Publications

2018
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. 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. 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. 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. 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. 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. 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. 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.
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. 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. 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. 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. 2015. Publisher's VersionAbstract
Gamma-ray bursts (GRBs) are the most energetic explosions in the Universe, thus providing a unique laboratory for the study of extreme astrophysical processes. In parallel, their large luminosity makes GRBs a premier probe of the early Universe. My thesis has explored and exploited both aspects of GRB science by addressing the following fundamental open questions: 1) what is the nature of the GRB ejecta?, 2) how does the GRB progenitor population evolve with redshift, and 3) how can GRBs be used to probe the high-redshift Universe? To answer these questions, 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 population to redshifts, z~9, and demonstrate the use of GRBs as probes of galaxy formation and evolution through the first galaxy mass-metallicity relation 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-moving ejecta, and proof that the GRB progenitor population does not evolve to the highest redshifts at which it has yet been observed. Building on the decade of observations by the Swift GRB mission, future observations and modeling of GRBs and their host galaxies will provide clues to these and other open questions in GRB science, allowing for the first statistical studies of their progenitors and host environments to the epoch of reionization and beyond.
Margutti R, Guidorzi C, Lazzati D, Milisavljevic D, Kamble A, Laskar T, Parrent J, Gehrels NC, Soderberg AM. Dust in the Wind: the Role of Recent Mass Loss in Long Gamma-Ray Bursts. The Astrophysical Journal. 2015;805. Publisher's VersionAbstract
We study the late-time (t\gt 0.5 days) X-ray afterglows of nearby (z\lt0.5) long gamma-ray bursts (GRBs) with Swift and identify a populationof explosions with slowly decaying, super-soft (photon index {{{Γ}}x}\gt 3) X-ray emission that is inconsistent with forwardshock synchrotron radiation associated with the afterglow. Theseexplosions also show larger-than-average intrinsic absorption(N{{H}x,i}\gt 6× {{10}21}c{{m}-2}) and prompt γ-ray emission with extremely longduration ({{T}90}\gt 1000 s). The chance association of thesethree rare properties (i.e., large N{{H}x,i}, super-soft{{{Γ }}x}, and extreme duration) in the same class ofexplosions is statistically unlikely. We associate these properties withthe turbulent mass-loss history of the progenitor star that enriched andshaped the circumburst medium. We identify a natural connection betweenN{{H}x,i}, {{{Γ }}x}, and {{T}90}in these sources by suggesting that the late-time super-soft X-raysoriginate from radiation reprocessed by material lost to the environmentby the stellar progenitor before exploding (either in the form of a dustecho or as reprocessed radiation from a long-lived GRB remnant), andthat the interaction of the explosion's shock/jet with the complexmedium is the source of the extremely long prompt emission. However,current observations do not allow us to exclude the possibility thatsuper-soft X-ray emitters originate from peculiar stellar progenitorswith large radii that only form in very dusty environments.
Laskar T, Berger E, Margutti R, Perley D, Zauderer BA, Sari R'em, Fong W-fai. Energy Injection in Gamma-Ray Burst Afterglows. The Astrophysical Journal. 2015;814. Publisher's VersionAbstract
We present multi-wavelength observations and modeling of gamma-raybursts (GRBs) that exhibit a simultaneous re-brightening in their X-rayand optical light curves, and are also detected at radio wavelengths. Weshow that the re-brightening episodes can be modeled by injection ofenergy into the blastwave and that in all cases the energy injectionrate falls within the theoretical bounds expected for a distribution ofenergy with ejecta Lorentz factor. Our measured values of thecircumburst density, jet opening angle, and beaming-corrected kineticenergy are consistent with the distribution of these parameters forlong-duration GRBs at both z˜ 1 and z≳ 6, suggesting that thejet launching mechanism and environment of these events are similar tothat of GRBs that do not have bumps in their light curves. However,events exhibiting re-brightening episodes have lower radiativeefficiencies than average, suggesting that a majority of the kineticenergy of the outflow is carried by slow-moving ejecta, which is furthersupported by steep measured distributions of the ejecta energy as afunction of Lorentz factor. We do not find evidence for reverse shocksover the energy injection period, implying that the onset of energyinjection is a gentle process. We further show that GRBs exhibitingsimultaneous X-ray and optical re-brightenings are likely the tail of adistribution of events with varying rates of energy injection, formingthe most extreme events in their class. Future X-ray observations of GRBafterglows with Swift and its successors will thus likely discoverseveral more such events, while radio follow-up and multi-wavelengthmodeling of similar events will unveil the role of energy injection inGRB afterglows.
