Person:

Berger, Edo

We present observations of the afterglows and host galaxies of three short-duration gamma-ray bursts (GRBs): 100625A, 101219A and 110112A. We find that GRB 100625A occurred in a z = 0.452 early-type galaxy with a stellar mass of ≈ 4.6×109 M⊙ and a stellar population age of ≈ 0.7 Gyr, and GRB 101219A originated in a starforming galaxy at z = 0.718 with a stellar mass of ≈ 1.4×109M⊙, a star formation rate of ≈ 16 M⊙ yr−1, and a stellar population age of ≈ 50 Myr. We also report the discovery of the optical afterglow of GRB 110112A, which lacks a coincident host galaxy to i & 26 mag and we cannot conclusively identify any field galaxy as a possible host. From afterglow modeling, the bursts have inferred circumburst densities of ≈ 10−4 − 1 cm−3, and isotropic-equivalent gamma-ray and kinetic energies of ≈ 1050 − 1051 erg. These three events highlight the diversity of galactic environments that host short GRBs. To quantify this diversity, we use the sample of 36 Swift short GRBs with robust associations to an environment (∼ 1/2 of 68 short bursts detected by Swift to May 2012) and classify bursts originating from four types of environments: late-type (≈ 50%), early-type (≈ 15%), inconclusive (≈ 20%), and “host-less” (lacking a coincident host galaxy to limits of & 26 mag; ≈ 15%). To find likely ranges for the true late- and early-type fractions, we assign each of the host-less bursts to either the late- or early-type category using probabilistic arguments, and consider the scenario that all hosts in the inconclusive category are early-type galaxies to set an upper bound on the early-type fraction. We calculate most likely ranges for the late- and early-type fractions of ≈ 60 − 80% and ≈ 20 − 40%, respectively. We find no clear trend between gamma-ray duration and host type. We also find no change to the fractions when excluding events recently claimed as possible contaminants from the long GRB/collapsar population. Our reported demographics are consistent with a short GRB rate driven by both stellar mass and star formation.

The inspiral and coalescence of double neutron star (NS-NS) and neutron star-black hole (NS-BH) binaries are likely to be detected by advanced networks of ground-based gravitational wave (GW) interferometers. Maximizing the science returns from such a discovery will require the identification and localization of an electromagnetic (EM) counterpart. Here we critically evaluate and compare several possible counterparts, including short-duration gamma-ray bursts (SGRBs), “orphan” optical and radio afterglows, and ∼ day-long optical transients powered by the radioactive decay of heavy nuclei synthesized in the merger ejecta (“kilonovae”). We assess the promise of each potential counterpart in terms of four “Cardinal Virtues”: detectability, high fraction, identifiability, and positional accuracy. For viewing angles within the half-opening angle of the jet (θobs . θj ) the SGRB and associated afterglow are easily detectable within the range of Advanced LIGO/Virgo if the jet energy (Ej ) and the circumburst density (n) are similar to those inferred from existing SGRB observations. For modest off-axis angles (θobs . 2θj), the orphan optical afterglow is detectable with LSST if Ej,50 n 7/8 0 & 0.002; the fraction of such events is ∼ 7θ 2 j ∼ 0.1. At even larger viewing angles (i.e., the majority of observers) the isotropic kilonova emission dominates, with a peak optical brightness of ∼ 19 − 22 mag within the Advanced LIGO/Virgo volume, detectable with LSST using a specialized 1-day cadence. Radio afterglow emission from an initially off-axis jet or from sub-relativistic ejecta is also isotropic, but peaks on a timescale of months-years; this signal is detectable provided that Ej,50 n 7/8 0 (v/c) 11/4 & 0.2 (for off-axis afterglows, v/c ∼ 1). However, existing SGRB afterglows do not satisfy this criterion, indicating a low probability of radio detections. Taking into account the search strategy for typical error regions of tens of square degrees, our primary conclusion is that SGRBs are the most useful EM counterparts to confirm the cosmic origin of a few GW events, and to test the association with NS-NS/NS-BH mergers. However, for the more ambitious goal of localizing and obtaining redshifts for a large sample of GW events, kilonovae are instead preferred. Off-axis optical afterglows will be detectable for at most ∼ 10% of all events, while radio afterglows are promising only for the unique combination of energetic relativistic ejecta in a high density medium, and even then will require hundreds of hours of EVLA time per event spread over months-years. Our main recommendations from this analysis are: (i) an all-sky γ-ray satellite is essential for temporal coincidence detections, and for GW searches of γ-ray triggered events; (ii) LSST should adopt a 1-day cadence follow-up strategy, ideally with ∼ 0.5 hr per pointing to cover GW error regions (the standard 4-day cadence and depth will severely limit the probability of a unique identification); and (iii) radio searches should only focus on the relativistic case, which requires observations for a few months.

