arXiv subjects

A. M. Soderberg

Publications and source records attributed to A. M. Soderberg.

At least 37 records · Page 2Linked to original sources

arXiv2013

Multi-Epoch High-Spectral-Resolution Observations of Neutral Sodium in 14 Type Ia Supernovae

One of the main questions concerning Type Ia supernovae is the nature of the binary companion of the exploding white dwarf. A major discriminant between different suggested models is the presence and physical properties of circumstellar material at the time of explosion. If present, this material will be ionized by the ultra-violet radiation of the explosion and later recombine. This ionization-recombination should manifest itself as time-variable absorption features that can be detected via multi-epoch high-spectral-resolution observations. Previous studies have shown that the strongest effect is seen in the neutral sodium D lines. We report on observations of neutral sodium absorption features observed in multi-epoch high-resolution spectra of 14 Type Ia supernova events. This is the first multi-epoch high-resolution study to include multiple SNe. No variability in line strength that can be associated with circumstellar material is detected. We find that ~18% of the events in the extended sample exhibit time-variable sodium features associated with circumstellar material. We explore the implication of this study on our understanding of the progenitor systems of Type Ia supernovae via the current Type Ia supernova multi-epoch high-spectral-resolution sample.

astro-ph.HE↗

arXiv2013

The UV-bright, Slowly Declining Transient PS1-11af as a Partial Tidal Disruption Event

We present the Pan-STARRS1 discovery of the long-lived and blue transient PS1-11af, which was also detected by GALEX 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 AGN 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 ~0.002M_sun, 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 VLA 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.

astro-ph.CO↗

arXiv2013

The signature of the central engine in the weakest relativistic explosions: GRB100316D

We present late-time radio and X-ray observations of the nearby sub-energetic Gamma-Ray Burst (GRB)100316D associated with supernova (SN) 2010bh. Our broad-band analysis constrains the explosion properties of GRB100316D to be intermediate between highly relativistic, collimated GRBs and the spherical, ordinary hydrogen-stripped SNe. We find that ~10^49 erg is coupled to mildly-relativistic (Gamma=1.5-2), quasi-spherical ejecta, expanding into a medium previously shaped by the progenitor mass-loss with rate Mdot ~10^-5 Msun yr^-1 (for wind velocity v_w = 1000 km s^-1). The kinetic energy profile of the ejecta argues for the presence of a central engine and identifies GRB100316D as one of the weakest central-engine driven explosions detected to date. Emission from the central engine is responsible for an excess of soft X-ray radiation which dominates over the standard afterglow at late times (t>10 days). We connect this phenomenology with the birth of the most rapidly rotating magnetars. Alternatively, accretion onto a newly formed black hole might explain the excess of radiation. However, significant departure from the standard fall-back scenario is required.

astro-ph.HE↗

arXiv2013

A Search for Fast Optical Transients in the Pan-STARRS1 Medium-Deep Survey: M Dwarf Flares, Asteroids, Limits on Extragalactic Rates, and Implications for LSST

[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 filter), 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.

astro-ph.HE↗

arXiv2013

The Fast and Furious Decay of the Peculiar Type Ic Supernova 2005ek

We present extensive multi-wavelength observations of the extremely rapidly declining Type Ic supernova, SN 2005ek. Reaching a peak magnitude of M_R = -17.3 and decaying by ~3 mag in the first 15 days post-maximum, SN 2005ek is among the fastest Type I supernovae observed to date. The spectra of SN 2005ek closely resemble those of normal SN Ic, but with an accelerated evolution. There is evidence for the onset of nebular features at only nine days post-maximum. Spectroscopic modeling reveals an ejecta mass of ~0.3 Msun that is dominated by oxygen (~80%), while the pseudo-bolometric light curve is consistent with an explosion powered by ~0.03 Msun of radioactive Ni-56. Although previous rapidly evolving events (e.g., SN 1885A, SN 1939B, SN 2002bj, SN 2010X) were hypothesized to be produced by the detonation of a helium shell on a white dwarf, oxygen-dominated ejecta are difficult to reconcile with this proposed mechanism. We find that the properties of SN 2005ek are consistent with either the edge-lit double detonation of a low-mass white dwarf or the iron-core collapse of a massive star, stripped by binary interaction. However, if we assume that the strong spectroscopic similarity of SN 2005ek to other SN Ic is an indication of a similar progenitor channel, then a white-dwarf progenitor becomes very improbable. SN 2005ek may be one of the lowest mass stripped-envelope core-collapse explosions ever observed. We find that the rate of such rapidly declining Type I events is at least 1-3% of the normal SN Ia rate.

