arXiv subjects

R. Chornock

Publications and source records attributed to R. Chornock.

At least 73 records · Page 4Linked to original sources

arXiv2017

The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/VIRGO GW170817. V. Rising X-ray Emission from an Off-Axis Jet

We report the discovery of rising X-ray emission from the binary neutron star (BNS) merger event GW170817. This is the first detection of X-ray emission from a gravitational-wave source. Observations acquired with the Chandra X-ray Observatory (CXO) at t~2.3 days post merger reveal no significant emission, with L_x<=3.2x10^38 erg/s (isotropic-equivalent). Continued monitoring revealed the presence of an X-ray source that brightened with time, reaching L_x\sim 9x10^39 erg/s at ~15.1 days post merger. We interpret these findings in the context of isotropic and collimated relativistic outflows (both on- and off-axis). We find that the broad-band X-ray to radio observations are consistent with emission from a relativistic jet with kinetic energy E_k~10^49-10^50 erg, viewed off-axis with theta_obs~ 20-40 deg. Our models favor a circumbinary density n~ 0.0001-0.01 cm-3, depending on the value of the microphysical parameter epsilon_B=10^{-4}-10^{-2}. A central-engine origin of the X-ray emission is unlikely. Future X-ray observations at $t\gtrsim 100$ days, when the target will be observable again with the CXO, will provide additional constraints to solve the model degeneracies and test our predictions. Our inferences on theta_obs are testable with gravitational wave information on GW170817 from Advanced LIGO/Virgo on the binary inclination.

astro-ph.HE↗

arXiv2017

The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. I. Dark Energy Camera Discovery of the Optical Counterpart

We present the Dark Energy Camera (DECam) discovery of the optical counterpart of the first binary neutron star merger detected through gravitational wave emission, GW170817. Our observations commenced 10.5 hours post-merger, as soon as the localization region became accessible from Chile. We imaged 70 deg$^2$ in the $i$ and $z$ bands, covering 93\% of the initial integrated localization probability, to a depth necessary to identify likely optical counterparts (e.g., a kilonova). At 11.4 hours post-merger we detected a bright optical transient located $10.6''$ from the nucleus of NGC\,4993 at redshift $z=0.0098$, consistent (for $H_0 = 70$\, km s$^{-1}$ Mpc$^{-1}$) with the distance of $40 \pm 8$\, Mpc reported by the LIGO Scientific Collaboration and the Virgo Collaboration (LVC). At detection the transient had magnitudes $i\approx 17.30$ and $z\approx 17.45$, and thus an absolute magnitude of $M_i = -15.7$, in the luminosity range expected for a kilonova. We identified 1,500 potential transient candidates. Applying simple selection criteria aimed at rejecting background events such as supernovae, we find the transient associated with NGC\,4993 as the only remaining plausible counterpart, and reject chance coincidence at the 99.5\% confidence level. We therefore conclude that the optical counterpart we have identified near NGC\,4993 is associated with GW170817. This discovery ushers in the era of multi-messenger astronomy with gravitational waves, and demonstrates the power of DECam to identify the optical counterparts of gravitational-wave sources.

astro-ph.HE↗

arXiv2017

The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/VIRGO GW170817. IV. Detection of Near-infrared Signatures of r-process Nucleosynthesis with Gemini-South

We present a near-infrared spectral sequence of the electromagnetic counterpart to the binary neutron star merger GW170817 detected by Advanced LIGO/Virgo. Our dataset comprises seven epochs of J+H spectra taken with FLAMINGOS-2 on Gemini-South between 1.5 and 10.5 days after the merger. In the initial epoch, the spectrum is dominated by a smooth blue continuum due to a high-velocity, lanthanide-poor blue kilonova component. Starting the following night, all subsequent spectra instead show features that are similar to those predicted in model spectra of material with a high concentration of lanthanides, including spectral peaks near 1.07 and 1.55 microns. Our fiducial model with 0.04 M_sun of ejecta, an ejection velocity of v=0.1c, and a lanthanide concentration of X_lan=1e-2 provides a good match to the spectra taken in the first five days, although it over-predicts the late-time fluxes. We also explore models with multiple fitting components, in each case finding that a significant abundance of lanthanide elements is necessary to match the broad spectral peaks that we observe starting at 2.5 d after the merger. These data provide direct evidence that binary neutron star mergers are significant production sites of even the heaviest r-process elements.

