Publications

2019
Margutti R, Metzger B D, Chornock R, Vurm I, Roth N, Grefenstette B W, Savchenko V, Cartier R, Steiner J F, Terreran G, et al. An Embedded X-Ray Source Shines through the Aspherical AT2018cow: Revealing the Inner Workings of the Most Luminous Fast-evolving Optical Transients. The Astrophysical Journal [Internet]. 2019;872 :18. Publisher's VersionAbstract
We present the first extensive radio to γ-ray observations of a fast-rising blue optical transient, AT 2018cow, over its first ∼100 days. AT 2018cow rose over a few days to a peak luminosity L pk ∼ 4 × 1044 erg s‑1, exceeding that of superluminous supernovae (SNe), before declining as L ∝ t ‑2. Initial spectra at δt ≲ 15 days were mostly featureless and indicated large expansion velocities v ∼ 0.1c and temperatures reaching T ∼ 3 × 104 K. Later spectra revealed a persistent optically thick photosphere and the emergence of H and He emission features with v ∼ 4000 km s‑1 with no evidence for ejecta cooling. Our broadband monitoring revealed a hard X-ray spectral component at E ≥ 10 keV, in addition to luminous and highly variable soft X-rays, with properties unprecedented among astronomical transients. An abrupt change in the X-ray decay rate and variability appears to accompany the change in optical spectral properties. AT 2018cow showed bright radio emission consistent with the interaction of a blast wave with v sh ∼ 0.1c with a dense environment (\dot{M}∼ {10}-3-{10}-4 {M}ȯ {yr}}-1 for v = 1000 km s‑1). While these properties exclude 56Ni-powered transients, our multiwavelength analysis instead indicates that AT 2018cow harbored a “central engine,” either a compact object (magnetar or black hole) or an embedded internal shock produced by interaction with a compact, dense circumstellar medium. The engine released ∼1050–1051.5 erg over ∼103–105 s and resides within low-mass fast-moving material with equatorial–polar density asymmetry (M ej,fast ≲ 0.3 M ). Successful SNe from low-mass H-rich stars (like electron-capture SNe) or failed explosions from blue supergiants satisfy these constraints. Intermediate-mass black holes are disfavored by the large environmental density probed by the radio observations.
Tanvir N R, Fynbo J P U, de Ugarte Postigo A, Japelj J, Wiersema K, Malesani D, Perley D A, Levan A J, Selsing J, Cenko S B, et al. The fraction of ionizing radiation from massive stars that escapes to the intergalactic medium. Monthly Notices of the Royal Astronomical Society [Internet]. 2019;483 :5380-5408. Publisher's VersionAbstract
Whether stars could have driven the reionization of the intergalactic medium depends critically on the proportion of ionizing radiation that escapes the galaxies in which it is produced. Spectroscopy of gamma-ray burst (GRB) afterglows can be used to estimate the opacity to extreme ultraviolet (EUV) radiation along the lines-of-sight to the bursts. Assuming that long-duration GRBs trace the locations of the massive stars dominating EUV production, the average escape fraction of ionizing radiation can be calculated independently of galaxy size or luminosity. Here we present a compilation of H I column density (N_{H I}) measures for 140 GRBs in the range 1.6 < z < 6.7. Although the sample is heterogeneous, in terms of spectral resolution and signal-to-noise ratio, fits to the Ly α absorption line provide robust constraints on N_{H I}, even for spectra of insufficient quality for other purposes. Thus we establish an escape fraction at the Lyman limit of <fesc> ≈ 0.005, with a 98 per cent confidence upper limit of <fesc> ≈ 0.015. This analysis suggests that stars provide a small contribution to the ionizing radiation budget at z < 5. At higher redshifts firm conclusions are limited by the small size of the GRB sample (7/140), but any decline in average H I column density seems to be modest. We also find no significant correlation of N_{H I} with galaxy UV luminosity or host stellar mass. We discuss in some detail potential biases and argue that, while not negligible, systematic errors in fesc are unlikely to be more than a factor ˜2 in either direction, and so would not affect the primary conclusions. Given that many GRB hosts are low-metallicity dwarf galaxies with high specific star-formation rates, these results present a particular problem for the hypothesis that such galaxies dominated the reionization of the Universe.
