arXiv subjects

J. S. Bloom

Publications and source records attributed to J. S. Bloom.

At least 55 records · Page 3Linked to original sources

arXiv2013

Evidence for Dust Destruction from the Early-time Colour Change of GRB 120119A

We present broadband observations and analysis of Swift gamma-ray burst (GRB) 120119A. Our early-time afterglow detections began under 15 s after the burst in the host frame (redshift z = 1.73), and they yield constraints on the burst energetics and local environment. Late-time afterglow observations of the burst show evidence for a moderate column of dust (A_V ~ 1.1 mag) similar to, but statistically distinct from, dust seen along Small Magellanic Cloud sightlines. Deep late-time observations reveal a dusty, rapidly star-forming host galaxy. Most notably, our early-time observations exhibit a significant red-to-blue colour change in the first ~200 s after the trigger at levels heretofore unseen in GRB afterglows. This colour change, which is coincident with the final phases of the prompt emission, is a hallmark prediction of the photodestruction of dust in GRB afterglows. We test whether dust-destruction signatures are significantly distinct from other sources of colour change, namely a change in the intrinsic spectral index β. We find that a time-varying power-law spectrum alone cannot adequately describe the observed colour change, and allowing for dust destruction (via a time-varying A_V) significantly improves the fit. While not definitively ruling out other possibilities, this event provides the best support yet for the direct detection of dust destruction in the local environment of a GRB.

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arXiv2013

An outburst from a massive star 40 days before a supernova explosion

Various lines of evidence suggest that very massive stars experience extreme mass-loss episodes shortly before they explode as a supernova. Interestingly, several models predict such pre-explosion outbursts. Establishing a causal connection between these mass-loss episodes and the final supernova explosion will provide a novel way to study pre-supernova massive-star evolution. Here we report on observations of a remarkable mass-loss event detected 40 days prior to the explosion of the Type IIn supernova SN 2010mc (PTF 10tel). Our photometric and spectroscopic data suggest that this event is a result of an energetic outburst, radiating at least 6x10^47 erg of energy, and releasing about 0.01 Solar mass at typical velocities of 2000 km/s. We show that the temporal proximity of the mass-loss outburst and the supernova explosion implies a causal connection between them. Moreover, we find that the outburst luminosity and velocity are consistent with the predictions of the wave-driven pulsation model and disfavor alternative suggestions.

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arXiv2012

An Independent Measurement of the Incidence of MgII Absorbers along Gamma-Ray Burst Sightlines: the End of the Mystery?

In 2006, Prochter et al. reported a statistically significant enhancement of very strong Mg II absorption systems intervening the sightlines to gamma-ray bursts (GRBs) relative to the in- cidence of such absorption along quasar sightlines. This counterintuitive result, has inspired a diverse set of astrophysical explanations (e.g. dust, gravitational lensing) but none of these has obviously resolved the puzzle. Using the largest set of GRB afterglow spectra available, we reexamine the purported enhancement. In an independent sample of GRB spectra with a survey path 3 times larger than Prochter et al., we measure the incidence per unit redshift of $\geq 1$Årest-frame equivalent width Mg II absorbers at $z \approx 1$ to be l(z)= 0.18 $\pm$ 0.06. This is fully consistent with current estimates for the incidence of such absorbers along quasar sightlines. Therefore, we do not confirm the original enhancement and suggest those results suffered from a statistical fluke. Signatures of the original result do remain in our full sample (l(z) shows an $\approx 1.5$ enhancement over l(z)QSO), but the statistical significance now lies at $\approx 90%$ c.l. Restricting our analysis to the subset of high-resolution spectra of GRB afterglows (which overlaps substantially with Prochter et al.), we still reproduce a statistically significant enhancement of Mg II absorption. The reason for this excess, if real, is still unclear since there is no connection between the rapid afterglow follow-up process with echelle (or echellette) spectrographs and the detectability of strong Mg II doublets. Only a larger sample of such high-resolution data will shed some light on this matter.

