We present rest-frame far-infrared (FIR) and optical observations of the host galaxy of GRB090423 at z=8.23 from the Atacama Large Millimeter Array (ALMA) and the Spitzer Space Telescope, respectively. The host remains undetected to 3-sigma limits of
Fnu(222 GHz)<33 microJy and Fnu(3.6 micron)<81 nJy. The FIR limit is about 20 times fainter than the luminosity of the local ULIRG Arp220, and comparable to the local starburst M82. Comparing to model spectral energy distributions we place a limit on the IR luminosity of L_IR(8-1000 micron)<3e10 Lsun, corresponding to a limit on the obscured star formation rate of SFR_IR<5 Msun/yr; for comparison, the limit on the unobscured star formation rate from Hubble Space Telescope rest-frame UV observations is SFR_UV<1 Msun/yr. We also place a limit on the host galaxy stellar mass of <5e7 Msun (for a stellar population age of 100 Myr and constant star formation rate). Finally, we compare our millimeter observations to those of field galaxies at z>4 (Lyman break galaxies, Ly-alpha emitters, and submillimeter galaxies), and find that our limit on the FIR luminosity is the most constraining to date, although the field galaxies have much larger rest-frame UV/optical luminosities than the host of GRB090423 by virtue of their selection techniques. We conclude that GRB host galaxies at z>4, especially those with measured interstellar medium metallicities from afterglow spectroscopy, are an attractive sample for future ALMA studies of high redshift obscured star formation.
Gamma-ray bursts (GRBs) display a bimodal duration distribution, with a separation between the short- and long-duration bursts at about 2 sec. The progenitors of long GRBs have been identified as massive stars based on their association with Type Ic
core-collapse supernovae, their exclusive location in star-forming galaxies, and their strong correlation with bright ultraviolet regions within their host galaxies. Short GRBs have long been suspected on theoretical grounds to arise from compact object binary mergers (NS-NS or NS-BH). The discovery of short GRB afterglows in 2005, provided the first insight into their energy scale and environments, established a cosmological origin, a mix of host galaxy types, and an absence of associated supernovae. In this review I summarize nearly a decade of short GRB afterglow and host galaxy observations, and use this information to shed light on the nature and properties of their progenitors, the energy scale and collimation of the relativistic outflow, and the properties of the circumburst environments. The preponderance of the evidence points to compact object binary progenitors, although some open questions remain. Based on this association, observations of short GRBs and their afterglows can shed light on the on- and off-axis electromagnetic counterparts of gravitational wave sources from the Advanced LIGO/Virgo experiments.
[Abridged] We present a search for fast optical transients (~0.5 hr-1 day) using repeated observations of the Pan-STARRS1 Medium-Deep Survey (PS1/MDS) fields. Our search takes advantage of the consecutive g/r-band observations (16.5 min in each filte
r), by requiring detections in both bands, with non-detections on preceding and subsequent nights. We identify 19 transients brighter than 22.5 AB mag (S/N>10). Of these, 11 events exhibit quiescent counterparts in the deep PS1/MDS templates that we identify as M4-M9 dwarfs. The remaining 8 transients exhibit a range of properties indicative of main-belt asteroids near the stationary point of their orbits. With identifications for all 19 transients, we place an upper limit of R_FOT(0.5hr)<0.12 deg^-2 d^-1 (95% confidence level) on the sky-projected rate of extragalactic fast transients at <22.5 mag, a factor of 30-50 times lower than previous limits; the limit for a timescale of ~day is R_FOT<2.4e-3 deg^-2 d^-1. To convert these sky-projected rates to volumetric rates, we explore the expected peak luminosities of fast optical transients powered by various mechanisms, and find that non-relativistic events are limited to M~-10 mag (M~-14 mag) for a timescale of ~0.5 hr (~day), while relativistic sources (e.g., GRBs, magnetar-powered transients) can reach much larger luminosities. The resulting volumetric rates are <13 (M~-10 mag), <0.05 (M~-14 mag) and <1e-6 Mpc^-3 yr^-1 (M~-24 mag), significantly above the nova, supernova, and GRB rates, respectively, indicating that much larger surveys are required to provide meaningful constraints. Motivated by the results of our search we discuss strategies for identifying fast optical transients in the LSST main survey, and reach the optimistic conclusion that the veil of foreground contaminants can be lifted with the survey data, without the need for expensive follow-up observations.