Alexander K D, Laskar T, Berger E, Johnson M D, Williams P K G, Dichiara S, Fong W, Gomboc A, Kobayashi S, Margutti R, et al. An Unexpectedly Small Emission Region Size Inferred from Strong High-frequency Diffractive Scintillation in GRB 161219B. The Astrophysical Journal [Internet]. 2019;870 :67. Publisher's VersionAbstract
We present Karl G. Jansky Very Large Array radio observations of the long gamma-ray burst GRB 161219B (z = 0.147) spanning 1–37 GHz. The data exhibit unusual behavior, including sharp spectral peaks and minutes-timescale large-amplitude variability centered at 20 GHz and spanning the full frequency range. We attribute this behavior to scattering of the radio emission by the turbulent ionized Galactic interstellar medium (ISM), including both diffractive and refractive scintillation. However, the scintillation is much stronger than predicted by a model of the Galactic electron density distribution (NE2001); from the measured variability timescale and decorrelation bandwidth we infer a scattering measure of SM ≈ (8–70) × 10‑4 kpc m‑20/3 (up to 25 times larger than predicted in NE2001) and a scattering screen distance of d scr ≈ 0.2–3 kpc. We infer an emission region size of {θ }s≈ 0.9{--}4 μas (≈ (1{--}4)× {10}16 cm) at ≈4 days, and find that prior to 8 days the source size is an order of magnitude smaller than model predictions for a uniformly illuminated disk or limb-brightened ring, indicating a slightly off-axis viewing angle or significant substructure in the emission region. Simultaneous multi-hour broadband radio observations of future GRB afterglows will allow us to characterize the scintillation more completely, and hence to probe the observer viewing angle, the evolution of the jet Lorentz factor, the structure of the afterglow emission regions, and ISM turbulence at high Galactic latitudes.
2018
Duffell P C, Laskar T. On the Deceleration and Spreading of Relativistic Jets. I. Jet Dynamics. The Astrophysical Journal [Internet]. 2018;865 :94. Publisher's VersionAbstract
Jet breaks in gamma-ray burst (GRB) afterglows provide a direct probe of their collimation angle. Modeling a jet break requires an understanding of the jet spreading process, whereby the jet transitions from a collimated outflow into the spherical Sedov–Taylor solution at late times. Currently, direct numerical calculations are the most accurate way to capture the deceleration and spreading process, as analytical models have previously given inaccurate descriptions of the dynamics. Here (in paper I) we present a new, semi-analytical model built empirically by performing relativistic numerical jet calculations and inferring the relationship between the Lorentz factor, opening angle, and shock radius. We then use the analytical model to calculate the Lorentz factor and jet opening angle as a function of the shock radius and compare to the numerical solutions. Our analytic model provides efficient means of computing synthetic GRB afterglow light curves and spectra, which is the focus of paper II.
Laskar T, Alexander K D, Berger E, Guidorzi C, Margutti R, Fong W-f., Kilpatrick C D, Milne P, Drout M R, Mundell C G, et al. First ALMA Light Curve Constrains Refreshed Reverse Shocks and Jet Magnetization in GRB 161219B. The Astrophysical Journal [Internet]. 2018;862 :94. Publisher's VersionAbstract
We present detailed multiwavelength observations of GRB 161219B at z = 0.1475, spanning the radio to X-ray regimes, and the first Atacama Large Millimeter/submillimeter Array (ALMA) light curve of a γ-ray burst (GRB) afterglow. The centimeter- and millimeter-band observations before 8.5 days require emission in excess of that produced by the afterglow forward shock (FS). These data are consistent with radiation from a refreshed reverse shock (RS) produced by the injection of energy into the FS, signatures of which are also present in the X-ray and optical light curves. We infer a constant-density circumburst environment with an extremely low density, {n}0≈ 3× {10}-4 {cm}}-3, and show that this is a characteristic of all strong RS detections to date. The Karl G. Lansky Very Large Array (VLA) observations exhibit unexpected rapid variability on roughly minute timescales, indicative of strong interstellar scintillation. The X-ray, ALMA, and VLA observations together constrain the jet break time, {t}jet}≈ 32 days, yielding a wide jet opening angle of {θ }jet}≈ 13^\circ , implying beaming-corrected γ-ray and kinetic energies of {E}γ ≈ 4.9× {10}48 erg and {E}{{K}}≈ 1.3× {10}50 erg, respectively. Comparing the RS and FS emission, we show that the ejecta are only weakly magnetized, with relative magnetization, {R}{{B}}≈ 1, compared to the FS. These direct, multifrequency measurements of a refreshed RS spanning the optical to radio bands highlight the impact of radio and millimeter data in probing the production and nature of GRB jets.