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arXiv2012

The Frequency of Debris Disks at White Dwarfs

We present near- and mid-infrared photometry and spectroscopy from PAIRITEL, IRTF, and Spitzer of a metallicity-unbiased sample of 117 cool, hydrogen-atmosphere white dwarfs from the Palomar-Green survey and find five with excess radiation in the infrared, translating to a 4.3+2.7-1.2% frequency of debris disks. This is slightly higher than, but consistent with the results of previous surveys. Using an initial-final mass relation, we apply this result to the progenitor stars of our sample and conclude that 1-7Msol stars have at least a 4.3% chance of hosting planets; an indirect probe of the intermediate-mass regime eluding conventional exoplanetary detection methods. Alternatively, we interpret this result as a limit on accretion timescales as a fraction of white dwarf cooling ages; white dwarfs accrete debris from several generations of disks for ~10Myr. The average total mass accreted by these stars ranges from that of 200km asteroids to Ceres-sized objects, indicating that white dwarfs accrete moons and dwarf planets as well as Solar System asteroid analogues.

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arXiv2012

Discovery of Bright Galactic R Coronae Borealis and DY Persei Variables: Rare Gems Mined from ACVS

We present the results of a machine-learning (ML) based search for new R Coronae Borealis (RCB) stars and DY Persei-like stars (DYPers) in the Galaxy using cataloged light curves from the All-Sky Automated Survey (ASAS) Catalog of Variable Stars (ACVS). RCB stars - a rare class of hydrogen-deficient carbon-rich supergiants - are of great interest owing to the insights they can provide on the late stages of stellar evolution. DYPers are possibly the low-temperature, low-luminosity analogs to the RCB phenomenon, though additional examples are needed to fully establish this connection. While RCB stars and DYPers are traditionally identified by epochs of extreme dimming that occur without regularity, the ML search framework more fully captures the richness and diversity of their photometric behavior. We demonstrate that our ML method can use newly discovered RCB stars to identify additional candidates within the same data set. Our search yields 15 candidates that we consider likely RCB stars/DYPers: new spectroscopic observations confirm that four of these candidates are RCB stars and four are DYPers. Our discovery of four new DYPers increases the number of known Galactic DYPers from two to six; noteworthy is that one of the new DYPers has a measured parallax and is m ~ 7 mag, making it the brightest known DYPer to date. Future observations of these new DYPers should prove instrumental in establishing the RCB connection. We consider these results, derived from a machine-learned probabilistic classification catalog, as an important proof-of-concept for the efficient discovery of rare sources with time-domain surveys.

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arXiv2012

PTF11kx: A Type-Ia Supernova with a Symbiotic Nova Progenitor

There is a consensus that Type-Ia supernovae (SNe Ia) arise from the thermonuclear explosion of white dwarf stars that accrete matter from a binary companion. However, direct observation of SN Ia progenitors is lacking, and the precise nature of the binary companion remains uncertain. A temporal series of high-resolution optical spectra of the SN Ia PTF 11kx reveals a complex circumstellar environment that provides an unprecedentedly detailed view of the progenitor system. Multiple shells of circumsteller are detected and the SN ejecta are seen to interact with circumstellar material (CSM) starting 59 days after the explosion. These features are best described by a symbiotic nova progenitor, similar to RS Ophiuchi.

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arXiv2012

X-ray emission from supernovae in dense circumstellar matter environments: A search for collisionless shocks

(Abridged). The optical light curve of some SNe may be powered by the outward diffusion of the energy deposited by the explosion shock in optically thick circumstellar matter (CSM). Recently, it was shown that the radiation-mediated and -dominated shock in an optically thick wind must transform into a collisionless shock and can produce hard X-rays. The X-rays are expected to peak at late times, relative to maximum visible light. Here we report on a search, using Swift and Chandra, for X-ray emission from 28 SNe that belong to classes whose progenitors are suspected to be embedded in dense CSM (IIn/Ibn/SLSN-I). Two SNe in our sample have X-ray properties that are roughly consistent with the expectation for X-rays from a collisionless shock in optically thick CSM. Therefore, we suggest that their optical light curves are powered by shock breakout in CSM. We show that two other events were too X-ray bright during the SN maximum optical light to be explained by the shock breakout model. We conclude that the light curves of some, but not all, type-IIn/Ibn SNe are powered by shock breakout in CSM. For the rest of the SNe in our sample, including all the SLSN-I events, our X-ray limits are not deep enough and were typically obtained at too early times to conclude about their nature. We argue that the optical light curves of SNe, for which the X-ray emission peaks at late times, are likely powered by the diffusion of shock energy from a dense CSM. We comment about the possibility to detect some of these events in radio.