2014
Berger E, Zauderer BA, Chary R-R, Laskar T, Chornock R, Tanvir NR, Stanway ER, Levan AJ, Levesque EM, Davies JE. ALMA Observations of the Host Galaxy of GRB 090423 at z = 8.23: Deep Limits on Obscured Star Formation 630 Million Years after the Big Bang. The Astrophysical Journal. 2014;796. Publisher's VersionAbstract
We present rest-frame far-infrared (FIR) and optical observations of thehost galaxy of GRB 090423 at z = 8.23 from the Atacama Large MillimeterArray (ALMA) and the Spitzer Space Telescope, respectively. The hostremains undetected to 3σ limits of F ν(222 GHz)<~ 33 μJy and F ν(3.6 μm) <~ 81 nJy. The FIRlimit is about 20 times fainter than the luminosity of the local ULIRGArp 220 and comparable to the local starburst M 82. Comparing this withmodel spectral energy distributions, we place a limit on the infrared(IR) luminosity of L IR(8-1000 μm) <~ 3 ×1010 L , corresponding to a limit on theobscured star formation rate of SFRIR ≲ 5 M yr-1. For comparison, the limit on theunobscured star formation rate from Hubble Space Telescope rest-frameultraviolet (UV) observations is SFRUV <~ 1 M yr-1. We also place a limit on the hostgalaxy stellar mass of M * <~ 5 × 107 M (for a stellar population age of 100 Myr and constantstar formation rate). Finally, we compare our millimeter observations tothose of field galaxies at z >~ 4 (Lyman break galaxies, Lyαemitters, and submillimeter galaxies) and find that our limit on the FIRluminosity is the most constraining to date, although the field galaxieshave much larger rest-frame UV/optical luminosities than the host of GRB090423 by virtue of their selection techniques. We conclude that GRBhost galaxies at z >~ 4, especially those with measured interstellarmedium metallicities from afterglow spectroscopy, are an attractivesample for future ALMA studies of high redshift obscured star formation.
Laskar T, Berger E, Tanvir N, Zauderer BA, Margutti R, Levan A, Perley D, Fong W-fai, Wiersema K, Menten K, et al. GRB 120521C at z ~ 6 and the Properties of High-redshift γ-Ray Bursts. The Astrophysical Journal. 2014;781. Publisher's VersionAbstract
We present optical, near-infrared, and radio observations of theafterglow of GRB 120521C. By modeling the multi-wavelength data set, wederive a photometric redshift of z ≈ 6.0, which we confirm with a lowsignal-to-noise ratio spectrum of the afterglow. We find that a modelwith a constant-density environment provides a good fit to the afterglowdata, with an inferred density of n <~ 0.05 cm-3.The radio observations reveal the presence of a jet break at tjet ≈ 7 d, corresponding to a jet opening angle ofθjet ≈ 3°. The beaming-corrected γ-rayand kinetic energies are E γ ≈ EK ≈ 3× 1050 erg. We quantify the uncertainties in ourresults using a detailed Markov Chain Monte Carlo analysis, which allowsus to uncover degeneracies between the physical parameters of theexplosion. To compare GRB 120521C to other high-redshift bursts in auniform manner we re-fit all available afterglow data for the two otherbursts at z >~ 6 with radio detections (GRBs 050904 and 090423). Wefind a jet break at t jet ≈ 15 d for GRB 090423, incontrast to previous work. Based on these three events, we find thatγ-ray bursts (GRBs) at z >~ 6 appear to explode inconstant-density environments, and exhibit a wide range of energies anddensities that span the range inferred for lower redshift bursts. On theother hand, we find a hint for narrower jets in the z >~ 6 bursts,potentially indicating a larger true event rate at these redshifts.Overall, our results indicate that long GRBs share a common progenitorpopulation at least to z ~ 8.
