaxies For Most Pre-Swift Long-Duration Gamma-Ray Bursts

We present new Jansky Very Large Array observations of five pre-Swift gamma-ray bursts for which an ultraluminous (SFR > 100 M_sun/yr) dusty host galaxy had previously been inferred from radio or submillimetre observations taken within a few years after the burst. In four of the five cases we no longer detect any source at the host location to limits much fainter than the original observations, ruling out the existence of an ultraluminous galaxy hosting any of these GRBs. We continue to detect a source at the position of GRB 980703, but it is much fainter than it was a decade ago and the inferred radio star-formation rate (~80 M_sun) is relatively modest. The radio flattening at 200-1000 days observed in the light curve of this GRB may have been caused by a decelerating counterjet oriented 180 degrees away from the viewer, although an unjetted wind model can also explain the data. Our results eliminate all well-established pre-Swift ULIRG hosts, and all cases for which an unobscured GRB was found in a galaxy dominated by heavily-obscured star-formation. When GRBs do occur in ULIRGs the afterglow is almost always observed to be heavily obscured, consistent with the large dust opacities and high dust covering fractions characteristic of these systems.

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arXiv2016

The Afterglow and Early-Type Host Galaxy of the Short GRB 150101B at z=0.1343

We present the discovery of the X-ray and optical afterglows of the short-duration GRB 150101B, pinpointing the event to an early-type host galaxy at z=0.1343 +/- 0.0030. This makes GRB 150101B the most nearby short GRB with an early-type host galaxy discovered to date. Fitting the spectral energy distribution of the host galaxy results in an inferred stellar mass of ~7x10^10 M_sol, stellar population age of ~2-2.5 Gyr, and star formation rate of <0.4 M_sol yr^-1. The host of GRB 150101B is one of the largest and most luminous short GRB host galaxies, with a B-band luminosity of ~4.3L* and half-light radius of ~8 kpc. GRB 150101B is located at a projected distance of 7.35 +/- 0.07 kpc from its host center, and lies on a faint region of its host rest-frame optical light. Its location, combined with the lack of associated supernova, is consistent with a NS-NS/NS-BH merger progenitor. From modeling the evolution of the broad-band afterglow, we calculate isotropic-equivalent gamma-ray and kinetic energies of ~1.3x10^49 erg and ~(6-14)x10^51 erg, respectively, a circumburst density of ~(0.8-4)x10^-5 cm^-3, and a jet opening angle of >9 deg. Using observations extending to ~30 days, we place upper limits of <(2-4)x10^41 erg s^-1 on associated kilonova emission. We compare searches following previous short GRBs to existing kilonova models, and demonstrate the difficulty of performing effective kilonova searches from cosmological short GRBs using current ground-based facilities. We show that at the Advanced LIGO/VIRGO horizon distance of 200 Mpc, searches reaching depths of ~23-24 AB mag are necessary to probe a meaningful range of kilonova models.

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