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arXiv2012

Evidence for a compact Wolf-Rayet progenitor for the Type Ic supernova PTF 10vgv

We present the discovery of PTF 10vgv, a Type Ic supernova detected by the Palomar Transient Factory, using the Palomar 48-inch telescope (P48). R-band observations of the PTF 10vgv field with P48 probe the supernova emission from its very early phases (about two weeks before R-band maximum), and set limits on its flux in the week prior to the discovery. Our sensitive upper limits and early detections constrain the post-shock-breakout luminosity of this event. Via comparison to numerical (analytical) models, we derive an upper-limit of R \lesssim 4.5 Rsun (R \lesssim 1 Rsun) on the radius of the progenitor star, a direct indication in favor of a compact Wolf-Rayet star. Applying a similar analysis to the historical observations of SN 1994I, yields R \lesssim 1/4 Rsun for the progenitor radius of this supernova.

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arXiv2011

The Luminous Infrared Host Galaxy of Short-Duration GRB 100206A

The known host galaxies of short-hard gamma-ray bursts (GRBs) to date are characterized by low to moderate star-formation rates and a broad range of stellar masses. In this paper, we positionally associate the recent unambiguously short-hard Swift GRB 100206A with a disk galaxy at redshift z=0.4068 that is rapidly forming stars at a rate of ~30 M_sun/yr, almost an order of magnitude higher than any previously identified short GRB host. Using photometry from Gemini, Keck, PAIRITEL, and WISE, we show that the galaxy is very red (g-K = 4.3 AB mag), heavily obscured (A_V ~ 2 mag), and has the highest metallicity of any GRB host to date (12 + log[O/H]_KD02 = 9.2): it is a classical luminous infrared galaxy (LIRG), with L_IR ~ 4 x 10^11 L_sun. While these properties could be interpreted to support an association of this GRB with very recent star formation, modeling of the broadband spectral energy distribution also indicates that a substantial stellar mass of mostly older stars is present. The current specific star-formation rate is modest (specific SFR ~ 0.5 Gyr^-1), the current star-formation rate is not substantially elevated above its long-term average, and the host morphology shows no sign of recent merger activity. Our observations are therefore equally consistent with an older progenitor, similar to what is inferred for other short-hard GRBs. Given the precedent established by previous short GRB hosts and the significant fraction of the Universe's stellar mass in LIRG-like systems at z >~0.3, an older progenitor represents the most likely origin of this event.

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arXiv2011

The dark GRB080207 in an extremely red host and the implications for GRBs in highly obscured environments

[Abridged] We present comprehensive X-ray, optical, near- and mid-infrared, and sub-mm observations of GRB 080207 and its host galaxy. The afterglow was undetected in the optical and near-IR, implying an optical to X-ray index <0.3, identifying GRB 080207 as a dark burst. Swift X-ray observations show extreme absorption in the host, which is confirmed by the unusually large optical extinction found by modelling the X-ray to nIR afterglow spectral energy distribution. Our Chandra observations obtained 8 days post-burst allow us to place the afterglow on the sky to sub-arcsec accuracy, enabling us to pinpoint an extremely red galaxy (ERO). Follow-up host observations with HST, Spitzer, Gemini, Keck and the James Clerk Maxwell Telescope (JCMT) provide a photometric redshift solution of z ~1.74 (+0.05,-0.06) (1 sigma), 1.56 < z < 2.08 at 2 sigma) for the ERO host, and suggest that it is a massive and morphologically disturbed ultra-luminous infrared galaxy (ULIRG) system, with L_FIR ~ 2.4 x 10^12 L_solar. These results add to the growing evidence that GRBs originating in very red hosts always show some evidence of dust extinction in their afterglows (though the converse is not true -- some extinguished afterglows are found in blue hosts). This indicates that a poorly constrained fraction of GRBs occur in very dusty environments. By comparing the inferred stellar masses, and estimates of the gas phase metallicity in both GRB hosts and sub-mm galaxies we suggest that many GRB hosts, even at z>2 are at lower metallicity than the sub-mm galaxy population, offering a likely explanation for the dearth of sub-mm detected GRB hosts. However, we also show that the dark GRB hosts are systematically more massive than those hosting optically bright events, perhaps implying that previous host samples are severely biased by the exclusion of dark events.