We present the optical discovery and sub-arcsecond optical and X-ray localization of the afterglow of the short GRB 120804A, as well as optical, near-IR, and radio detections of its host galaxy. X-ray observations with Swift/XRT, Chandra, and XMM-Newton extending to δt ≈ 19 d reveal a single power law decline. The optical afterglow is faint and comparison to the X-ray flux indicates that GRB 120804A is “dark”, with a restframe extinction of A host V ≈ 2.5 mag (at z ≈ 1.3). The intrinsic neutral hydrogen column density inferred from the X-ray spectrum, NH,int(z = 1.3) ≈ 2×1022 cm−2, is commensurate with the large extinction. The host galaxy exhibits red optical/near-IR colors. Equally important, JVLA observations at ≈ 0.9−11 d reveal a constant flux density of Fν(5.8GHz) = 35 ± 4 µJy and an optically-thin spectrum, unprecedented for GRB afterglows, but suggestive instead of emission from the host galaxy. The optical/near-IR and radio fluxes are well fit with the scaled spectral energy distribution of the local ultra-luminous infrared galaxy (ULIRG) Arp 220 at z ≈ 1.3, with a resulting star formation rate of ≈ 300 M⊙ yr−1. The inferred extinction and small projected offset (2.2 ± 1.2 kpc) are also consistent with the ULIRG scenario, as is the presence of a companion galaxy at a separation of about 11 kpc. The limits on radio afterglow emission, in conjunction with the observed X-ray and optical emission, require a circumburst density of n ∼ 10−3 cm−3 an isotropic-equivalent energy scale of Eγ,iso ≈ EK,iso ≈ 7×1051 erg, and a jet opening angle of θj & 8 ◦. The expected fraction of luminous infrared galaxies in the short GRB host sample is ∼ 0.01 − 0.3 (for pure stellar mass and star formation weighting, respectively). Thus, the observed fraction of 2 events in about 25 hosts (GRBs 120804A and 100206A), provides additional support to our previous conclusion that short GRBs track both stellar mass and star formation activity

We present a detailed investigation of Hubble Space Telescope rest-frame UV/optical observations of 22 short gamma-ray burst (GRB) host galaxies and sub-galactic environments. Utilizing the high angular resolution and depth of HST we characterize the host galaxy morphologies, measure precise projected physical and hostnormalized offsets between the bursts and host centers, and calculate the locations of the bursts with respect to their host light distributions (rest-frame UV and optical). We calculate a median short GRB projected physical offset of 4.5 kpc, about 3.5 times larger than that for long GRBs, and find that ≈ 25% of short GRBs have offsets of & 10 kpc. When compared to their host sizes, the median offset is 1.5 half-light radii (re), about 1.5 times larger than the values for long GRBs, core-collapse supernovae, and Type Ia supernovae. In addition, ≈ 20% of short GRBs having offsets of & 5re, and only ≈ 25% are located within 1re. We further find that short GRBs severely under-represent their hosts’ rest-frame optical and UV light, with ≈ 30 − 45% of the bursts located in regions of their host galaxies that have no detectable stellar light, and ≈ 55% in the regions with no UV light. Therefore, short GRBs do not occur in regions of star formation or even stellar mass. This demonstrates that the progenitor systems of short GRBs must migrate from their birth sites to their eventual explosion sites, a signature of kicks in compact object binary systems. Utilizing the full sample of offsets, we estimate natal kick velocities of ≈ 20 − 140 km s−1. These independent lines of evidence provide the strongest support to date that short GRBs result from the merger of compact object binaries (NS-NS/NS-BH).