astro-ph.HE↗

arXiv2013

PS1-10bzj: A Fast, Hydrogen-Poor Superluminous Supernova in a Metal Poor Host Galaxy

We present observations and analysis of PS1-10bzj, a superluminous supernova (SLSN) discovered in the Pan-STARRS Medium Deep Survey at a redshift z = 0.650. Spectroscopically, PS1-10bzj is similar to the hydrogen-poor SLSNe 2005ap and SCP 06F6, though with a steeper rise and lower peak luminosity (M_bol = -21.4 mag) than previous events. We construct a bolometric light curve, and show that while PS1-10bzj's energetics were less extreme than previous events, its luminosity still cannot be explained by radioactive nickel decay alone. We explore both a magnetar spin-down and circumstellar interaction scenario and find that either can fit the data. PS1-10bzj is located in the Extended Chandra Deep Field South and the host galaxy is imaged in a number of surveys, including with the Hubble Space Telescope. The host is a compact dwarf galaxy (M_B ~ -18 mag, diameter < 800 pc), with a low stellar mass (M_* ~ 2.4 * 10^7 M_sun), young stellar population (τ_* ~ 5 Myr), and a star formation rate of ~ 2-3 M_sun/yr. The specific star formation rate is the highest seen in a SLSN host so far (~ 100 Gyr^{-1}). We detect the [O III]λ4363 line, and find a low metallicity: 12+(O/H) = 7.8 +/- 0.2 (~ 0.1 Z_sun). Together, this indicates that at least some of the progenitors of SLSNe come from young, low-metallicity populations.

astro-ph.HE↗

arXiv2013

A panchromatic view of the restless SN2009ip reveals the explosive ejection of a massive star envelope

The 2012 explosion of SN2009ip raises questions about our understanding of the late stages of massive star evolution. Here we present a comprehensive study of SN2009ip during its remarkable re-brightening(s). 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 VLA, Swift, Fermi, HST and XMM) constrain SN2009ip to be a low energy (E~ 10^50 erg for an ejecta mass ~ 0.5 Msun) and likely 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 5x10^14 cm with M~0.1 Msun, ejected by the precursor outburst ~40 days before the major explosion. We interpret the NIR excess of emission as signature of dust vaporization of material located further out (R>4x 10^15 cm), 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, that later interacted with material deposited in the surroundings by previous eruptions. Future observations will reveal if the luminous blue variable (LBV) progenitor star survived. Irrespective of whether the explosion was terminal, SN2009ip brought to light the existence of new channels for sustained episodic mass-loss, the physical origin of which has yet to be identified.

astro-ph.HE↗

arXiv2013

A Reverse Shock in GRB 130427A

We present extensive radio and millimeter observations of the unusually bright GRB 130427A at z=0.340, spanning 0.67 to 12 days after the burst. Taken in conjunction with detailed multi-band UV, optical, NIR, and X-ray observations we find that the broad-band afterglow emission is 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 emission dominates in the X-rays and at >0.1 days in the UV/optical/NIR. We further find that the optical and X-ray data require a Wind circumburst environment, pointing to a massive star progenitor. Using the combined forward and reverse shock emission we find that the parameters of the burst are an isotropic kinetic energy of E_Kiso~2e53 erg, a mass loss rate of Mdot~3e-8 Msun/yr (for a wind velocity of 1,000 km/s), and a Lorentz factor at the deceleration time of Gamma(200s)~130. Due to the low density and large isotropic energy, the absence of a jet break to ~15 days places only a weak constraint on the opening angle of theta_j>2.5 deg, and therefore a total energy of E_gamma+E_K>1.2e51 erg, similar to other GRBs. The reverse shock emission is detectable in this burst due to the low circumburst density, which leads to a slow cooling shock. We speculate that this is a required property for the detectability of reverse shocks in the radio and millimeter bands. Following on GRB 130427A as a benchmark event, observations of future GRBs with the exquisite sensitivity of VLA and ALMA, coupled with detailed modeling of the reverse and forward shock contributions will test this hypothesis.

astro-ph.HE↗

arXiv2013

PS1-12sk is a Peculiar Supernova From a He-rich Progenitor System in a Brightest Cluster Galaxy Environment