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arXiv2011

Constraining GRB Emission Physics with Extensive Early-Time, Multiband Follow-up

Understanding the origin and diversity of emission processes responsible for Gamma-ray Bursts (GRBs) remains a pressing challenge. While prompt and contemporaneous panchromatic observations have the potential to test predictions of the internal-external shock model, extensive multiband imaging has been conducted for only a few GRBs. We present rich, early-time, multiband datasets for two \swift\ events, GRB 110205A and GRB 110213A. The former shows optical emission since the early stages of the prompt phase, followed by the steep rising in flux up to ~1000s after the burst ($t^{-α}$ with $α=-6.13 \pm 0.75$). We discuss this feature in the context of the reverse-shock scenario and interpret the following single power-law decay as being forward-shock dominated. Polarization measurements, obtained with the RINGO2 instrument mounted on the Liverpool Telescope, also provide hints on the nature of the emitting ejecta. The latter event, instead, displays a very peculiar optical to near-infrared lightcurve, with two achromatic peaks. In this case, while the first peak is probably due to the onset of the afterglow, we interpret the second peak to be produced by newly injected material, signifying a late-time activity of the central engine.

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arXiv2011

PTF 10bzf (SN 2010ah): a broad-line Ic supernova discovered by the Palomar Transient Factory

We present the discovery and follow-up observations of a broad-line type-Ic supernova (SN), PTF 10bzf (SN 2010ah), detected by the Palomar Transient Factory (PTF) on 2010 February 23. The SN distance is \cong 218 Mpc, greater than GRB 980425 / SN 1998bw and GRB 060218 / SN 2006aj, but smaller than the other SNe firmly associated with gamma-ray bursts (GRBs). We conducted a multi-wavelength follow-up campaign with Palomar-48 inch, Palomar 60-inch, Gemini-N, Keck, Wise, Swift, the Allen Telescope Array, CARMA, WSRT, and EVLA. Here we compare the properties of PTF 10bzf with those of SN 1998bw and other broad-line SNe. The optical luminosity and spectral properties of PTF 10bzf suggest that this SN is intermediate, in kinetic energy and amount of 56Ni, between non GRB-associated SNe like 2002ap or 1997ef, and GRB-associated SNe like 1998bw. No X-ray or radio counterpart to PTF 10bzf was detected. X-ray upper-limits allow us to exclude the presence of an underlying X-ray afterglow as luminous as that of other SN-associated GRBs like GRB 030329 or GRB 031203. Early-time radio upper-limits do not show evidence for mildly-relativistic ejecta. Late-time radio upper-limits rule out the presence of an underlying off-axis GRB, with energy and wind density similar to the SN-associated GRB 030329 and GRB 031203. Finally, by performing a search for a GRB in the time window and at the position of PTF 10bzf, we find that no GRB in the IPN catalog could be associated with this SN.