Lunnan R, Chornock R, Berger E, Jones D O, Rest A, Czekala I, Dittmann J, Drout M R, Foley R J, Fong W, et al. Hydrogen-poor Superluminous Supernovae from the Pan-STARRS1 Medium Deep Survey. The Astrophysical Journal [Internet]. 2018;852 :81. Publisher's VersionAbstract
We present light curves and classification spectra of 17 hydrogen-poor superluminous supernovae (SLSNe) from the Pan-STARRS1 Medium Deep Survey (PS1 MDS). Our sample contains all objects from the PS1 MDS sample with spectroscopic classification that are similar to either of the prototypes SN 2005ap or SN 2007bi, without an explicit limit on luminosity. With a redshift range 0.3< z< 1.6, PS1 MDS is the first SLSN sample primarily probing the high-redshift population; our multifilter PS1 light curves probe the rest-frame UV emission, and hence the peak of the spectral energy distribution. We measure the temperature evolution and construct bolometric light curves, and find peak luminosities of (0.5{--}5)× {10}44 erg s-1 and lower limits on the total radiated energies of (0.3{--}2)× {10}51 erg. The light curve shapes are diverse, with both rise and decline times spanning a factor of ˜5 and several examples of double-peaked light curves. When correcting for the flux-limited nature of our survey, we find a median peak luminosity at 4000 Å of {M}4000=-21.1 {mag} and a spread of σ =0.7 {mag}.
Tanvir N R, Laskar T, Levan A J, Perley D A, Zabl J, Fynbo J P U, Rhoads J, Cenko S B, Greiner J, Wiersema K, et al. The Properties of GRB 120923A at a Spectroscopic Redshift of z~7.8. The Astrophysical Journal [Internet]. 2018;865 :107. Publisher's VersionAbstract
Gamma-ray bursts (GRBs) are powerful probes of early stars and galaxies, during and potentially even before the era of reionization. Although the number of GRBs identified at z ≳ 6 remains small, they provide a unique window on typical star-forming galaxies at that time, and thus are complementary to deep field observations. We report the identification of the optical drop-out afterglow of Swift GRB 120923A in near-infrared Gemini-North imaging, and derive a redshift of z={7.84}-0.12+0.06 from Very Large Telescope/X-shooter spectroscopy. At this redshift the peak 15–150 keV luminosity of the burst was 3.2 × 1052 erg s‑1, and in this sense it was a rather typical long-duration GRB in terms of rest frame luminosity. This burst was close to the Swift/Burst Alert Telescope detection threshold, and the X-ray and near-infrared afterglow were also faint. We present ground- and space-based follow-up observations spanning from X-ray to radio, and find that a standard external shock model with a constant-density circumburst environment of density n ≈ 4 × 10‑2 cm‑3 gives a good fit to the data. The near-infrared light curve exhibits a sharp break at t ≈ 3.4 days in the observer frame which, if interpreted as being due to a jet, corresponds to an opening angle of {θ }jet}≈ 5^\circ . The beaming-corrected γ-ray energy is then {E}γ ≈ 2× {10}50 erg, while the beaming-corrected kinetic energy is lower, {E}{{K}}≈ {10}49 erg, suggesting that GRB 120923A was a comparatively low kinetic energy event. We discuss the implications of this event for our understanding of the high-redshift population of GRBs and their identification.