Lunnan R, Chornock R, Berger E, Laskar T, Fong W, Rest A, Sanders NE, Challis PM, Drout MR, Foley RJ, et al. Hydrogen-poor Superluminous Supernovae and Long-duration Gamma-Ray Bursts Have Similar Host Galaxies. The Astrophysical Journal. 2014;787. Publisher's VersionAbstract
We present optical spectroscopy and optical/near-IR photometry of 31host galaxies of hydrogen-poor superluminous supernovae (SLSNe),including 15 events from the Pan-STARRS1 Medium Deep Survey. Our samplespans the redshift range 0.1 <~ z <~ 1.6, and is the firstcomprehensive host galaxy study of this specific subclass of cosmicexplosions. Combining the multi-band photometry and emission-linemeasurements, we determine the luminosities, stellar masses, starformation rates, and metallicities. We find that, as a whole, the hostsof SLSNe are a low-luminosity (langMB rang ≈ -17.3 mag),low stellar mass (langM *rang ≈ 2 × 108 M) population, with a high median specific star formationrate (langsSFRrang ≈ 2 Gyr-1). The median metallicity ofour spectroscopic sample is low, 12 + log (O/H) ≈ 8.35 ≈ 0.45 Z, although at least one host galaxy has solarmetallicity. The host galaxies of H-poor SLSNe are statisticallydistinct from the hosts of GOODS core-collapse SNe (which cover asimilar redshift range), but resemble the host galaxies of long-durationgamma-ray bursts (LGRBs) in terms of stellar mass, SFR, sSFR, andmetallicity. This result indicates that the environmental causes leadingto massive stars forming either SLSNe or LGRBs are similar, and inparticular that SLSNe are more effectively formed in low metallicityenvironments. We speculate that the key ingredient is large core angularmomentum, leading to a rapidly spinning magnetar in SLSNe and anaccreting black hole in LGRBs.
Fong W, Berger E, Metzger BD, Margutti R, Chornock R, Migliori G, Foley RJ, Zauderer BA, Lunnan R, Laskar T, et al. Short GRB 130603B: Discovery of a Jet Break in the Optical and Radio Afterglows, and a Mysterious Late-time X-Ray Excess. The Astrophysical Journal. 2014;780. Publisher's VersionAbstract
We present radio, optical/NIR, and X-ray observations of the afterglowof the short-duration Swift and Konus-Wind GRB 130603B, and uncover abreak in the radio and optical bands at ≈0.5 day after the burst,best explained as a jet break with an inferred jet opening angle of≈4°-8°. GRB 130603B is only the third short GRB with a radioafterglow detection to date, and represents the first time that a jetbreak has been evident in the radio band. We model the temporalevolution of the spectral energy distribution to determine the burstexplosion properties and find an isotropic-equivalent kinetic energy of≈(0.6-1.7) × 1051 erg and a circumburst density of≈5 × 10-3-30 cm-3. From theinferred opening angle of GRB 130603B, we calculate beaming-correctedenergies of E γ ≈ (0.5-2) × 1049erg and E K ≈ (0.1-1.6) × 1049 erg. Alongwith previous measurements and lower limits we find a median openingangle of ≈10°. Using the all-sky observed rate of 10Gpc-3 yr-1, this implies a true shortGRB rate of ≈20 yr-1 within 200 Mpc, the AdvancedLIGO/VIRGO sensitivity range for neutron star binary mergers. Finally,we uncover evidence for significant excess emission in the X-rayafterglow of GRB 130603B at >~ 1 day and conclude that the additionalenergy component could be due to fall-back accretion or spin-down energyfrom a magnetar formed following the merger.
