Person:

Zauderer, B

Active galactic nuclei, which are powered by long-term accretion onto central supermassive black holes, produce1 relativistic jets with lifetimes of at least one million years, and the observation of the birth of such a jet is therefore unlikely. Transient accretion onto a supermassive black hole, for example through the tidal disruption2, 3 of a stray star, thus offers a rare opportunity to study the birth of a relativistic jet. On 25 March 2011, an unusual transient source (Swift J164449.3+573451) was found4, potentially representing5, 6 such an accretion event. Here we report observations spanning centimetre to millimetre wavelengths and covering the first month of evolution of a luminous radio transient associated with Swift J164449.3+573451. The radio transient coincides7 with the nucleus of an inactive galaxy. We conclude that we are seeing a newly formed relativistic outflow, launched by transient accretion onto a million-solar-mass black hole. A relativistic outflow is not predicted in this situation, but we show that the tidal disruption of a star naturally explains the observed high-energy properties and radio luminosity and the inferred rate of such events. The weaker beaming in the radio-frequency spectrum relative to γ-rays or X-rays suggests that radio searches may uncover similar events out to redshifts of z ≈ 6.

We present optical observations of supernova SN 2014C, which underwent an unprecedented slow metamorphosis from H-poor type Ib to H-rich type IIn over the course of one year. The observed spectroscopic evolution is consistent with the supernova having exploded in a cavity before encountering a massive shell of the progenitor star's stripped hydrogen envelope. Possible origins for the circumstellar shell include a brief Wolf–Rayet fast wind phase that overtook a slower red supergiant wind, eruptive ejection, or confinement of circumstellar material by external influences of neighboring stars. An extended high velocity Hα absorption feature seen in near-maximum light spectra implies that the progenitor star was not completely stripped of hydrogen at the time of core collapse. Archival pre-explosion Subaru Telescope Suprime-Cam and Hubble Space Telescope Wide Field Planetary Camera 2 images of the region obtained in 2009 show a coincident source that is most likely a compact massive star cluster in NGC 7331 that hosted the progenitor system. By comparing the emission properties of the source with stellar population models that incorporate interacting binary stars we estimate the age of the host cluster to be 30–300 Myr, and favor ages closer to 30 Myr in light of relatively strong Hα emission. SN 2014C is the best observed member of a class of core-collapse supernovae that fill the gap between events that interact strongly with dense, nearby environments immediately after explosion and those that never show signs of interaction. Better understanding of the frequency and nature of this intermediate population can contribute valuable information about the poorly understood final stages of stellar evolution.

We present continued multi-frequency radio observations of the relativistic tidal disruption event Swift J164449.3+573451 (Sw 1644+57) extending to δt ≈ 600 d. The data were obtained with the JVLA and AMI Large Array as part of our on-going study of the jet energetics and the density structure of the parsecscale environment around the disrupting supermassive black hole (SMBH). We combine these data with public Swift/XRT and Chandra X-ray observations over the same time-frame to show that the jet has undergone a dramatic transition starting at ≈ 500 d, with a sharp decline in the X-ray flux by about a factor of 170 on a timescale of δt/t . 0.2 (and by a factor of 15 in δt/t ≈ 0.05). The rapid decline rules out a forward shock origin (direct or reprocessing) for the X-ray emission at . 500 d, and instead points to internal dissipation in the inner jet. On the other hand, our radio data uniquely demonstrate that the low X-ray flux measured by Chandra at ≈ 610 d is consistent with emission from the forward shock. Furthermore, the Chandra data are inconsistent with thermal emission from the accretion disk itself since the expected temperature of ∼ 30 − 60 eV and inner radius of ∼ 2 − 10Rs cannot accommodate the observed flux level or the detected emission at & 1 keV. We associate the rapid decline with a turn off of the relativistic jet when the mass accretion rate dropped below ∼ M˙ Edd ≈ 0.006 M⊙ yr−1 (for a 3 × 106 M⊙ black hole and order unity efficiency) indicating that the peak accretion rate was about 330M˙ Edd, and the total accreted mass by δt ≈ 500 d is about 0.15 M⊙. From the radio data we further find significant flattening in the integrated energy of the forward shock at δt & 250 d with Ej,iso ≈ 2 × 1054 erg (Ej ≈ 1052 erg for a jet opening angle, θj = 0.1) following a rise by about a factor of 15 at ≈ 30 − 250 d. Projecting forward, we predict that the emission in the radio and X-ray bands will evolve in tandem with similar decline rates.