We report on our discovery and observations of the Pan-STARRS1 supernova (SN) PS1-12sk, a transient with properties that indicate atypical star formation in its host galaxy cluster or pose a challenge to popular progenitor system models for this class of explosion. The optical spectra of PS1-12sk classify it as a Type Ibn SN (c.f. SN 2006jc), dominated by intermediate-width (3x10^3 km/s) and time variable He I emission. Our multi-wavelength monitoring establishes the rise time dt = 9-23 days and shows an NUV-NIR SED with temperature > 17x10^3 K and a peak rise magnitude of Mz = -18.9 mag. SN Ibn spectroscopic properties are commonly interpreted as the signature of a massive star (17 - 100 M_sun) explosion within a He-enriched circumstellar medium. However, unlike previous Type Ibn supernovae, PS1-12sk is associated with an elliptical brightest cluster galaxy, CGCG 208-042 (z = 0.054) in cluster RXC J0844.9+4258. The expected probability of an event like PS1-12sk in such environments is low given the measured infrequency of core-collapse SNe in red sequence galaxies compounded by the low volumetric rate of SN Ibn. Furthermore, we find no evidence of star formation at the explosion site to sensitive limits (Sigma Halpha < 2x10^-3 M_sun/yr/kpc^2). We therefore discuss white dwarf binary systems as a possible progenitor channel for SNe Ibn. We conclude that PS1-12sk represents either a fortuitous and statistically unlikely discovery, evidence for a top-heavy IMF in galaxy cluster cooling flow filaments, or the first clue suggesting an alternate progenitor channel for Type Ibn SNe.

astro-ph.CO↗

arXiv2013

PS1-10afx at z=1.388: Pan-STARRS1 Discovery of a New Type of Superluminous Supernova

We present the Pan-STARRS1 discovery of PS1-10afx, a unique hydrogen-deficient superluminous supernova (SLSN) at z=1.388. The light curve peaked at z_P1=21.7 mag, making PS1-10afx comparable to the most luminous known SNe, with M_u = -22.3 mag. Our extensive optical and NIR observations indicate that the bolometric light curve of PS1-10afx rose on the unusually fast timescale of ~12 d to the extraordinary peak luminosity of 4.1e44 erg/s (M_bol = -22.8 mag) and subsequently faded rapidly. Equally important, the SED is unusually red for a SLSN, with a color temperature of 6800 K near maximum light, in contrast to previous H-poor SLSNe, which are bright in the UV. The spectra more closely resemble those of a normal SN Ic than any known SLSN, with a photospheric velocity of 11,000 km/s and evidence for line blanketing in the rest-frame UV. Despite the fast rise, these parameters imply a very large emitting radius (>5e15 cm). We demonstrate that no existing theoretical model can satisfactorily explain this combination of properties: (i) a nickel-powered light curve cannot match the combination of high peak luminosity with the fast timescale; (ii) models powered by the spindown energy of a rapidly-rotating magnetar predict significantly hotter and faster ejecta; and (iii) models invoking shock breakout through a dense circumstellar medium cannot explain the observed spectra or color evolution. The host galaxy is well detected in pre-explosion imaging with a luminosity near L*, a star formation rate of 15 M_sun/yr, and is fairly massive (2e10 M_sun), with a stellar population age of 1e8 yr, also in contrast to the dwarf hosts of known H-poor SLSNe. PS1-10afx is distinct from known examples of SLSNe in its spectra, colors, light-curve shape, and host galaxy properties, suggesting that it resulted from a different channel than other hydrogen-poor SLSNe.

astro-ph.CO↗

arXiv2013

Multi-Wavelength Observations of Supernova 2011ei: Time-Dependent Classification of Type IIb and Ib Supernovae and Implications for their Progenitors

We present X-ray, UV/optical, and radio observations of the stripped-envelope, core-collapse supernova (SN) 2011ei, one of the least luminous SNe IIb or Ib observed to date. Our observations begin with a discovery within 1 day of explosion and span several months afterward. Early optical spectra exhibit broad, Type II-like hydrogen Balmer profiles that subside rapidly and are replaced by Type Ib-like He-rich features on the timescale of one week. High-cadence monitoring of this transition suggests that absorption attributable to a high velocity (> 12,000 km/s) H-rich shell is not rare in Type Ib events. Radio observations imply a shock velocity of v = 0.13c and a progenitor star mass-loss rate of 1.4 x 10^{-5} Msun yr^{-1} (assuming wind velocity v_w=10^3 km/s). This is consistent with independent constraints from deep X-ray observations with Swift-XRT and Chandra. Overall, the multi-wavelength properties of SN 2011ei are consistent with the explosion of a lower-mass (3-4 Msun), compact (R* <= 1x10^{11} cm), He core star. The star retained a thin hydrogen envelope at the time of explosion, and was embedded in an inhomogeneous circumstellar wind suggestive of modest episodic mass-loss. We conclude that SN 2011ei's rapid spectral metamorphosis is indicative of time-dependent classifications that bias estimates of explosion rates for Type IIb and Ib objects, and that important information about a progenitor star's evolutionary state and mass-loss immediately prior to SN explosion can be inferred from timely multi-wavelength observations.