2013
Berger E, Zauderer BA, Levan A, Margutti R, Laskar T, Fong W, Mangano V, Fox DB, Tunnicliffe RL, Chornock R, et al. The Afterglow and ULIRG Host Galaxy of the Dark Short GRB 120804A. The Astrophysical Journal. 2013;765. Publisher's VersionAbstract
We present the optical discovery and subarcsecond optical and X-raylocalization of the afterglow of the short GRB 120804A, as well asoptical, near-IR, and radio detections of its host galaxy. X-rayobservations with Swift/XRT, Chandra, and XMM-Newton extending toδt ≈ 19 days reveal a single power-law decline. The opticalafterglow is faint, and comparison to the X-ray flux indicates that GRB120804A is "dark," with a rest-frame extinction of A host V ≈ 2.5 mag (at z = 1.3). The intrinsic neutral hydrogencolumn density inferred from the X-ray spectrum, N H, int(z =1.3) ≈ 2 × 1022 cm-2, iscommensurate with the large extinction. The host galaxy exhibits redoptical/near-IR colors. Equally important, JVLA observations at≈0.9-11 days reveal a constant flux density of F ν(5.8GHz) = 35 ± 4 μJy and an optically thin spectrum,unprecedented for GRB afterglows, but suggestive instead of emissionfrom the host galaxy. The optical/near-IR and radio fluxes are well fitwith the scaled spectral energy distribution of the local ultraluminousinfrared galaxy (ULIRG) Arp 220 at z ≈ 1.3, with a resulting starformation rate of x ≈ 300 M yr-1.The inferred extinction and small projected offset (2.2 ± 1.2kpc) are also consistent with the ULIRG scenario, as is the presence ofa companion galaxy at the same redshift and with a separation of about11 kpc. The limits on radio afterglow emission, in conjunction with theobserved X-ray and optical emission, require a circumburst density of n~ 10-3 cm-3, an isotropic-equivalentenergy scale of E γ, iso ≈ E K, iso ≈7 × 1051 erg, and a jet opening angle of θj >~ 11°. The expected fraction of luminous infraredgalaxies in the short GRB host sample is ~0.01 and ~0.25 (for purestellar mass and star formation weighting, respectively). Thus, theobserved fraction of two events in about 25 hosts (GRBs 120804A and100206A) appears to support our previous conclusion that short GRBstrack both stellar mass and star formation activity.
Fong W, Berger E, Chornock R, Margutti R, Levan AJ, Tanvir NR, Tunnicliffe RL, Czekala I, Fox DB, Perley DA, et al. Demographics of the Galaxies Hosting Short-duration Gamma-Ray Bursts. The Astrophysical Journal. 2013;769. Publisher's VersionAbstract
We present observations of the afterglows and host galaxies of threeshort-duration gamma-ray bursts (GRBs): 100625A, 101219A, and 110112A.We find that GRB 100625A occurred in a z = 0.452 early-type galaxy witha stellar mass of ≈4.6 × 109 M and a stellar population age of ≈0.7 Gyr, and GRB 101219A originatedin a star-forming galaxy at z = 0.718 with a stellar mass of ≈1.4× 109 M , a star formation rate of≈16 M yr-1, and a stellarpopulation age of ≈50 Myr. We also report the discovery of theoptical afterglow of GRB 110112A, which lacks a coincident host galaxyto i >~ 26 mag, and we cannot conclusively identify any field galaxyas a possible host. From afterglow modeling, the bursts have inferredcircumburst densities of ≈10-4-1cm-3 and isotropic-equivalent gamma-ray and kineticenergies of ≈1050-1051 erg. These three eventshighlight the diversity of galactic environments that host short GRBs.To quantify this diversity, we use the sample of 36 Swift short GRBswith robust associations to an environment (~1/2 of 68 short burstsdetected by Swift to 2012 May) and classify bursts originating from four
Chornock R, Berger E, Fox DB, Lunnan R, Drout MR, Fong W-fai, Laskar T, Roth KC. GRB 130606A as a Probe of the Intergalactic Medium and the Interstellar Medium in a Star-forming Galaxy in the First Gyr after the Big Bang. The Astrophysical Journal. 2013;774. Publisher's VersionAbstract
We present high signal-to-noise ratio Gemini and MMT spectroscopy of theoptical afterglow of the gamma-ray burst (GRB) 130606A at redshift z =5.913, discovered by Swift. This is the first high-redshift GRBafterglow to have spectra of comparable quality to those of z ≈ 6quasars. The data exhibit a smooth continuum at near-infraredwavelengths that is sharply cut off blueward of 8410 Å due toabsorption from Lyα at redshift z ≈ 5.91, with some fluxtransmitted through the Lyα forest between 7000 and 7800 Å.We use column densities inferred from metal absorption lines toconstrain the metallicity of the host galaxy between a lower limit of[Si/H] >~ -1.7 and an upper limit of [S/H] <~ -0.5 setby the non-detection of S II absorption. We demonstrate consistencybetween the dramatic evolution in the transmission fraction of Lyαseen in this spectrum over the redshift range z = 4.9-5.85 with thatpreviously measured from observations of high-redshift quasars. There isan extended redshift interval of Δz = 0.12 in the Lyα forestat z = 5.77 with no detected transmission, leading to a 3σ upper

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