astro-ph.HE↗

arXiv2017

The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/VIRGO GW170817. VI. Radio Constraints on a Relativistic Jet and Predictions for Late-Time Emission from the Kilonova Ejecta

We present Very Large Array (VLA) and Atacama Large Millimeter/sub-millimeter Array ALMA radio observations of GW\,170817, the first Laser Interferometer Gravitational-wave Observatory (LIGO)/Virgo gravitational wave (GW) event from a binary neutron star merger and the first GW event with an electromagnetic (EM) counterpart. Our data include the first observations following the discovery of the optical transient at both the centimeter ($13.7$ hours post merger) and millimeter ($2.41$ days post merger) bands. We detect faint emission at 6 GHz at 19.47 and 39.23 days after the merger, but not in an earlier observation at 2.46 d. We do not detect cm/mm emission at the position of the optical counterpart at frequencies of 10-97.5 GHz at times ranging from 0.6 to 30 days post merger, ruling out an on-axis short gamma-ray burst (SGRB) for energies $\gtrsim 10^{48}$ erg. For fiducial SGRB parameters, our limits require an observer viewer angle of $\gtrsim 20^{\circ}$. The radio and X-ray data can be jointly explained as the afterglow emission from an SGRB with a jet energy of $\sim 10^{49}-10^{50}$ erg that exploded in a uniform density environment with $n\sim 10^{-4}-10^{-2}$ cm$^{-3}$, viewed at an angle of $\sim 20^{\circ}-40^{\circ}$ from the jet axis. Using the results of our light curve and spectral modeling, in conjunction with the inference of the circumbinary density, we predict the emergence of late-time radio emission from the deceleration of the kilonova (KN) ejecta on a timescale of $\sim 5-10$ years that will remain detectable for decades with next-generation radio facilities, making GW\,170817 a compelling target for long-term radio monitoring.

astro-ph.HE↗

arXiv2017

How Many Kilonovae Can Be Found in Past, Present, and Future Survey Datasets?

The discovery of a kilonova (KN) associated with the Advanced LIGO (aLIGO)/Virgo event GW170817 opens up new avenues of multi-messenger astrophysics. Here, using realistic simulations, we provide estimates of the number of KNe that could be found in data from past, present and future surveys without a gravitational-wave trigger. For the simulation, we construct a spectral time-series model based on the DES-GW multi-band light-curve from the single known KN event, and we use an average of BNS rates from past studies of $10^3 \rm{Gpc}^{-3}/\rm{year}$, consistent with the $1$ event found so far. Examining past and current datasets from transient surveys, the number of KNe we expect to find for ASAS-SN, SDSS, PS1, SNLS, DES, and SMT is between 0 and $0.3$. We predict the number of detections per future survey to be: 8.3 from ATLAS, 10.6 from ZTF, 5.5/69 from LSST (the Deep Drilling / Wide Fast Deep), and 16.0 from WFIRST. The maximum redshift of KNe discovered for each survey is z = 0.8 for WFIRST, z = 0.25 for LSST and z = 0.04 for ZTF and ATLAS. For the LSST survey, we also provide contamination estimates from Type Ia and Core-collapse supernovae: after light-curve and template-matching requirements, we estimate a background of just 2 events. More broadly, we stress that future transient surveys should consider how to optimize their search strategies to improve their detection efficiency, and to consider similar analyses for GW follow-up programs.

astro-ph.IM↗

arXiv2017

iPTF15eqv: Multi-wavelength Exposé of a Peculiar Calcium-rich Transient

The progenitor systems of the class of "Ca-rich transients" is a key open issue in time domain astrophysics. These intriguing objects exhibit unusually strong calcium line emissions months after explosion, fall within an intermediate luminosity range, are often found at large projected distances from their host galaxies, and may play a vital role in enriching galaxies and the intergalactic medium. Here we present multi-wavelength observations of iPTF15eqv in NGC 3430, which exhibits a unique combination of properties that bridge those observed in Ca-rich transients and Type Ib/c supernovae. iPTF15eqv has among the highest [Ca II]/[O I] emission line ratios observed to date, yet is more luminous and decays more slowly than other Ca-rich transients. Optical and near-infrared photometry and spectroscopy reveal signatures consistent with the supernova explosion of a < 10 solar mass star that was stripped of its H-rich envelope via binary interaction. Distinct chemical abundances and ejecta kinematics suggest that the core collapse occurred through electron capture processes. Deep limits on possible radio emission made with the Jansky Very Large Array imply a clean environment ($n <$ 0.1 cm$^{-3}$) within a radius of $\sim 10^{17}$ cm. Chandra X-ray Observatory observations rule out alternative scenarios involving tidal disruption of a white dwarf by a black hole, for masses > 100 solar masses). Our results challenge the notion that spectroscopically classified Ca-rich transients only originate from white dwarf progenitor systems, complicate the view that they are all associated with large ejection velocities, and indicate that their chemical abundances may vary widely between events.