Warren D C, Barkov M V, Ito H, Nagataki S, Laskar T. Synchrotron self-absorption in GRB afterglows: the effects of a thermal electron population. Monthly Notices of the Royal Astronomical Society [Internet]. 2018;480 :4060-4068. Publisher's VersionAbstract
In the standard synchrotron afterglow model, a power law of electrons is responsible for all aspects of photon production and absorption. Recent numerical work has shown that the vast majority of particles in the downstream medium are actually `thermal' particles, which were shock heated but did not enter the Fermi acceleration process (the name stands in contrast to the non-thermal high-energy tail, rather than connoting a Maxwellian distribution). There are substantial differences at optical and higher energies when these thermal electrons participate in the afterglow, but early work along these lines ignored the radio end of the electromagnetic spectrum. We report here on an extension of previous Monte Carlo simulations of gamma-ray burst afterglows. The model now includes the synchrotron self-absorption (SSA) process and so can simulate afterglows across the entire EM spectrum, and several orders of magnitude in time. In keeping with earlier work, inclusion of the thermal electrons increases the SSA frequency by a factor of 30, and the radio intensity by a factor of 100. Furthermore, these changes happen with no modification to the late optical or X-ray afterglow. Our results provide very strong evidence that thermal electrons must be considered in any multiwavelength model for afterglows.
Laskar T, Berger E, Chornock R, Margutti R, Fong W-f., Zauderer B A. A VLA Study of High-redshift GRBs. I. Multiwavelength Observations and Modeling of GRB 140311A. The Astrophysical Journal [Internet]. 2018;858 :65. Publisher's VersionAbstract
We present the first results from a recently concluded study of GRBs at z ≳ 5 with the Karl G. Jansky Very Large Array (VLA). Spanning 1 to 85.5 GHz and 7 epochs from 1.5 to 82.3 days, our observations of GRB 140311A are the most detailed joint radio and millimeter observations of a GRB afterglow at z ≳ 5 to date. In conjunction with optical/near-IR and X-ray data, the observations can be understood in the framework of radiation from a single blast wave shock with energy {E}{{K},{iso}}≈ 8.5× {10}53 erg expanding into a constant density environment with density, {n}0≈ 8 {cm}}-3. The X-ray and radio observations require a jet break at {t}jet}≈ 0.6 days, yielding an opening angle of {θ }jet}≈ 4^\circ and a beaming-corrected blast wave kinetic energy of {E}{{K}}≈ 2.2× {10}50 erg. The results from our radio follow-up and multiwavelength modeling lend credence to the hypothesis that detected high-redshift GRBs may be more tightly beamed than events at lower redshift. We do not find compelling evidence for reverse shock emission, which may be related to fast cooling driven by the moderately high circumburst density.
Laskar T, Berger E, Margutti R, Zauderer B A, Williams P K G, Fong W-f., Sari R, Alexander K D, Kamble A. A VLA Study of High-redshift GRBs. II. The Complex Radio Afterglow of GRB 140304A: Shell Collisions and Two Reverse Shocks. The Astrophysical Journal [Internet]. 2018;859 :134. Publisher's VersionAbstract
We present detailed multifrequency, multiepoch radio observations of GRB 140304A at z = 5.283 from 1 to 86 GHz and from 0.45 to 89 days. The radio and millimeter data exhibit unusual multiple spectral components, which cannot be simply explained by standard forward and reverse shock scenarios. Through detailed multiwavelength analysis spanning radio to X-rays, we constrain the forward shock parameters to E k,iso ≈ 4.9 × 1054erg, {A}≈ 2.6 × 10-2, {ɛ }{{e}} ≈ 2.5 × 10-2, {ɛ }{{B}} ≈ 5.9 × 10-2, p ≈ 2.6, and {θ }jet} ≈ 1.°1, yielding a beaming-corrected γ-ray and kinetic energy, {E}γ ≈ 2.3 × 1049 erg and {E}{{K}} ≈ 9.5 × 1050 erg, respectively. We model the excess radio emission as due to a combination of a late-time reverse shock (RS) launched by a shell collision, which also produces a rebrightening in the X-rays at ≈0.26 days, and either a standard RS or diffractive interstellar scintillation (ISS). Under the standard RS interpretation, we invoke consistency arguments between the forward and reverse shocks to derive a deceleration time, t dec ≈ 100 s, the ejecta Lorentz factor, Γ(t dec) ≈ 300, and a low RS magnetization, R B ≈ 0.6. Our observations highlight both the power of radio observations in capturing RS emission and thus constraining the properties of GRB ejecta and central engines and the challenge presented by ISS in conclusively identifying RS emission in GRB radio afterglows.