GRB 080913 and GRB 090423 are the most distant gamma-ray bursts (GRBs) known to-date, with spectroscopically determined redshifts of z = 6 .7 and z = 8.1, respectively. The detection of bursts at this early epoch of the Universe significantly constrains the nature of GRBs and their progenitors. We perform population synthesis studies of the formation and evolution of early stars, and calculate the resulting formation rates of short and long-duration GRBs at high redshift. The peak of the GRB rate from Population II stars occurs at z ∼ 7 for a model with efficient/fast mixing of metals, while it is found at z∼ 3 for an inefficient/slow metallicity evolution model. We show that in the redshift range 6 ∼ < z ∼< 10 essentially all GRBs originate from Population II stars, regardless of metallicity evolution model. These stars (having small, but non-zero metallicity) are the most likely progenitors for both long GRBs (collapsars) and short GRBs (NS-NS or BH-NS mergers) at this epoch. Although the predicted intrinsic rates of long and short GRBs are similar at these high redshifts, observational selection effects lead to higher (factor of ∼ 10) observed rates for long GRBs. We conclude that the two recently observed highz GRB events are most likely long GRBs originating from Population II collapsars.

We present X-ray, optical, near-infrared (IR), and radio observations of GRBs 110709B and 111215A, as well as optical and near-IR observations of their host galaxies. The combination of X-ray detections and deep optical/near-IR limits establish both bursts as “dark”. Sub-arcsecond positions enabled by radio detections lead to robust host galaxy associations, with optical detections that indicate z . 4 (110709B) and z ≈ 1.8 − 2.7 (111215A). We therefore conclude that both bursts are dark due to substantial rest-frame extinction. Using the radio and X-ray data for each burst we find that GRB 110709B requires A host V & 5.3 mag and GRB 111215A requires A host V & 8.5 mag (z = 2). These are among the largest extinction values inferred for dark bursts to date. The two bursts also exhibit large neutral hydrogen column densities of NH,int & 1022 cm−2 (z = 2) as inferred from their X-ray spectra, in agreement with the trend for dark GRBs. Moreover, the inferred values are in agreement with the Galactic AV −NH relation, unlike the bulk of the GRB population. Finally, we find that for both bursts the afterglow emission is best explained by a collimated outflow with a total beamingcorrected energy of Eγ + EK ≈ (7 − 9) × 1051 erg (z = 2) expanding into a wind medium with a high density, M˙ ≈ (6 − 20) × 10−5 M⊙ yr−1 (n ≈ 100 − 350 cm−3 at ≈ 1017 cm). While the energy release is typical of long GRBs, the inferred density may be indicative of larger mass loss rates for GRB progenitors in dusty (and hence metal rich) environments. This study establishes the critical role of radio observations in demonstrating the origin and properties of dark GRBs. Observations with the JVLA and ALMA will provide a sample with subarcsecond positions and robust host associations that will help to shed light on obscured star formation and the role of metallicity in GRB progenitors.

We investigate the environment of the nearby (d ≈ 40 Mpc) broad-lined Type Ic supernova SN 2009bb. This event was observed to produce a relativistic outflow likely powered by a central accreting compact object. While such a phenomenon was previously observed only in long-duration gamma-ray bursts (LGRBs), no LGRB was detected in association with SN 2009bb. Using an optical spectrum of the SN 2009bb explosion site, we determine a variety of ISM properties for the host environment, including metallicity, young stellar population age, and star formation rate. We compare the SN explosion site properties to observations of LGRB and broad-lined SN Ic host environments on optical emission line ratio diagnostic diagrams. Based on these analyses, we find that the SN 2009bb explosion site has a metallicity between 1.7Z⊙ and 3.5Z⊙, in agreement with other broad-lined SN Ic host environments and at odds with the low-redshift LGRB host environments and recently proposed maximum metallicity limits for relativistic explosions. We consider the implications of these findings and the impact that SN 2009bb’s unusual explosive properties and environment have on our understanding of the key physical ingredient that enables some SNe to produce a relativistic outflow.