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arXiv2011

A faint optical flash in dust-obscured GRB 080603A - implications for GRB prompt emission mechanisms

We report the detection of a faint optical flash by the 2-m Faulkes Telescope North simultaneously with the second of two prompt gamma-ray pulses in INTEGRAL gamma-ray burst (GRB) 080603A, beginning at t_rest = 37 s after the onset of the GRB. This optical flash appears to be distinct from the subsequent emerging afterglow emission, for which we present comprehensive broadband radio to X-ray light curves to 13 days post-burst and rigorously test the standard fireball model. The intrinsic extinction toward GRB 080603A is high (A_V,z = 0.8 mag), and the well-sampled X-ray-to-near-infrared spectral energy distribution is interesting in requiring an LMC2 extinction profile, in contrast to the majority of GRBs. Comparison of the gamma-ray and extinction-corrected optical flux densities of the flash rules out an inverse-Compton origin for the prompt gamma-rays; instead, we suggest that the optical flash could originate from the inhomogeneity of the relativistic flow. In this scenario, a large velocity irregularity in the flow produces the prompt gamma-rays, followed by a milder internal shock at a larger radius that would cause the optical flash. Flat gamma-ray spectra, roughly F propto nu^-0.1, are observed in many GRBs. If the flat spectrum extends down to the optical band in GRB 080603A, the optical flare could be explained as the low-energy tail of the gamma-ray emission. If this is indeed the case, it provides an important clue to understanding the nature of the emission process in the prompt phase of GRBs and highlights the importance of deep (R> 20 mag), rapid follow-up observations capable of detecting faint, prompt optical emission.

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arXiv2011

Automating Discovery and Classification of Transients and Variable Stars in the Synoptic Survey Era

The rate of image acquisition in modern synoptic imaging surveys has already begun to outpace the feasibility of keeping astronomers in the real-time discovery and classification loop. Here we present the inner workings of a framework, based on machine-learning algorithms, that captures expert training and ground-truth knowledge about the variable and transient sky to automate 1) the process of discovery on image differences and, 2) the generation of preliminary science-type classifications of discovered sources. Since follow-up resources for extracting novel science from fast-changing transients are precious, self-calibrating classification probabilities must be couched in terms of efficiencies for discovery and purity of the samples generated. We estimate the purity and efficiency in identifying real sources with a two-epoch image-difference discovery algorithm for the Palomar Transient Factory (PTF) survey. Once given a source discovery, using machine-learned classification trained on PTF data, we distinguish between transients and variable stars with a 3.8% overall error rate (with 1.7% errors for imaging within the Sloan Digital Sky Survey footprint). At >96% classification efficiency, the samples achieve 90% purity. Initial classifications are shown to rely primarily on context-based features, determined from the data itself and external archival databases. In the ~one year since autonomous operations, this discovery and classification framework has led to several significant science results, from outbursting young stars to subluminous Type IIP supernovae to candidate tidal disruption events. We discuss future directions of this approach, including the possible roles of crowdsourcing and the scalability of machine learning to future surveys such a the Large Synoptical Survey Telescope (LSST).

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arXiv2011

An extremely luminous panchromatic outburst from the nucleus of a distant galaxy

Variable X-ray and gamma-ray emission is characteristic of the most extreme physical processes in the Universe, and studying the sources of these energetic photons has been a major driver in astronomy for the past 50 years. Here we present multiwavelength observations of a unique gamma-ray selected transient, discovered by Swift, which was accompanied by bright emission across the electromagnetic spectrum, and whose properties are unlike any previously observed source. We pinpoint the event to the center of a small, star-forming galaxy at redshift z=0.3534. Its high-energy emission has lasted much longer than any gamma-ray burst, while its peak luminosity was about 100 times higher than the brightest active galactic nuclei. The association of the outburst with the center of its host galaxy suggests that this phenomenon has its origin in a new, rare mechanism associated with a massive black hole in the nucleus of a galaxy.