astro-ph.HE↗

arXiv2017

An Empirical Study of Contamination in Deep, Rapid, and Wide-Field Optical Follow-Up of Gravitational Wave Events

We present an empirical study of contamination in deep, rapid, and wide-field optical follow-up searches of GW sources from aLIGO. We utilize dedicated observations during four nights of imaging with DECam. Our search covered $\sim56$ deg$^2$, with two visits per night separated by $\approx 3$~hours, in $i$- and $z$-band, followed by an additional set of $griz$ images three weeks later to serve as reference images for subtraction, and for the purpose of identifying galaxy and stellar counterparts for any transient sources. We achieve $5σ$ point-source limiting magnitudes of $i \approx 23.5$ and $z \approx 22.4$ mag in the coadded single-epoch images. We conduct a search for transient objects that can mimic the $i-z$ color behavior of both red ($i-z > 0.5$~mag) and blue ($i-z < 0$~mag) kilonova emission, finding 11 and 10 contaminants, respectively. Independent of color, we identify 48 transients of interest. Additionally, we leverage the rapid cadence of our observations to search for sources with characteristic timescales of $\approx1$ day and $\approx3$ hours, finding no potential contaminants. We assess the efficiency of our pipeline and search methodology with injected point sources, finding that we are 90\% (60\%) efficient when searching for red (blue) kilonova-like sources to a limiting magnitude of $i \lesssim 22.5$ mag. Applying these efficiencies, we derive sky rates for kilonova contaminants in the red and blue regimes of $\mathcal{R}_{\rm red} \approx 0.16$ deg$^{-2}$ and $\mathcal{R}_{\rm blue} \approx 0.80$ deg$^{-2}$. The total contamination rate, independent of color, is $\mathcal{R}_{\rm all} \approx 1.79$ deg$^{-2}$. We compare our derived results to optical follow-up searches of the GW events GW150914 and GW151226 and comment on the outlook for GW follow-up searches as additional GW detectors (e.g., KAGRA, LIGO India) come online in the next decade.

astro-ph.HE↗

arXiv2017

Measuring the Properties of Dark Energy with Photometrically Classified Pan-STARRS Supernovae. I. Systematic Uncertainty from Core-Collapse Supernova Contamination

The Pan-STARRS (PS1) Medium Deep Survey discovered over 5,000 likely supernovae (SNe) but obtained spectral classifications for just 10% of its SN candidates. We measured spectroscopic host galaxy redshifts for 3,147 of these likely SNe and estimate that $\sim$1,000 are Type Ia SNe (SNe Ia) with light-curve quality sufficient for a cosmological analysis. We use these data with simulations to determine the impact of core-collapse SN (CC SN) contamination on measurements of the dark energy equation of state parameter, $w$. Using the method of Bayesian Estimation Applied to Multiple Species (BEAMS), distances to SNe Ia and the contaminating CC SN distribution are simultaneously determined. We test light-curve based SN classification priors for BEAMS as well as a new classification method that relies upon host galaxy spectra and the association of SN type with host type. By testing several SN classification methods and CC SN parameterizations on large SN simulations, we estimate that CC SN contamination gives a systematic error on $w$ ($σ_w^{CC}$) of 0.014, 29% of the statistical uncertainty. Our best method gives $σ_w^{CC} = 0.004$, just 8% of the statistical uncertainty, but could be affected by incomplete knowledge of the CC SN distribution. This method determines the SALT2 color and shape coefficients, $α$ and $β$, with $\sim$3% bias. However, we find that some variants require $α$ and $β$ to be fixed to known values for BEAMS to yield accurate measurements of $w$. Finally, the inferred abundance of bright CC SNe in our sample is greater than expected based on measured CC SN rates and luminosity functions.

astro-ph.CO↗

arXiv2017

Results from a systematic survey of X-ray emission from Hydrogen-poor Superluminous Supernovae