astro-ph.HE↗

arXiv2013

Type Iax Supernovae: A New Class of Stellar Explosion

We describe observed properties of the Type Iax class of supernovae (SNe Iax), consisting of SNe observationally similar to its prototypical member, SN 2002cx. The class currently has 25 members, and we present optical photometry and/or optical spectroscopy for most of them. SNe Iax are spectroscopically similar to SNe Ia, but have lower maximum-light velocities (2000 < |v| < 8000 km/s), typically lower peak magnitudes (-14.2 > M_V,peak > -18.9 mag), and most have hot photospheres. Relative to SNe Ia, SNe Iax have low luminosities for their light-curve shape. There is a correlation between luminosity and light-curve shape, similar to that of SNe Ia, but offset from that of SNe Ia and with larger scatter. Despite a host-galaxy morphology distribution that is highly skewed to late-type galaxies without any SNe Iax discovered in elliptical galaxies, there are several indications that the progenitor stars are white dwarfs (WDs): evidence of C/O burning in their maximum-light spectra, low ejecta masses, strong Fe lines in their late-time spectra, a lack of X-ray detections, and deep limits on massive stars and star formation at the SN sites. However, two SNe Iax show strong He lines in their spectra. The progenitor system and explosion model that best fits all of the data is a binary system of a C/O WD that accretes matter from a He star and has a significant deflagration. At least some of the time, this explosion will not disrupt the WD. We estimate that in a given volume there are 31^+17_-13 SNe Iax for every 100 SNe Ia, and for every 1 M_sun of iron generated by SNe Ia at z = 0, SNe Iax generate 0.052^+0.017_-0.014 M_sun. Being the largest class of peculiar SNe, thousands of SNe Iax will be discovered by LSST. Future detailed observations of SNe Iax should further our understanding of both their progenitor systems and explosions as well as those of SNe Ia.

astro-ph.SR↗

arXiv2012

Radio Monitoring of the Tidal Disruption Event Swift J164449.3+573451. II. The Relativistic Jet Shuts Off and a Transition to Forward Shock X-ray/Radio Emission

We present continued multi-frequency radio observations of the relativistic tidal disruption event Sw1644+57 extending to dt~600 d. The data were obtained with the JVLA and AMI Large Array. 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 dt/t<0.2. 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 2-10 R_s 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 Mdot_Edd~0.006 Msun/yr (for a 3x10^6 Msun black hole and order unity efficiency) indicating that the peak accretion rate was about 330 Mdot_Edd, and the total accreted mass by 500 d is about 0.15 Msun. From the radio data we further find significant flattening in the integrated energy of the forward shock at >250 d with E_j,iso~2x10^54 erg (E_j~10^52$ erg for a jet opening angle, theta_j=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.

astro-ph.HE↗

arXiv2012

Illuminating the Darkest Gamma-Ray Bursts with Radio Observations

We present X-ray, optical, near-infrared, 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-infrared 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 1.8 < z < 2.7 (111215A). Using the radio and X-ray data for each burst we find that GRB 110709B requires A_V > 5.3 mag and GRB 111215A requires A_V > 8.5 mag (z=2), among the largest extinction values inferred for dark bursts to-date. The two bursts also exhibit large neutral hydrogen column densities (N_H > 10^22/cm^2; z=2) as inferred from their X-ray spectra, in agreement with the trend for dark GRBs. Finally, we find that for both bursts the afterglow emission is best explained by a collimated outflow with a total beaming-corrected energy of E_gamma+E_K ~ 7-9 x 10^51 erg (z=2) expanding into a wind medium with a high density (n~100-350 cm^-3 at 10^17 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 determining the origin and properties of dark GRBs. Observations with the JVLA and ALMA will provide a sample with sub-arcsecond positions and robust host associations that will help shed light on obscured star formation and the role of metallicity in GRB progenitors.