2017
Alexander K D, Laskar T, Berger E, Guidorzi C, Dichiara S, Fong W, Gomboc A, Kobayashi S, Kopac D, Mundell C G, et al. A Reverse Shock and Unusual Radio Properties in GRB 160625B. The Astrophysical Journal [Internet]. 2017;848 :69. Publisher's VersionAbstract
We present multi-wavelength observations and modeling of the exceptionally bright long γ-ray burst GRB 160625B. The optical and X-ray data are well fit by synchrotron emission from a collimated blastwave with an opening angle of {θ }j≈ 3\buildrel{\circ}\over{.} 6 and kinetic energy of {E}K≈ 2× {10}51 erg, propagating into a low-density (n≈ 5× {10}-5 cm-3) medium with a uniform profile. The forward shock is sub-dominant in the radio band; instead, the radio emission is dominated by two additional components. The first component is consistent with emission from a reverse shock, indicating an initial Lorentz factor of {{{Γ }}}0≳ 100 and an ejecta magnetization of {R}B≈ 1{--}100. The second component exhibits peculiar spectral and temporal evolution and is most likely the result of scattering of the radio emission by the turbulent Milky Way interstellar medium (ISM). Such scattering is expected in any sufficiently compact extragalactic source and has been seen in GRBs before, but the large amplitude and long duration of the variability seen here are qualitatively more similar to extreme scattering events previously observed in quasars, rather than normal interstellar scintillation effects. High-cadence, broadband radio observations of future GRBs are needed to fully characterize such effects, which can sensitively probe the properties of the ISM and must be taken into account before variability intrinsic to the GRB can be interpreted correctly.
Ressler S M, Laskar T. Thermal Electrons in Gamma-Ray Burst Afterglows. The Astrophysical Journal [Internet]. 2017;845 :150. Publisher's VersionAbstract
To date, nearly all multi-wavelength modeling of long-duration γ-ray bursts has ignored synchrotron radiation from the significant population of electrons expected to pass the shock without acceleration into a power-law distribution. We investigate the effect of including the contribution of thermal, non-accelerated electrons to synchrotron absorption and emission in the standard afterglow model, and show that these thermal electrons provide an additional source of opacity to synchrotron self-absorption, and yield an additional emission component at higher energies. The extra opacity results in an increase in the synchrotron self-absorption frequency by factors of 10-100 for fiducial parameters. The nature of the additional emission depends on the details of the thermal population, but is generally observed to yield a spectral peak in the optical brighter than radiation from the nonthermal population by similar factors a few seconds after the burst, remaining detectable at millimeter and radio frequencies several days later.
2016
Fong W, Margutti R, Chornock R, Berger E, Shappee B J, Levan A J, Tanvir N R, Smith N, Milne P A, Laskar T, et al. The Afterglow and Early-type Host Galaxy of the Short GRB 150101B at z = 0.1343. The Astrophysical Journal [Internet]. 2016;833 :151. Publisher's VersionAbstract
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 gamma-ray burst (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 ≈ 7× {10}10 {M}⊙ , stellar population age of ≈2-2.5 Gyr, and star formation rate of ≲0.4 M  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 ≈ 4.3{L}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 an NS-NS/NS-BH merger progenitor. From modeling the evolution of the broadband afterglow, we calculate isotropic-equivalent gamma-ray and kinetic energies of ≈ 1.3× {10}49 erg and ≈ (6{--}14)× {10}51 erg, respectively, a circumburst density of ≈ (0.8{--}4)× {10}-5 cm-3, and a jet opening angle of ≳9°. Using observations extending to ≈30 days, we place upper limits of ≲ (2{--}4)× {10}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.
