Publications

2013
Zauderer BA, Berger E, Margutti R, Levan AJ, Olivares E. F, Perley DA, Fong W, Horesh A, Updike AC, Greiner J, et al. Illuminating the Darkest Gamma-Ray Bursts with Radio Observations. The Astrophysical Journal. 2013;767. Publisher's VersionAbstract
We present X-ray, optical, near-infrared (IR), and radio observations ofgamma-ray bursts (GRBs) 110709B and 111215A, as well as optical andnear-IR observations of their host galaxies. The combination of X-raydetections and deep optical/near-IR limits establish both bursts as"dark." Sub-arcsecond positions enabled by radio detections lead torobust host galaxy associations, with optical detections that indicate z<~ 4 (110709B) and z ≈ 1.8-2.9 (111215A). We therefore concludethat both bursts are dark due to substantial rest-frame extinction.Using the radio and X-ray data for each burst we find that GRB 110709Brequires A_V^host≳ 5.3 mag and GRB 111215A requiresA_V^host≳ 8.5 mag (assuming z = 2). These are among the largestextinction values inferred for dark bursts to date. The two bursts alsoexhibit large neutral hydrogen column densities of N H, int>~ 1022 cm-2 (z = 2) as inferred fromtheir X-ray spectra, in agreement with the trend for dark GRBs.Moreover, the inferred values are in agreement with the GalacticAV -N H relation, unlike the bulk of the GRBpopulation. Finally, we find that for both bursts the afterglow emissionis best explained by a collimated outflow with a total beaming-correctedenergy of E γ + EK ≈ (7-9) ×1051 erg (z = 2) expanding into a wind medium with a highdensity, \dot{M}≈ (6{--}20)× 10^{-5} M yr-1 (n ≈ 100-350 cm-3 at≈1017 cm). While the energy release is typical of longGRBs, the inferred density may be indicative of larger mass-loss ratesfor GRB progenitors in dusty (and hence metal rich) environments. Thisstudy establishes the critical role of radio observations indemonstrating the origin and properties of dark GRBs. Observations withthe JVLA and ALMA will provide a sample with sub-arcsecond positions androbust host associations that will help to shed light on obscured starformation and the role of metallicity in GRB progenitors.
Laskar T, Berger E, Zauderer BA, Margutti R, Soderberg AM, Chakraborti S, Lunnan R, Chornock R, Chandra P, Ray A. A Reverse Shock in GRB 130427A. The Astrophysical Journal. 2013;776. Publisher's VersionAbstract
We present extensive radio and millimeter observations of the unusuallybright GRB 130427A at z = 0.340, spanning 0.67-12 days after the burst.We combine these data with detailed multi-band UV, optical, NIR, andSwift X-ray observations and find that the broadband afterglow emissionis composed of distinct reverse shock and forward shock contributions.The reverse shock emission dominates in the radio/millimeter and at<~ 0.1 days in the UV/optical/NIR, while the forward shock emissiondominates in the X-rays and at >~ 0.1 days in the UV/optical/NIR. Wefurther find that the optical and X-ray data require a wind circumburstenvironment, pointing to a massive star progenitor. Using the combinedforward and reverse shock emission, we find that the parameters of theburst include an isotropic kinetic energy of E K, iso ≈ 2× 1053 erg, a mass loss rate of \dot{M}\approx 3\times10^{-8} M yr-1 (for a wind velocityof 1000 km s-1), and a Lorentz factor at thedeceleration time of Γ(200 s) ≈ 130. Due to the low density andlarge isotropic energy, the absence of a jet break to ≈15 days placesonly a weak constraint on the opening angle, θj >~2.°5, and therefore a total energy of E γ +EK >~ 1.2 × 1051 erg, similar to othergamma-ray bursts (GRBs). The reverse shock emission is detectable inthis burst due to the low circumburst density, which leads to a slowcooling shock. We speculate that this property is required for thedetectability of reverse shocks in radio and millimeter bands. Followingon GRB 130427A as a benchmark event, observations of future GRBs withthe exquisite sensitivity of the Very Large Array and ALMA, coupled withdetailed modeling of the reverse and forward shock contributions, willtest this hypothesis.
2011
Laskar T, Berger E, Chary R-R. Exploring the Galaxy Mass-metallicity Relation at z ~ 3-5. The Astrophysical Journal. 2011;739. Publisher's VersionAbstract
Long-duration gamma-ray bursts (GRBs) provide a premier tool forstudying high-redshift star-forming galaxies thanks to their extremebrightness and association with massive stars. Here we use GRBs to studythe galaxy stellar mass-metallicity (M *-Z) relation at z ~3-5, where conventional direct metallicity measurements are extremelychallenging. We use the interstellar medium metallicities of long GRBhosts derived from afterglow absorption spectroscopy, in conjunctionwith host galaxy stellar masses determined from deep Spitzer 3.6 μmobservations of 20 GRB hosts. We detect about 1/4 of the hosts with MAB(I) ≈ -21.5 to -22.5 mag and place a limit of MAB(I) >~ -19 mag on the remaining hosts from a stackinganalysis. Using these observations, we present the first rest-frameoptical luminosity distribution of long GRB hosts at z >~ 3 and findthat it is similar to the distribution of long GRB hosts at z ~ 1. Incomparison to Lyman-break galaxies at the same redshift, GRB hosts aregenerally fainter, but the sample is too small to rule out an overallsimilar luminosity function. On the other hand, the GRB hosts appear tobe more luminous than the population of Lyα emitters at z ~ 3-4.Using a conservative range of mass-to-light ratios for simple stellarpopulations (with ages of 70 Myr to ~2 Gyr), we infer the host stellarmasses and present mass-metallicity measurements at z ~ 3-5 (langzrang≈ 3.5). We find that the detected GRB hosts, with M * ≈2 × 1010 M sun, display a wide range ofmetallicities, but that the mean metallicity at this mass scale, Z ≈0.3 Z sun, is lower than measurements at z <~ 3. Combinedwith stacking of the non-detected hosts with M * <~ 3× 109 M sun and Z <~ 0.1 Zsun, we find tentative evidence for the existence of an M*-Z relation at z ~ 3.5 and continued evolution of thisrelation to systematically lower metallicities from z ~ 2.