We present ground-based optical and Hubble Space Telescope optical and near-IR observations of the short-hard GRB130603B 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~8-32 hr post-burst and has a spectral index of beta=-1.5 (F_nu t^alpha*nu^beta), leading to an expected near-IR brightness at the time of the first HST observation of m(F160W)>29.3 AB mag. Instead, the detected source has m(F160W)=25.8+/-0.2 AB mag, corresponding to a rest-frame absolute magnitude of M(J)=-15.2 mag. The upper limit in the HST optical observations is m(F606W)>27.7 AB mag (3-sigma), 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_ej~0.03-0.08 Msun for v_ej=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).
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-New
ton to ~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 rest-frame extinction of A_V~2.5 mag (at z~1.3). The intrinsic neutral hydrogen column density inferred from the X-ray spectrum, N_H~2x10^22 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 5.8 GHz flux density 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 Msun/yr. 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 ~10^-3 cm^-3 an isotropic-equivalent energy scale of E_gamma,iso ~ E_K,iso ~ 7x10^51 erg, and a jet opening angle of >8 deg. 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 the Pan-STARRS1 discovery and light curves, and follow-up MMT and Gemini spectroscopy of an ultra-luminous supernova (ULSN; dubbed PS1-11bam) at a redshift of z=1.566 with a peak brightness of M_UV=-22.3 mag. PS1-11bam is one of the highes
t redshift spectroscopically-confirmed SNe known to date. The spectrum is characterized by broad absorption features typical of previous ULSNe (e.g., CII, SiIII), and by strong and narrow MgII and FeII absorption lines from the interstellar medium (ISM) of the host galaxy, confirmed by an [OII]3727 emission line at the same redshift. The equivalent widths of the FeII2600 and MgII2803 lines are in the top quartile of the quasar intervening absorption system distribution, but are weaker than those of gamma-ray burst intrinsic absorbers (i.e., GRB host galaxies). We also detect the host galaxy in pre-explosion Pan-STARRS1 data and find that its UV spectral energy distribution is best fit with a young stellar population age of tau~15-45 Myr and a stellar mass of M sim (1.1-2.6)x10^9 M_sun (for Z=0.05-1 Z_sun). The star formation rate inferred from the UV continuum and [OII]3727 emission line is ~10 M_sun/yr, higher than in any previous ULSN host. PS1-11bam provides the first direct demonstration that ULSNe can serve as probes of the interstellar medium in distant galaxies. At the present, the depth and red sensitivity of PS1 are uniquely suited to finding such events at cosmologically interesting redshifts (z~1-2); the future combination of LSST and 30-m class telescopes promises to extend this technique to z~4.
We present continued radio observations of the tidal disruption event SwiftJ164449.3+573451 extending to sim216 days after discovery. The data are part of a long-term program to monitor the expansion and energy scale of the relativistic outflow, and
to trace the parsec-scale environment around a previously-dormant supermassive black hole (SMBH). The new observations reveal a significant change in the radio evolution starting at sim1 month, with a brightening at all frequencies that requires an increase in the energy by about an order of magnitude, and an overall density profile around the SMBH of rho propto r^{-3/2} (0.1-1.2 pc) with a significant flattening at rsim0.4-0.6 pc. The increase in energy cannot be explained with continuous injection from an L propto t^{-5/3} tail, which is observed in the X-rays. Instead, we conclude that the relativistic jet was launched with a wide range of Lorentz factors, obeying E(>Gamma) propto Gamma^{-2.5}. The similar ratio of duration to dynamical timescale for Sw1644+57 and GRBs suggests that this result may be applicable to GRBs as well. The radial density profile may be indicative of Bondi accretion, with the inferred flattening at rsim0.5 pc in good agreement with the Bondi radius for a sim10^6 M_sun black hole. The density at sim0.5 pc is about a factor of 30 times lower than inferred for the Milky Way galactic center, potentially due to a smaller number of mass-shedding massive stars. From our latest observations (sim216 d) we find that the jet energy is E_{iso}sim5x10^{53} erg (E_jsim2.4x10^{51} erg for theta_j=0.1), the radius is rsim1.2 pc, the Lorentz factor is Gammasim2.2, the ambient density is nsim0.2 cm^{-3}, and the projected size is r_{proj}sim25 microarcsec. Assuming no future changes in the observed evolution we predict that the radio emission from Sw1644+57 should be detectable with the EVLA for several decades, and will be resolvable with VLBI in a few years.