Laskar T, Alexander K D, Berger E, Fong W-f., Margutti R, Shivvers I, Williams P K G, Kopa\v c D, Kobayashi S, Mundell C, et al. A Reverse Shock in GRB 160509A. The Astrophysical Journal [Internet]. 2016;833 :88. Publisher's VersionAbstract
We present optical spectroscopy, ultraviolet-to-infrared imaging, and X-ray observations of the intermediate luminosity optical transient (ILOT) SN 2010da in NGC 300 (d = 1.86 Mpc) spanning from -6 to +6 years relative to the time of outburst in 2010. Based on the light-curve and multi-epoch spectral energy distributions of SN 2010da, we conclude that the progenitor of SN 2010da is a ≈10-12 M  yellow supergiant possibly transitioning into a blue-loop phase. During outburst, SN 2010da had a peak absolute magnitude of M bol ≲ -10.4 mag, dimmer than other ILOTs and supernova impostors. We detect multi-component hydrogen Balmer, Paschen, and Ca II emission lines in our high-resolution spectra, which indicate a dusty and complex circumstellar environment. Since the 2010 eruption, the star has brightened by a factor of ≈5 and remains highly variable in the optical. Furthermore, we detect SN 2010da in archival Swift and Chandra observations as an ultraluminous X-ray source (L X ≈ 6 × 1039 erg s-1). We additionally attribute He II 4686 Å and coronal Fe emission lines in addition to a steady X-ray luminosity of ≈1037 erg s-1 to the presence of a compact companion.
Perley D A, Krühler T, Schulze S, de Ugarte Postigo A, Hjorth J, Berger E, Cenko S B, Chary R, Cucchiara A, Ellis R, et al. The Swift Gamma-Ray Burst Host Galaxy Legacy Survey. I. Sample Selection and Redshift Distribution. The Astrophysical Journal [Internet]. 2016;817 :7. Publisher's VersionAbstract
We present rest-frame near-IR (NIR) luminosities and stellar masses for a large and uniformly selected population of gamma-ray burst (GRB) host galaxies using deep Spitzer Space Telescope imaging of 119 targets from the Swift GRB Host Galaxy Legacy Survey spanning 0.03 < z < 6.3, and we determine the effects of galaxy evolution and chemical enrichment on the mass distribution of the GRB host population across cosmic history. We find a rapid increase in the characteristic NIR host luminosity between z ˜ 0.5 and z ˜ 1.5, but little variation between z ˜ 1.5 and z ˜ 5. Dust-obscured GRBs dominate the massive host population but are only rarely seen associated with low-mass hosts, indicating that massive star-forming galaxies are universally and (to some extent) homogeneously dusty at high redshift while low-mass star-forming galaxies retain little dust in their interstellar medium. Comparing our luminosity distributions with field surveys and measurements of the high-z mass-metallicity relation, our results have good consistency with a model in which the GRB rate per unit star formation is constant in galaxies with gas-phase metallicity below approximately the solar value but heavily suppressed in more metal-rich environments. This model also naturally explains the previously reported “excess” in the GRB rate beyond z ≳ 2 metals stifle GRB production in most galaxies at z < 1.5 but have only minor impact at higher redshifts. The metallicity threshold we infer is much higher than predicted by single-star models and favors a binary progenitor. Our observations also constrain the fraction of cosmic star formation in low-mass galaxies undetectable to Spitzer to be small at z < 4.