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 the Pan-STARRS1 discovery of the long-lived and blue transient PS1-11af, which was also detected by Galaxy Evolution Explorer with coordinated observations in the near-ultraviolet (NUV) band. PS1-11af is associated with the nucleus of an early type galaxy at redshift z = 0.4046 that exhibits no evidence for star formation or active galactic nucleus activity. Four epochs of spectroscopy reveal a pair of transient broad absorption features in the UV on otherwise featureless spectra. Despite the superficial similarity of these features to P-Cygni absorptions of supernovae (SNe), we conclude that PS1-11af is not consistent with the properties of known types of SNe. Blackbody fits to the spectral energy distribution are inconsistent with the cooling, expanding ejecta of a SN, and the velocities of the absorption features are too high to represent material in homologous expansion near a SN photosphere. However, the constant blue colors and slow evolution of the luminosity are similar to previous optically selected tidal disruption events (TDEs). The shape of the optical light curve is consistent with models for TDEs, but the minimum accreted mass necessary to power the observed luminosity is only ∼0.002 M , which points to a partial disruption model. A full disruption model predicts higher bolometric luminosities, which would require most of the radiation to be emitted in a separate component at high energies where we lack observations. In addition, the observed temperature is lower than that predicted by pure accretion disk models for TDEs and requires reprocessing to a constant, lower temperature. Three deep non-detections in the radio with the Very Large Array over the first two years after the event set strict limits on the production of any relativistic outflow comparable to Swift J1644+57, even if off-axis.

We report the detection of radio emission from the unusually active L5e + T7 binary 2MASS J13153094−2649513AB made with the Australian Telescope Compact Array. Observations at 5.5 GHz reveal an unresolved source with a continuum flux of 370±50 µJy, corresponding to a radio luminosity of Lrad = νLν = (9±3)×1023 erg s−1 and log10 Lrad/Lbol = −5.44±0.22. No detection is made at 9.0 GHz to a 5σ limit of 290 µJy, consistent with a power law spectrum Sν ∝ ν−α with α & 0.5. The emission is quiescent, with no evidence of variability or bursts over 3 hr of observation, and no measurable polarization (V/I < 34%). 2MASS J1315−2649AB is one of the most radio-luminous ultracool dwarfs detected in quiescent emission to date, comparable in strength to other cool sources detected in outburst. Its detection indicates no decline in radio flux through the mid-L dwarfs. It is unique among L dwarfs in having strong and persistent Hα and radio emission, indicating the coexistence of a cool, neutral photosphere (low electron density) and a highly active chromosphere (high electron density and active heating). These traits, coupled with the system’s mature age and substellar secondary, makes 2MASS J1315−2649AB an important test for proposed radio emission mechanisms in ultracool dwarfs.

The double explosion of SN 2009ip in 2012 raises questions about our understanding of the late stages of massive star evolution. Here we present a comprehensive study of SN 2009ip during its remarkable rebrightenings. High-cadence photometric and spectroscopic observations from the GeV to the radio band obtained from a variety of ground-based and space facilities (including the Very Large Array, Swift, Fermi, Hubble Space Telescope, and XMM) constrain SN 2009ip to be a low energy (E ~ 1050 erg for an ejecta mass ~0.5 M ☉) and asymmetric explosion in a complex medium shaped by multiple eruptions of the restless progenitor star. Most of the energy is radiated as a result of the shock breaking out through a dense shell of material located at ~5 × 1014 cm with M ~ 0.1 M ☉, ejected by the precursor outburst ~40 days before the major explosion. We interpret the NIR excess of emission as signature of material located further out, the origin of which has to be connected with documented mass-loss episodes in the previous years. Our modeling predicts bright neutrino emission associated with the shock break-out if the cosmic-ray energy is comparable to the radiated energy. We connect this phenomenology with the explosive ejection of the outer layers of the massive progenitor star, which later interacted with material deposited in the surroundings by previous eruptions. Future observations will reveal if the massive luminous progenitor star survived. Irrespective of whether the explosion was terminal, SN 2009ip brought to light the existence of new channels for sustained episodic mass loss, the physical origin of which has yet to be identified.

