We investigate the afterglow properties and large-scale environments of several short-duration gamma-ray bursts (GRBs) with subarcsecond optical afterglow positions but no bright coincident host ...galaxies. The purpose of this joint study is to robustly assess the possibility of significant offsets, a hallmark of the compact object binary merger model. Five such events exist in the current sample of 20 short bursts with optical afterglows, and we find that their optical, X-ray, and Delta *g-ray emission are systematically fainter. These differences may be due to lower circumburst densities (by about an order of magnitude), to higher redshifts (by Delta *Dz 0.5-1), or to lower energies (by about a factor of 3), although in the standard GRB model the smaller Delta *g-ray fluences cannot be explained by lower densities. To study the large-scale environments, we use deep optical observations to place limits on underlying hosts and to determine probabilities of chance coincidence for galaxies near each burst. In four of the five cases, the lowest probabilities of chance coincidence (P(30-75 kpc, while for the faint hosts the assumption of z 1 leads to offsets of ~15 kpc. Alternatively, the limits at the burst positions (26 mag) can be explained by typical short GRB host galaxies (L 0.1-1 L*) at z 2-3. Thus, two possibilities exist: (1) ~1/4 of short GRBs explode ~50 kpc or ~15 kpc from the centers of z ~ 0.3 or z 1 galaxies, respectively, and have fainter afterglows due to the resulting lower densities; or (2) ~1/4 of short GRBs occur at z 2 and have fainter afterglows due to their higher redshifts. The high-redshift scenario leads to a bimodal redshift distribution, with peaks at z ~ 0.5 and z ~ 3. The large offset scenario leads to an offset distribution that is well matched by theoretical predictions of NS-NS/NS-BH binary kicks, or by a hybrid population with globular cluster NS-NS binaries at large offsets and primordial binaries at offsets of 10 kpc (indicative of negligible kicks). Deeper constraints on any coincident galaxies to 28 mag (using the Hubble Space Telescope) will allow us to better exclude the high-redshift scenario.
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 host-normalized 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 approx =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, approx =20% of short GRBs having offsets of > ~5re, and only approx =25% are located within 1re. We further find that short GRBs severely under-represent their hosts' rest-frame optical and UV light, with approx =30%-45% of the bursts located in regions of their host galaxies that have no detectable stellar light, and approx =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 approx =20-140 km s super(-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).
The final inspiral of double neutron star and neutron-star-black-hole 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 of an electromagnetic 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 counterpart in terms of four "Cardinal Virtues": detectability, high fraction, identifiability, and positional accuracy. Taking into account the search strategy for typical error regions of tens of square degrees, we conclude that SGRBs are the most useful to confirm the cosmic origin of a few GW events, and to test the association with neutron star 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 are detectable for at most tens of percent of events, while radio afterglows are promising only for energetic relativistic ejecta in a high-density medium. Our main recommendations are: (1) an all-sky gamma-ray satellite is essential for temporal coincidence detections, and for GW searches of gamma-ray-triggered events; (2) the Large Synoptic Survey Telescope should adopt a one-day cadence follow-up strategy, ideally with 0.5 hr per pointing to cover GW error regions; and (3) radio searches should focus on the relativistic case, which requires observations for a few months.
The association of some short-duration gamma-ray bursts (GRBs) with elliptical galaxies established that their progenitors, unlike those of long GRBs, belong to an old stellar population. However, ...the majority of short GRBs appear to occur in star forming galaxies, raising the possibility that some progenitors are related to recent star formation activity. Here, we present optical spectroscopy of these hosts and measure their luminosities, star formation rates, and metallicities. We find luminosities of LB 0.1-1.5 L *, star formation rates of SFR 0.2-6 M yr-1, and metallicities of 12 + log(O/H) 8.5-8.9 (Z 0.6-1.6 Z ). A detailed comparison to the hosts of long GRBs reveals systematically higher luminosities, lower specific star formation rates (SFR/LB ) by about an order of magnitude, and higher metallicities by about 0.6 dex. The Kolmogorov-Smirnov probability that the short and long GRB hosts are drawn from the same underlying galaxy distribution is only ~10-3. Short GRB hosts exhibit excellent agreement with the specific star formation rates and the luminosity-metallicity relation of field galaxies at z ~ 0.1-1. We thus conclude that short GRB hosts are not dominated by young stellar populations like long GRB hosts. Instead, short GRB hosts appear to be drawn uniformly from the underlying field galaxy distribution, indicating that the progenitors have a wide age distribution of several Gyr.