We present ground-based optical and Hubble Space Telescope optical and near-IR observations of the shorthard GRB 130603B at z = 0.356, which demonstrate the presence of excess near-IR emission matching the expected brightness and color of an r-process powered transient (a “kilonova”). The early afterglow fades rapidly with α . −2.6 at t ≈ 8 − 32 hr post-burst and has a spectral index of β ≈ −1.5 (Fν ∝ tανβ), leading to an expected near-IR brightness at the time of the first HST observation of mF160W(t = 9.4d) & 29.3 AB mag. Instead, the detected source has mF160W = 25.8±0.2 AB mag, corresponding to a rest-frame absolute magnitude of MJ ≈ −15.2 mag. The upper limit in the HST optical observations is mF606W & 27.7 AB mag (3σ), indicating an unusually red color of V − H & 1.9 mag. Comparing the observed near-IR luminosity to theoretical models of kilonovae produced by ejecta from the merger of an NS-NS or NS-BH binary, we infer an ejecta mass of Mej ≈ 0.03 − 0.08 M⊙ for vej ≈ 0.1 − 0.3c. The inferred mass matches the expectations from numerical merger simulations. The presence of a kilonova provides the strongest evidence to date that short GRBs are produced by compact object mergers, and provides initial insight on the ejected mass and the primary role that compact object merger may play in the r-process. Equally important, it demonstrates that gravitational wave sources detected by Advanced LIGO/Virgo will be accompanied by optical/near-IR counterparts with unusually red colors, detectable by existing and upcoming large wide-field facilities (e.g., Pan-STARRS, DECam, Subaru, LSST).

The first direct detection of gravitational waves (GW) by the ground-based Advanced LIGO/Virgo interferometers is expected to occur within the next few years. These interferometers are designed to detect the mergers of compact object binaries composed of neutron stars and/or black holes to a fiducial distance of ∼ 200 Mpc and a localization region of ∼ 100 deg2. To maximize the science gains from such GW detections it is essential to identify electromagnetic (EM) counterparts. Among the wide range of proposed counterparts, the most promising is optical/IR emission powered by the radioactive decay of r-process elements synthesized in the neutron-rich merger ejecta – a “kilonova”. Here we present detailed simulated observations that encompass a range of strategies for kilonova searches during GW follow-up. We utilize these simulations to assess both the detectability of kilonovae and our ability to distinguish them from a wide range of contaminating transients in the large GW localization regions. We find that if pre-existing deep template images for the GW localization region are available, then nightly observations to a depth of i ≈ 24 mag and z ≈ 23 mag are required to achieve a 95% detection rate; observations that commence within ∼ 12 hours of trigger will also capture the kilonova peak and provide stronger constraints on the ejecta properties. We also find that kilonovae can be robustly separated from other known and hypothetical types of transients utilizing cuts on color (i − z & 0.3 mag) and rise time (trise . 4 days). In the absence of a pre-existing template the observations must reach ∼ 1 mag deeper to achieve the same kilonova detection rate, but robust rejection of contaminants can still be achieved. Motivated by the results of our simulations we discuss the expected performance of current and future wide-field telescopes in achieving these observational goals, and find that prior to LSST the Dark Energy Camera (DECam) on the Blanco 4-m telescope and Hyper Suprime-Cam (HSC) on the Subaru 8-m telescope offer the best kilonova discovery potential.

We report the discovery of radio afterglow emission from the gamma-ray burst GRB 090423, which exploded at a redshift of 8.3, making it the object with the highest known redshift in the Universe. By combining our radio measurements with existing X-ray and infrared observations, we estimate the kinetic energy of the afterglow, the geometry of the outflow and the density of the circumburst medium. Our best fit model is a quasi-spherical, high-energy explosion in a low, constant-density medium. GRB 090423 had a similar energy release to the other well-studied high redshift GRB 050904 (z = 6.26), but their circumburst densities differ by two orders of magnitude. We compare the properties of GRB 090423 with a sample of GRBs at moderate redshifts. We find that the high energy and afterglow properties of GRB 090423 are not sufficiently different from other GRBs to suggest a different kind of progenitor, such as a Population III star. However, we argue that it is not clear that the afterglow properties alone can provide convincing identification of Population III progenitors. We suggest that the millimeter and centimeter radio detections of GRB 090423 at early times contained emission from a reverse shock component. This has important implications for the detection of high redshift GRBs by the next generation of radio facilities.