astro-ph.HE↗

arXiv2012

The Afterglow and Environment of the Short GRB111117A

We present multi-wavelength observations of the afterglow of the short GRB111117A, and follow-up observations of its host galaxy. From rapid optical and radio observations we place limits of r \gtrsim 25.5 mag at \deltat \approx 0.55 d and F_nu(5.8 GHz) < 18 \muJy at \deltat \approx 0.50 d, respectively. However, using a Chandra observation at t~3.0 d we locate the absolute position of the X-ray afterglow to an accuracy of 0.22" (1 sigma), a factor of about 6 times better than the Swift-XRT position. This allows us to robustly identify the host galaxy and to locate the burst at a projected offset of 1.25 +/- 0.20" from the host centroid. Using optical and near-IR observations of the host galaxy we determine a photometric redshift of z=1.3 (+0.3,-0.2), one of the highest for any short GRB, and leading to a projected physical offset for the burst of 10.5 +/- 1.7 kpc, typical of previous short GRBs. At this redshift, the isotropic gamma-ray energy is E_{gamma,iso} \approx 3\times10^51 erg (rest-frame 23-2300 keV) with a peak energy of E_{pk} \approx 850-2300 keV (rest-frame). In conjunction with the isotropic X-ray energy, GRB111117A appears to follow our recently-reported E_x,iso-E_gamma,iso-E_pk universal scaling. Using the X-ray data along with the optical and radio non-detections we find that for a blastwave kinetic energy of E_{K,iso} \approx E_{gamma,iso}, the circumburst density is n_0 \sim 3x10^(-4)-1 cm^-3 (for a range of epsilon_B=0.001-0.1). Similarly, from the non-detection of a break in the X-ray light curve at t<3 d, we infer a minimum opening angle for the outflow of theta_j> 3-10 degrees (depending on the circumburst density). We conclude that Chandra observations of short GRBs are effective at determining precise positions and robust host galaxy associations in the absence of optical and radio detections.

astro-ph.HE↗

arXiv2012

Ultra-Luminous Supernovae as a New Probe of the Interstellar Medium in Distant Galaxies

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 highest 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.

astro-ph.CO↗

arXiv2012

VLBI Observations of the Nearby Type IIb Supernova 2011dh

We report on phase-referenced VLBI radio observations of the Type IIb supernova 2011dh, at times t = 83 days and 179 days after the explosion and at frequencies, respectively, of 22.2 and 8.4 GHz. We detected SN 2011dh at both epochs. At the first epoch only an upper limit on SN 2011dh's angular size was obtained, but at the second epoch, we determine the angular radius SN 2011dh's radio emission to be 0.25 +- 0.08 mas by fitting a spherical shell model directly to the visibility measurements. At a distance of 8.4 Mpc this angular radius corresponds to a time-averaged (since t=0) expansion velocity of the forward shock of 21000 +- 7000 km/s. Our measured values of the radius of the emission region are in excellent agreement with those derived from fitting synchrotron self-absorbed models to the radio spectral energy distribution, providing strong confirmation for the latter method of estimating the radius. We find that SN 2011dh's radius evolves in a power-law fashion, with R proportional to t^(0.92 +- 0.10).

astro-ph.HE↗

arXiv2012

EVLA Observations of the Radio Evolution of SN 2011dh

We report on Expanded Very Large Array (EVLA) observations of the Type IIb supernova 2011dh, performed over the first 100 days of its evolution and spanning 1-40 GHz in frequency. The radio emission is well-described by the self-similar propagation of a spherical shockwave, generated as the supernova ejecta interact with the local circumstellar environment. Modeling this emission with a standard synchrotron self-absorption (SSA) model gives an average expansion velocity of v \approx 0.1c, supporting the classification of the progenitor as a compact star (R_* \approx 10^11 cm). We find that the circumstellar density is consistent with a ρ \propto r^-2 profile. We determine that the progenitor shed mass at a constant rate of \approx 3 \times 10^-5 M_\odot / yr, assuming a wind velocity of 1000 km / s (values appropriate for a Wolf-Rayet star), or \approx 7 \times 10^-7 M_\odot / yr assuming 20 km / s (appropriate for a yellow supergiant [YSG] star). Both values of the mass-loss rate assume a converted fraction of kinetic to magnetic energy density of ε_B = 0.1. Although optical imaging shows the presence of a YSG, the rapid optical evolution and fast expansion argue that the progenitor is a more compact star - perhaps a companion to the YSG. Furthermore, the excellent agreement of the radio properties of SN 2011dh with the SSA model implies that any YSG companion is likely in a wide, non-interacting orbit.

astro-ph.HE↗