We present the results from a sensitive X-ray survey of 26 nearby hydrogen-poor superluminous supernovae (SLSNe-I) with Swift, Chandra and XMM. This dataset constrains the SLSN evolution from a few days until ~2000 days after explosion, reaching a luminosity L_x~10^40 erg/s and revealing the presence of significant X-ray emission at the location of PTF12dam. No SLSN-I is detected above L_x~10^41 erg/s, suggesting that the luminous X-ray emission L_x~10^45 erg/s associated with SCP60F6 is not common among SLSNe-I. We constrain the presence of off-axis GRB jets, ionization breakouts from magnetar central engines and the density in the sub-pc environments of SLSNe-I through Inverse Compton emission. The deepest limits rule out the weakest uncollimated GRB outflows, suggesting that IF the similarity of SLSNe-I with GRB/SNe extends to their fastest ejecta, then SLSNe-I are either powered by energetic jets pointed far away from our line of sight theta>30 deg, or harbor failed jets that do not successfully break through the stellar envelope. Furthermore, IF a magnetar central engine is responsible for the exceptional luminosity of SLSNe-I, our X-ray analysis favors large magnetic fields B>2x10^(14) G and ejecta masses M_ej>3 Msun in agreement with optical/UV studies. Finally, we constrain the pre-explosion mass-loss rate of stellar progenitors of SLSNe-I. For PTF12dam we infer Mdot<2x10^(-5) Msun/yr, suggesting that the SN shock interaction with the CSM is unlikely to supply the main source of energy powering the optical transient and that some SLSN-I progenitors end their life as compact stars surrounded by a low-density medium similar to long GRBs and Type Ib/c SNe.

astro-ph.HE↗

arXiv2017

PS16dtm: A Tidal Disruption Event in a Narrow-line Seyfert 1 Galaxy

[Abridged] We present observations of PS16dtm, a luminous transient that occurred at the nucleus of a known Narrow-line Seyfert 1 galaxy hosting a 10$^6$ M$_\odot$ black hole. The transient was previously claimed to be a Type IIn SLSN due to its luminosity and hydrogen emission lines. The light curve shows that PS16dtm brightened by about two magnitudes in ~50 days relative to the archival host brightness and then exhibited a plateau phase for about 100 days followed by the onset of fading in the UV. During the plateau PS16dtm showed no color evolution, maintained a blackbody temperature of 1.7 x 10$^4$ K, and radiated at approximately $L_{Edd}$ of the SMBH. The spectra exhibit multi-component hydrogen emission lines and strong FeII emission, show little evolution with time, and closely resemble the spectra of NLS1s while being distinct from those of Type IIn SNe. Moreover, PS16dtm is undetected in the X-rays to a limit an order of magnitude below an archival X-ray detection of its host galaxy. These observations strongly link PS16dtm to activity associated with the SMBH and are difficult to reconcile with a SN origin or any known form of AGN variability, and therefore we argue that it is a TDE in which the accretion of the stellar debris powers the rise in the continuum and excitation of the pre-existing broad line region, while providing material that obscures the X-ray emitting region of the pre-existing AGN accretion disk. A detailed TDE model fit to the light curve indicates that PS16dtm will remain bright for several years; we further predict that the X-ray emission will reappear on a similar timescale as the accretion rate declines. Finally, we place PS16dtm in the context of other TDEs and find that TDEs in AGN galaxies are an order of magnitude more efficient and reach Eddington luminosities, likely due to interaction of the stellar debris with the pre-existing accretion disk.

astro-ph.HE↗

arXiv2017

An ultraviolet excess in the superluminous supernova Gaia16apd reveals a powerful central engine

Since the discovery of superluminous supernovae (SLSNe) in the last decade, it has been known that these events exhibit bluer spectral energy distributions than other supernova subtypes, with significant output in the ultraviolet. However, the event Gaia16apd seems to outshine even the other SLSNe at rest-frame wavelengths below $\sim 3000$ Å. Yan et al (2016) have recently presented HST UV spectra and attributed the UV flux to low metallicity and hence reduced line blanketing. Here we present UV and optical light curves over a longer baseline in time, revealing a rapid decline at UV wavelengths despite a typical optical evolution. Combining the published UV spectra with our own optical data, we demonstrate that Gaia16apd has a much hotter continuum than virtually any SLSN at maximum light, but it cools rapidly thereafter and is indistinguishable from the others by $\sim 10$-15 days after peak. Comparing the equivalent widths of UV absorption lines with those of other events, we show that the excess UV continuum is a result of a more powerful central power source, rather than a lack of UV absorption relative to other SLSNe or an additional component from interaction with the surrounding medium. These findings strongly support the central-engine hypothesis for hydrogen-poor SLSNe. An explosion ejecting $M_{\rm ej} = 4 (0.2/κ)$ M$_\odot$, where $κ$ is the opacity in cm$^2$g$^{-1}$, and forming a magnetar with spin period $P=2$ ms, and $B=2\times10^{14}$ G (lower than other SLSNe with comparable rise-times) can consistently explain the light curve evolution and high temperature at peak. The host metallicity, $Z=0.18$ Z$_\odot$, is comparable to other SLSNe.