Perley D A, Tanvir N R, Hjorth J, Laskar T, Berger E, Chary R, de Ugarte Postigo A, Fynbo J P U, Krühler T, Levan A J, et al. The Swift GRB Host Galaxy Legacy Survey. II. Rest-frame Near-IR Luminosity Distribution and Evidence for a Near-solar Metallicity Threshold. The Astrophysical Journal [Internet]. 2016;817 :8. Publisher's VersionAbstract
We introduce the Swift Gamma-Ray Burst Host Galaxy Legacy Survey (“SHOALS”), a multi-observatory high-redshift galaxy survey targeting the largest unbiased sample of long-duration gamma-ray burst (GRB) hosts yet assembled (119 in total). We describe the motivations of the survey and the development of our selection criteria, including an assessment of the impact of various observability metrics on the success rate of afterglow-based redshift measurement. We briefly outline our host galaxy observational program, consisting of deep Spitzer/IRAC imaging of every field supplemented by similarly deep, multicolor optical/near-IR photometry, plus spectroscopy of events without preexisting redshifts. Our optimized selection cuts combined with host galaxy follow-up have so far enabled redshift measurements for 110 targets (92%) and placed upper limits on all but one of the remainder. About 20% of GRBs in the sample are heavily dust obscured, and at most 2% originate from z\gt 5.5. Using this sample, we estimate the redshift-dependent GRB rate density, showing it to peak at z˜ 2.5 and fall by at least an order of magnitude toward low (z = 0) redshift, while declining more gradually toward high (z˜ 7) redshift. This behavior is consistent with a progenitor whose formation efficiency varies modestly over cosmic history. Our survey will permit the most detailed examination to date of the connection between the GRB host population and general star-forming galaxies, directly measure evolution in the host population over cosmic time and discern its causes, and provide new constraints on the fraction of cosmic star formation occurring in undetectable galaxies at all redshifts.
Villar VA, Berger E, Chornock R, Margutti R, Laskar T, Brown PJ, Blanchard PK, Czekala I, Lunnan R, Reynolds MT. The Intermediate Luminosity Optical Transient SN 2010da: The Progenitor, Eruption, and Aftermath of a Peculiar Supergiant High-mass X-Ray Binary. The Astrophysical Journal [Internet]. 2016;830. Publisher's VersionAbstract
We present optical spectroscopy, ultraviolet-to-infrared imaging, andX-ray observations of the intermediate luminosity optical transient(ILOT) SN 2010da in NGC 300 (d = 1.86 Mpc) spanning from ‑6 to +6years relative to the time of outburst in 2010. Based on the light-curveand multi-epoch spectral energy distributions of SN 2010da, we concludethat the progenitor of SN 2010da is a ≈10–12 M yellow supergiant possibly transitioning into a blue-loop phase. Duringoutburst, SN 2010da had a peak absolute magnitude of M bol≲ ‑10.4 mag, dimmer than other ILOTs and supernova impostors.We detect multi-component hydrogen Balmer, Paschen, and Ca ii emissionlines in our high-resolution spectra, which indicate a dusty and complexcircumstellar environment. Since the 2010 eruption, the star hasbrightened by a factor of ≈5 and remains highly variable in theoptical. Furthermore, we detect SN 2010da in archival Swift and Chandraobservations as an ultraluminous X-ray source (L X ≈ 6× 1039 erg s‑1). We additionallyattribute He ii 4686 Å and coronal Fe emission lines in additionto a steady X-ray luminosity of ≈1037 ergs‑1 to the presence of a compact companion.