Radio detections of ultracool dwarfs provide insight into their magnetic fields and the dynamos that maintain them, especially at the very bottom of the main sequence, where other activity indicators dramatically weaken. Until recently, radio emission was only detected in the M and L dwarf regimes, but this has changed with the Arecibo detection of rapid polarized flares from the T6.5 dwarf 2MASS J10475385+2124234. Here, we report the detection of quasi-quiescent radio emission from this source at 5.8 GHz using the Karl G. Jansky Very Large Array. The spectral luminosity is Lν = (2.2 ± 0.7) × 1012 erg s−1 Hz−1, a factor of ∼100 times fainter than the Arecibo flares. Our detection is the lowest-luminosity yet achieved for an ultracool dwarf. Although the emission is fully consistent with being steady, unpolarized, and broadband, we find tantalizing hints for variability. We exclude the presence of short-duration flares as seen by Arecibo, although this is not unexpected given estimates of the duty cycle. Follow-up observations of this object will offer the potential to constrain its rotation period, electron density, and the strength and configuration of the magnetic field. Equally important, follow-up will address the question of whether the electron cyclotron maser instability, which is thought to produce the flares seen by Arecibo, also operates in the very different parameter regime of the emission we detect, or whether instead this ultracool dwarf exhibits both maser and gyrosynchrotron radiation, potentially originating from substantially different locations.

We present broad-band observations of the afterglow and environment of the short GRB 111020A. An extensive X-ray light curve from Swift/XRT, XMM-Newton and Chandra, spanning ∼ 100 seconds to 10 days after the burst, reveals a significant break at δt ≈2 days with pre- and post-break decline rates of αX,1 ≈ −0.78 and αX,2 . −1.7, respectively. Interpreted as a jet break, we infer a collimated outflow with an opening angle of θj ≈ 3 − 8◦. The resulting beaming-corrected γ-ray (10 − 1000 keV band) and blastwave kinetic energies are (2−3)×1048 erg and (0.3−2)×1049 erg, respectively, with the range depending on the unknown redshift of the burst. We report a radio afterglow limit of <39 µJy (3σ) from EVLA observations which, along with our finding that νc < νX , constrains the circumburst density to n0 ∼ 0.01 − 0.1 cm−3. Optical observations provide an afterglow limit of i & 24.4 mag at 18 hours after the burst, and reveal a potential host galaxy with i ≈ 24.3 mag. The sub-arcsecond localization from Chandra provides a precise offset of 0.80′′ ±0.11′′ (1σ) from this galaxy corresponding to an offset of 5−7 kpc for z = 0.5−1.5. We find a high excess neutral Hydrogen column density of (7.5 ± 2.0) × 1021 cm−2 (z = 0). Our observations demonstrate that a growing fraction of short GRBs are collimated which may lead to a true event rate of & 100 − 1000 Gpc−3 yr−1, in good agreement with the NS-NS merger rate of ≈ 200 − 3000 Gpc−3 yr−1. This consistency is promising for coincident short GRB-gravitational wave searches in the forthcoming era of Advanced LIGO/VIRGO.

We present a comprehensive catalog and analysis of broad-band afterglow observations for 103 short duration gamma-ray bursts (GRBs), comprised of all short GRBs from November 2004 to March 2015 with prompt follow-up observations in the X-ray, optical, near-infrared and/or radio bands. These afterglow observations have uncovered 71 X-ray detections, 30 optical/NIR detections, and 4 radio detections. Employing the standard afterglow synchrotron model, we perform joint probability analyses for a subset of 38 short GRBs with wellsampled light curves to infer the burst isotropic-equivalent energies and circumburst densities. For this subset, we find median isotropic-equivalent γ-ray and kinetic energies of Eγ,iso ≈ 2 × 1051 erg, and EK,iso ≈ (1 − 3) × 1051 erg, respectively, depending on the values of the model input parameters. We further find that short GRBs occur in low-density environments, with a median density of n ≈ (3 − 15) × 10−3 cm−3 , and that ≈ 80 − 95% of bursts have densities of n . 1 cm−3. We investigate trends between the circumburst densities and host galaxy properties, and find that events located at large projected offsets of & 10 effective radii from their hosts exhibit particularly low densities of n . 10−4 cm−3 , consistent with an IGM-like environment. Using late-time afterglow data for 11 events, we find a median jet opening angle of θj = 16±10◦ . We also calculate a median beaming factor of fb ≈ 0.04, leading to a beaming-corrected total energy release of Etrue ≈ 1.6 × 1050 erg. Furthermore, we calculate a beaming-corrected event rate of ℜtrue = 270+1580 −180 Gpc−3 yr−1 , or ≈ 8 +47 −5 yr−1 within a 200 Mpc volume, the Advanced LIGO/Virgo typical detection distance for NS-NS binaries.