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arXiv2011

Progenitor Diagnostics for Stripped Core-Collapse Supernovae: Measured Metallicities at Explosion Sites

Metallicity is expected to influence not only the lives of massive stars but also the outcome of their deaths as supernovae (SNe) and as gamma-ray bursts (GRBs). However, there are surprisingly few direct measurements of the local metallicities of different flavors of core-collapse SNe. Here we present the largest existing set of host-galaxy spectra with H II region emission lines at the sites of 35 stripped-envelope core-collapse SNe. We derive local oxygen abundances in a robust manner in order to constrain the SN Ib/c progenitor population. We obtain spectra at the SN sites, include SNe from targeted and untargeted surveys, and perform the abundance determinatinos using three different oxygen-abundance calibrations. The sites of SNe Ic (the demise of the most heavily stripped stars having lost both the H and He layers) are systematically more metal rich than those of SNe Ib (arising from stars that retained their He layer) in all calibrations. A Kolmogorov-Smirnov test yields the very low probability of 1% that SN Ib and SN Ic environment abundances, which are different on average by ~0.2 dex (in the Pettini & Pagel scale), are drawn from the same parent population. Broad-lined SNe Ic (without GRBs) occur at metallicities between those of SNe Ib and SNe Ic. Lastly, we find that the host-galaxy central oxygen abundance is not a good indicator of the local SN metallicity; hence, large-scale SN surveys need to obtain local abundance measurements in order to quantify the impact of metallicity on stellar death.

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arXiv2011

The Subluminous and Peculiar Type Ia Supernova PTF09dav

PTF09dav is a peculiar subluminous type Ia supernova (SN) discovered by the Palomar Transient Factory (PTF). Spectroscopically, it appears superficially similar to the class of subluminous SN1991bg-like SNe, but it has several unusual features which make it stand out from this population. Its peak luminosity is fainter than any previously discovered SN1991bg-like SN Ia (M_B -15.5), but without the unusually red optical colors expected if the faint luminosity were due to extinction. The photospheric optical spectra have very unusual strong lines of Sc II and Mg I, with possible Sr II, together with stronger than average Ti II and low velocities of ~6000 km/s. The host galaxy of PTF09dav is ambiguous. The SN lies either on the extreme outskirts (~41kpc) of a spiral galaxy, or in an very faint (M_R>-12.8) dwarf galaxy, unlike other 1991bg-like SNe which are invariably associated with massive, old stellar populations. PTF09dav is also an outlier on the light-curve-width--luminosity and color--luminosity relations derived for other sub-luminous SNe Ia. The inferred 56Ni mass is small (0.019+/-0.003Msun), as is the estimated ejecta mass of 0.36Msun. Taken together, these properties make PTF09dav a remarkable event. We discuss various physical models that could explain PTF09dav. Helium shell detonation or deflagration on the surface of a CO white-dwarf can explain some of the features of PTF09dav, including the presence of Sc and the low photospheric velocities, but the observed Si and Mg are not predicted to be very abundant in these models. We conclude that no single model is currently capable of explaining all of the observed signatures of PTF09dav.

astro-ph.CO↗

arXiv2011

New Observations of the Very Luminous Supernova 2006gy: Evidence for Echoes

Supernova (SN) 2006gy was a hydrogen-rich core-collapse SN that remains one of the most luminous optical supernovae ever observed. The total energy budget (> 2 x 10^51 erg radiated in the optical alone) poses many challenges for standard SN theory. We present new ground-based near-infrared (NIR) observations of SN 2006gy, as well as a single epoch of Hubble Space Telescope (HST) imaging obtained more than two years after the explosion. Our NIR data taken around peak optical emission show an evolution that is largely consistent with a cooling blackbody, with tentative evidence for a growing NIR excess starting at day ~100. Our late-time Keck adaptive optics (AO) NIR image, taken on day 723, shows little change from previous NIR observations taken around day 400. Furthermore, the optical HST observations show a reduced decline rate after day 400, and the SN is bluer on day 810 than it was at peak. This late-time decline is inconsistent with Co56 decay, and thus is problematic for the various pair-instability SN models used to explain the nature of SN 2006gy. The slow decline of the NIR emission can be explained with a light echo, and we confirm that the late-time NIR excess is the result of a massive (>10 Msun) dusty shell heated by the SN peak luminosity. The late-time optical observations require the existence of a scattered light echo, which may be generated by the same dust that contributes to the NIR echo. Both the NIR and optical echoes originate in the proximity of the progenitor, ~10^18 cm for the NIR echo and <~10-40 pc for the optical echo, which provides further evidence that the progenitor of SN 2006gy was a very massive star.

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