[Abridged] We present a new radio survey of about 100 late-M and L dwarfs undertaken with the VLA. The sample was chosen to explore the role of rotation in the radio activity of ultracool dwarfs. Combining the new sample with results from our previou
s studies and from the literature, we compile the largest sample to date of ultracool dwarfs with radio observations and measured rotation velocities (167 objects). In the spectral type range M0-M6 we find a radio activity-rotation relation, with saturation at log(L_rad/L_bol) 10^(-7.5) above vsini~5 km/s, similar to the relation in H-alpha and X-rays. However, at spectral types >M7 the ratio of radio to bolometric luminosity increases regardless of rotation velocity, and the scatter in radio luminosity increases. In particular, while the most rapid rotators (vsini>20 km/s) exhibit super-saturation in X-rays and H-alpha, this effect is not seen in the radio. We also find that ultracool dwarfs with vsini>20 km/s have a higher radio detection fraction by about a factor of 3 compared to objects with vsini<10 km/s. When measured in terms of the Rossby number (Ro), the radio activity-rotation relation follows a single trend and with no apparent saturation from G to L dwarfs and down to Ro~10^-3; in X-rays and H-alpha there is clear saturation at Ro<0.1, with super-saturation beyond M7. A similar trend is observed for the radio surface flux (L_rad/R^2) as a function of Ro. The continued role of rotation in the overall level of radio activity and in the fraction of active sources, and the single trend of L_rad/L_bol and L_rad/R^2 as a function of Ro from G to L dwarfs indicates that rotation effects are important in regulating the topology or strength of magnetic fields in at least some fully-convective dwarfs. The fact that not all rapid rotators are detected in the radio provides additional support to the idea of dual dynamo states.
We present multi-epoch radio and optical observations of the M7 dwarf 2MASS J13142039+1320011. We detect a ~1 mJy source at 1.43, 4.86, 8.46 and 22.5 GHz, making it the most luminous radio emission over the widest frequency range detected from an ult
racool dwarf to date. A 10 hr VLA observation reveals that the radio emission varies sinusoidally with a period of 3.89+/-0.05 hr, and an amplitude of ~30% at 4.86 GHz and ~20% at 8.46 GHz. The periodicity is also seen in circular polarization, where at 4.86 GHz the polarization reverses helicity from left- to right-handed in phase with the total intensity. An archival detection in the FIRST survey indicates that the radio emission has been stable for at least a decade. We also detect periodic photometric variability in several optical filters with a period of 3.79 hr, and measure a rotation velocity of vsini=45+/-5 km/s, in good agreement with the radio and optical periods. The period and rotation velocity allow us to place a lower limit on the radius of the source of >0.12 R_sun, about 30% larger than theoretical expectations. The properties of the radio emission can be explained with a simple model of a magnetic dipole mis-aligned relative to the stellar rotation axis, with the sinusoidal variations and helicity reversal due to the rotation of the magnetic poles relative to our line of sight. The long-term stability of the radio emission indicates that the magnetic field (and hence the dynamo) is stable on a much longer timescale than the convective turn-over time of ~0.2 yr. If the radio emission is due to the electron cyclotron maser process, the inferred magnetic field strength reaches at least 8 kG.
We present spectroscopic observations of GRB091127 (z=0.490) at the peak of the putative associated supernova, SN2009nz. Subtracting a late-time spectrum of the host galaxy, we isolate the contribution of SN2009nz and uncover broad features typical o
f nearby GRB-SNe. This establishes unambiguously that GRB091127 was accompanied by a broad-lined Type Ic SN, and links a cosmological long burst with a standard energy release (E_gamma,iso ~ 1.1e52 erg) to a massive star progenitor. The spectrum of SN2009nz closely resembles that of SN2006aj, with SN2003dh also providing an acceptable match, but has significantly narrower features than SNe 1998bw and 2010bh, indicative of a lower expansion velocity. The photospheric velocity inferred from the SiII 6355 absorption feature, v_ph ~ 17,000 km/s, is indeed closer to that of SNe 2006aj and 2003dh than to the other GRB-SNe. Combining the measured velocity with the light curve peak brightness and width, we estimate the following explosion parameters: M_Ni ~ 0.35 M_sun, E_K ~ 2.3e51 erg, and M_ej ~ 1.4 M_sun, similar to those of SN2006aj. These properties indicate that SN2009nz follows a trend of lower M_Ni for GRB-SNe with lower E_K and M_ej. Equally important, since GRB091127 is a typical cosmological burst, the similarity of SN2009nz to SN2006aj either casts doubt on the claim that XRF060218/SN2006aj was powered by a neutron star, or indicates that the nature of the central engine is encoded in the SN properties but not in the prompt emission. Future spectra of GRB-SNe at z > 0.3, including proper subtraction of the host galaxy contribution, will shed light on the full dispersion of SN properties for standard long GRBs, on the relation between SNe associated with sub-energetic and standard GRBs, and on a potential dispersion in the associated SN types.