2010
Kruse EA, Berger E, Knapp GR, Laskar T, Gunn JE, Loomis CP, Lupton RH, Schlegel DJ. Chromospheric Variability in Sloan Digital Sky Survey M Dwarfs. II. Short-timescale Hα Variability. The Astrophysical Journal. 2010;722 :1352-1359. Publisher's VersionAbstract
We present the first comprehensive study of short-timescalechromospheric Hα variability in M dwarfs using the individual 15minute spectroscopic exposures for 52, 392 objects from the SloanDigital Sky Survey. Our sample contains about103-104 objects per spectral type bin in the rangeM0-M9, with a typical number of three exposures per object (ranging upto a maximum of 30 exposures). Using this extensive data set, we find
Laskar T, Fabian AC, Blundell KM, Erlund MC. Inverse-Compton X-rays from giant radio galaxies at z ~ 1. Monthly Notices of the Royal Astronomical Society. 2010;401 :1500-1504. Publisher's VersionAbstract
We report XMM-Newton observations of three FR II radio galaxies atredshifts between 0.85 and 1.34, which show extended diffuse X-rayemission within the radio lobes, likely due to inverse-Comptonup-scattering of the cosmic microwave background. Under this assumption,through spectrum fitting together with archival Very Large Array radioobservations, we derive an independent estimate of the magnetic field inthe radio lobes of 3C 469.1 and compare it with the equipartition value.We find concordance between these two estimates as long as the turnoverin the energy distribution of the particles occurs at a Lorentz factorin excess of ~250. We determine the total energy in relativisticparticles in the radio-emitting lobes of all three sources to rangebetween 3 × 1059 and 8 × 1059erg. Thenuclei of these X-ray sources are heavily-absorbed powerful activegalactic nuclei.
2009
Laskar T, Soderblom DR, Valenti JA, Stauffer JR. The Metallicity of the HD 98800 System. The Astrophysical Journal. 2009;698 :660-665. Publisher's VersionAbstract
Pre-main-sequence (PMS) binaries and multiples enable critical tests ofstellar models if masses, metallicities, and luminosities of thecomponent stars are known. We have analyzed high-resolution, highsignal-to-noise echelle spectra of the quadruple-star system HD 98800and using spectrum synthesis computed fits to the composite spectrum fora full range of plausible stellar parameters for the components. Weconsistently find that subsolar metallicity yields fits with lowerχ2 values, with an overall best fit of [M/H] = -0.20± 0.10. This metallicity appears to be consistent with PMSevolutionary tracks for the measured masses and luminosities of thecomponents of HD 98800 but additional constraints on the system andmodeling are needed.
Soderblom DR, Laskar T, Valenti JA, Stauffer JR, Rebull LM. The Metallicity of the Pleiades. The Astronomical Journal. 2009;138 :1292-1295. Publisher's VersionAbstract
We have measured the abundances of Fe, Si, Ni, Ti, and Na in 20 Pleiadesstars with T eff values near solar and with low vsin i usinghigh-resolution, high signal-to-noise echelle spectra. We have validatedour procedures by also analyzing 10 field stars of a range oftemperatures and metallicities that were observed by Valenti andFischer. Our result for the Pleiades is [Fe/H] = +0.03 ± 0.02± 0.05 (statistical and systematic). The average of publishedmeasurements for the Pleiades is +0.042 ± 0.021.
2008
Gupta S, Saini TD, Laskar T. Direction dependence and non-Gaussianity in the high-redshift supernova data. Monthly Notices of the Royal Astronomical Society. 2008;388 :242-246. Publisher's VersionAbstract
The most detailed constraints on the accelerating expansion of theuniverse and the nature of dark energy are derived from thehigh-redshift supernova data, assuming that the luminosity distanceversus redshift relation is isotropic and the errors in the measurementsare Gaussian. There is a possibility that there is a systematicdirection dependence in the data, either due to uncorrected, knownphysical processes or because there are tiny departures from thecosmological principle, making the universe slightly anisotropic. Toinvestigate this possibility, we introduce a statistic based on extremevalue theory and apply it to the gold data sets from Riess et al. Ouranalysis indicate a systematic, directional dependence in the supernovadata in both sets, which using the bootstrap distribution comes to about80 per cent level of confidence for Riess et al. and 90 per cent forRiess et al. Equally importantly, we show that while the 2007 data fitΛ cold dark matter (ΛCDM) model better than the 2004 data,the level of non-Gaussianity, quantified by departures of our statisticfrom the Gaussian predictions has become worse. In fact, we find thatRiess et al. data lie totally outside the distribution obtained byassuming the noise to be Gaussian.
Nath BB, Laskar T, Shull JM. Dust Sputtering by Reverse Shocks in Supernova Remnants. The Astrophysical Journal. 2008;682. Publisher's VersionAbstract
We consider the sputtering of dust grains, believed to be formed incooling supernovae ejecta, under the influence of reverse shocks. In theregime of self-similar evolution of reverse shocks, we can follow theevolution of ejecta density and temperature analytically as a functionof time in different parts of the ejecta, and calculate the sputteringrate of graphite and silicate grains embedded in the ejecta as theyencounter the reverse shock. Through analytic (one-dimensional)

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