astro-ph.SR↗

arXiv2017

Growing evidence that SNe Iax are not a one-parameter family: the case of PS1-12bwh

In this study, we present observations of a type Iax supernova, PS1-12bwh, discovered during the Pan-STARRS1 3$π$-survey. Our analysis was driven by previously unseen pre-maximum, spectroscopic heterogeneity. While the light curve and post-maximum spectra of PS1-12bwh are virtually identical to those of the well-studied type Iax supernova, SN 2005hk, the $-$2 day spectrum of PS1-12bwh does not resemble SN 2005hk at a comparable epoch; instead, we found it to match a spectrum of SN 2005hk taken over a week earlier ($-$12 day). We are able to rule out the cause as being incorrect phasing, and argue that it is not consistent with orientation effects predicted by existing explosion simulations. To investigate the potential source of this difference, we performed radiative transfer modelling of both supernovae. We found that the pre-maximum spectrum of PS1-12bwh is well matched by a synthetic spectrum generated from a model with a lower density in the high velocity ($\gtrsim$6000 km~s$^{-1}$) ejecta than SN 2005hk. The observed differences between SN 2005hk and PS1-12bwh may therefore be attributed primarily to differences in the high velocity ejecta alone, while comparable densities for the lower velocity ejecta would explain the nearly identical post-maximum spectra. These two supernovae further highlight the diversity within the SNe Iax class, as well as the challenges in spectroscopically identifying and phasing these objects, especially at early epochs.

astro-ph.HE↗

arXiv2016

Slowly fading super-luminous supernovae that are not pair-instability explosions

Super-luminous supernovae that radiate more than 10^44 ergs per second at their peak luminosity have recently been discovered in faint galaxies at redshifts of 0.1-4. Some evolve slowly, resembling models of 'pair-instability' supernovae. Such models involve stars with original masses 140-260 times that of the Sun that now have carbon-oxygen cores of 65-30 solar masses. In these stars, the photons that prevent gravitational collapse are converted to electron-positron pairs, causing rapid contraction and thermonuclear explosions. Many solar masses of 56Ni are synthesized; this isotope decays to 56Fe via 56Co, powering bright light curves. Such massive progenitors are expected to have formed from metal-poor gas in the early Universe. Recently, supernova 2007bi in a galaxy at redshift 0.127 (about 12 billion years after the Big Bang) with a metallicity one-third that of the Sun was observed to look like a fading pair-instability supernova. Here we report observations of two slow-to-fade super-luminous supernovae that show relatively fast rise times and blue colours, which are incompatible with pair-instability models. Their late-time light-curve and spectral similarities to supernova 2007bi call the nature of that event into question. Our early spectra closely resemble typical fast-declining super-luminous supernovae, which are not powered by radioactivity. Modelling our observations with 10-16 solar masses of magnetar-energized ejecta demonstrates the possibility of a common explosion mechanism. The lack of unambiguous nearby pair-instability events suggests that their local rate of occurrence is less than 6x10^-6 times that of the core-collapse rate.

astro-ph.CO↗

arXiv2016

PS1-14bj: A Hydrogen-Poor Superluminous Supernova With a Long Rise and Slow Decay