Perley DA, Krühler T, Schulze S, de Ugarte Postigo A, Hjorth J, Berger E, Cenko SB, Chary R, Cucchiara A, Ellis R, et al. The Swift Gamma-Ray Burst Host Galaxy Legacy Survey. I. Sample Selection and Redshift Distribution. The Astrophysical Journal [Internet]. 2016;817. Publisher's VersionAbstract
We introduce the Swift Gamma-Ray Burst Host Galaxy Legacy Survey(“SHOALS”), a multi-observatory high-redshift galaxy surveytargeting the largest unbiased sample of long-duration gamma-ray burst(GRB) hosts yet assembled (119 in total). We describe the motivations ofthe survey and the development of our selection criteria, including anassessment of the impact of various observability metrics on the successrate of afterglow-based redshift measurement. We briefly outline ourhost galaxy observational program, consisting of deep Spitzer/IRACimaging of every field supplemented by similarly deep, multicoloroptical/near-IR photometry, plus spectroscopy of events withoutpreexisting redshifts. Our optimized selection cuts combined with hostgalaxy follow-up have so far enabled redshift measurements for 110
Perley DA, Tanvir NR, Hjorth J, Laskar T, Berger E, Chary R, de Ugarte Postigo A, Fynbo JPU, Krühler T, Levan AJ, et al. The Swift GRB Host Galaxy Legacy Survey. II. Rest-frame Near-IR Luminosity Distribution and Evidence for a Near-solar Metallicity Threshold. The Astrophysical Journal [Internet]. 2016;817. Publisher's VersionAbstract
We present rest-frame near-IR (NIR) luminosities and stellar masses fora large and uniformly selected population of gamma-ray burst (GRB) hostgalaxies using deep Spitzer Space Telescope imaging of 119 targets fromthe Swift GRB Host Galaxy Legacy Survey spanning 0.03 < z < 6.3,and we determine the effects of galaxy evolution and chemical enrichmenton the mass distribution of the GRB host population across cosmichistory. We find a rapid increase in the characteristic NIR hostluminosity between z ˜ 0.5 and z ˜ 1.5, but little variationbetween z ˜ 1.5 and z ˜ 5. Dust-obscured GRBs dominate themassive host population but are only rarely seen associated withlow-mass hosts, indicating that massive star-forming galaxies areuniversally and (to some extent) homogeneously dusty at high redshiftwhile low-mass star-forming galaxies retain little dust in theirinterstellar medium. Comparing our luminosity distributions with fieldsurveys and measurements of the high-z mass-metallicity relation,our results have good consistency with a model in which the GRB rate perunit star formation is constant in galaxies with gas-phase metallicitybelow approximately the solar value but heavily suppressed in moremetal-rich environments. This model also naturally explains thepreviously reported “excess” in the GRB rate beyond z ≳2 metals stifle GRB production in most galaxies at z < 1.5 but haveonly minor impact at higher redshifts. The metallicity threshold weinfer is much higher than predicted by single-star models and favors abinary progenitor. Our observations also constrain the fraction ofcosmic star formation in low-mass galaxies undetectable to Spitzer to besmall at z < 4.
2015
Laskar T. The Diversity and Versatility of Gamma-Ray Bursts. Ph.D. Thesis [Internet]. 2015. Publisher's VersionAbstract
Gamma-ray bursts (GRBs) are the most energetic explosions in theUniverse, thus providing a unique laboratory for the study of extremeastrophysical processes. In parallel, their large luminosity makes GRBsa premier probe of the early Universe. My thesis has explored andexploited both aspects of GRB science by addressing the followingfundamental open questions: 1) what is the nature of the GRB ejecta?, 2)how does the GRB progenitor population evolve with redshift, and 3) howcan GRBs be used to probe the high-redshift Universe? To answer thesequestions, I present the first multi-wavelength detection and modelingof a GRB reverse shock, a comprehensive analysis of the plateau phase ofGRB light curves, studies of the evolution of the progenitor populationto redshifts, z~9, and demonstrate the use of GRBs as probes of galaxyformation and evolution through the first galaxy mass-metallicityrelation at z~3-5. I find support for baryonic ejecta in GRB 130427A,evidence that GRB jets contain a large amount of energy in slow-movingejecta, and proof that the GRB progenitor population does not evolve tothe highest redshifts at which it has yet been observed. Building on thedecade of observations by the Swift GRB mission, future observations andmodeling of GRBs and their host galaxies will provide clues to these andother open questions in GRB science, allowing for the first statisticalstudies of their progenitors and host environments to the epoch ofreionization and beyond.

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