The first precise localization of a fast radio burst (FRB) sheds light on the nature of these mysterious bursts and the physical mechanisms that power them. Increasing the sample of FRBs with robust ...host galaxy associations is the key impetus behind ongoing and upcoming searches and facilities. Here, we quantify the robustness of FRB host galaxy associations as a function of localization area and galaxy apparent magnitude. We also explore the use of FRB dispersion measures to constrain the source redshift, thereby reducing the number of candidate hosts. We use these results to demonstrate that even in the absence of a unique association, a constraint can be placed on the maximum luminosity of a host galaxy as a function of localization and dispersion measure (DM). We find that localizations of are required for a chance coincidence probability of for dwarf galaxies at if some hosts have luminosities of , then localizations of up to may suffice at . Constraints on the redshift from the DM only marginally improve the association probability unless the DM is low, pc cm−3. This approach also relies on the determination of galaxy redshifts, which is challenging at if the hosts are dwarf galaxies. Finally, interesting limits on the maximum host luminosity require localizations of at . Even a few such localizations will explain the nature of FRB progenitors, their possible diversity, and their use as cosmological tools.
I present radio observations of 90 dwarf stars and brown dwarfs of spectral type M5-T8. Three sources exhibit radio activity, in addition to the six objects previously detected in quiescence and ...outburst, leading to an overall detection rate of 610% for objects later than M7. The inferred magnetic field strengths are 610 super(2) G in quiescence and nearly 1 kG during flares, while the majority of the nondetected objects have B<50 G. Depending on the configuration and size of the magnetic loops, the surface fields may approach 1 kG even in quiescence, at most a factor of a few smaller than in early M dwarfs. With the larger sample of sources I find continued evidence for (1) a sharp transition around spectral type of M7 from a radio of radio to X-ray luminosity of log (L sub(R)/L sub(X)) 6-15.5 to >-12, (2) increased radio activity (L sub(R)/L sub(bol)) with later spectral type, in contrast to Ha and X-ray observations, and (3) an overall drop in the fraction of active sources from 630% for M dwarfs to 65% for L dwarfs, consistent with Ha and X-ray observations. Taken together, these trends suggest that some late M and L dwarfs are capable of generating 0.1-1 kG magnetic fields, but the overall drop in the fraction of such objects likely reflects changes in the structure of the chromospheres and coronae, possibly due to increasingly neutral atmospheres and/or a transition to a turbulent dynamo. These possibilities can best be tested through simultaneous observations, which can trace the effect of magnetic dissipation in a direct, rather than a statistical, manner. Still, a more extended radio survey currently holds the best promise for measuring the magnetic field properties of a large number of dwarf stars.
We present ground-based optical and Hubble Space Telescope (HST) optical and near-IR observations of the short-hard 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 alpha <, ~ -2.6 at t approximate 8-32 hr post-burst and has a spectral index of beta approximate -1.5 (F sub(v) proportional, variant t super( alpha ) v super( beta )), leading to an expected near-IR brightness at the time of the first HST observation of m sub(F160W)(t = 9.4 days) > ~ 29.3 AB mag. Instead, the detected source has m sub(F160W) = 25.8 + or - 0.2 AB mag, corresponding to a rest-frame absolute magnitude of M sub(J) approximate -15.2 mag. The upper limit in the HST optical observations is m sub(F606W) > ~ 27.7 AB mag (3sigma), 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 M sub(ej) approximate 0.03-0.08 M sub(middot in circle) for upsilon sub(ej) approximate 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).
ABSTRACT The first direct detection of gravitational waves (GWs) 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 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 hr 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 on the Blanco 4 m telescope and Hyper Suprime-Cam on the Subaru 8 m telescope offer the best potential for kilonova discovery.
We report the discovery of rising X-ray emission from the binary neutron star merger event GW170817. This is the first detection of X-ray emission from a gravitational-wave (GW) source. Observations ...acquired with the Chandra X-ray Observatory (CXO) at t 2.3 days post-merger reveal no significant emission, with L x 3.2 × 10 38 erg s − 1 (isotropic-equivalent). Continued monitoring revealed the presence of an X-ray source that brightened with time, reaching L x 9 × 10 38 erg s − 1 at 15.1 days post-merger. We interpret these findings in the context of isotropic and collimated relativistic outflows (both on- and off-axis). We find that the broadband X-ray to radio observations are consistent with emission from a relativistic jet with kinetic energy E k ∼ 10 49 − 50 erg , viewed off-axis with θ obs ∼ 20 ° - 40 ° . Our models favor a circumbinary density n ∼ 10 − 4 - 10 − 2 cm − 3 , depending on the value of the microphysical parameter ϵ B = 10 − 4 - 10 − 2 . A central-engine origin of the X-ray emission is unlikely. Future X-ray observations at t 100 days, when the target will be observable again with the CXO, will provide additional constraints to solve the model degeneracies and test our predictions. Our inferences on θ obs are testable with GW information on GW170817 from advanced LIGO/Virgo on the binary inclination.
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