We present photometry and spectroscopy of PS1-14bj, a hydrogen-poor superluminous supernova (SLSN) at redshift $z=0.5215$ discovered in the last months of the Pan-STARRS1 Medium Deep Survey. PS1-14bj stands out by its extremely slow evolution, with an observed rise of $\gtrsim 125$ rest-frame days, and exponential decline out to $\sim 250$ days past peak at a measured rate of $0.01~{\rm mag~day}^{-1}$, consistent with fully-trapped $^{56}$Co decay. This is the longest rise time measured in a SLSN to date, and the first SLSN to show a rise time consistent with pair-instability supernova (PISN) models. Compared to other slowly-evolving SLSNe, it is spectroscopically similar to the prototype SN2007bi at maximum light, though lower in luminosity ($L_{\rm peak} \simeq 4.6 \times 10^{43} {\rm erg s}^{-1}$) and with a flatter peak than previous events. PS1-14bj shows a number of peculiar properties, including a near-constant color temperature for $>200$ days past peak, and strong emission lines from [O III] $λ$5007 and [O III] $λ$4363 with a velocity width of $\sim$3400 km/s, in its late-time spectra. These both suggest there is a sustained source of heating over very long timescales, and are incompatible with a simple $^{56}$Ni-powered/PISN interpretation. A modified magnetar model including emission leakage at late times can reproduce the light curve, in which case the blue continuum and [O III] features are interpreted as material heated and ionized by the inner pulsar wind nebula becoming visible at late times. Alternatively, the late-time heating could be due to interaction with a shell of H-poor circumstellar material.

astro-ph.HE↗

arXiv2016

X-rays from the location of the Bactrian Transient ASASSN-15lh

We present the detection of persistent soft X-ray radiation with L_x ~ 10^41-10^42 erg/s at the location of the extremely luminous, double-humped transient ASASSN-15lh as revealed by Chandra and Swift. We interpret this finding in the context of observations from our multiwavelength campaign, which revealed the presence of weak narrow nebular emission features from the host-galaxy nucleus and clear differences with respect to superluminous supernova optical spectra. Significant UV flux variability on short time-scales detected at the time of the re-brightening disfavors the shock interaction scenario as the source of energy powering the long-lived UV emission, while deep radio limits exclude the presence of relativistic jets propagating into a low-density environment. We propose a model where the extreme luminosity and double-peaked temporal structure of ASASSN-15lh is powered by a central source of ionizing radiation that produces a sudden change of the ejecta opacity at later times. As a result, UV radiation can more easily escape, producing the second bump in the light-curve. We discuss different interpretations for the intrinsic nature of the ionizing source. We conclude that, IF the X-ray source is physically associated with the optical-UV transient, ASASSN-15lh most likely represents the tidal disruption of a main-sequence star by the most massive spinning black hole detected to date. In this case, ASASSN-15lh and similar events discovered in the future would constitute the most direct probes of very massive, dormant, spinning, supermassive black holes in galaxies. Future monitoring of the X-rays may allow us to distinguish between the supernova and TDE hypothesis.

astro-ph.HE↗

arXiv2016

The GALEX Time Domain Survey. II. Wavelength-Dependent Variability of Active Galactic Nuclei in the Pan-STARRS1 Medium Deep Survey

We analyze the wavelength-dependent variability of a sample of spectroscopically confirmed active galactic nuclei (AGN) selected from near-UV ($NUV$) variable sources in the GALEX Time Domain Survey that have a large amplitude of optical variability (difference-flux S/N $>$ 3) in the Pan-STARRS1 Medium Deep Survey (PS1 MDS). By matching GALEX and PS1 epochs in 5 bands ($NUV$, $g_{P1}$, $r_{P1}$, $i_{P1}$, $z_{P1}$) in time, and taking their flux difference, we create co-temporal difference-flux spectral energy distributions ($Δf$SEDs) using two chosen epochs for each of the 23 objects in our sample on timescales of about a year. We confirm the "bluer-when-brighter" trend reported in previous studies, and measure a median spectral index of the $Δf$SEDs of $α_λ$ = 2.1 that is consistent with an accretion disk spectrum. We further fit the $Δf$SEDs of each source with a standard accretion disk model in which the accretion rate changes from one epoch to the other. In our sample, 17 out of 23 ($\sim$74 %) sources are well described by this variable accretion-rate disk model, with a median average characteristic disk temperature $\bar{T}^*$ of $1.2\times 10^5$~K that is consistent with the temperatures expected given the distribution of accretion rates and black hole masses inferred for the sample. Our analysis also shows that the variable accretion rate model is a better fit to the $Δf$SEDs than a simple power law.

astro-ph.HE↗

arXiv2016

A DECam Search for an Optical Counterpart to the LIGO Gravitational Wave Event GW151226

We report the results of a Dark Energy Camera (DECam) optical follow-up of the gravitational wave (GW) event GW151226, discovered by the Advanced LIGO detectors. Our observations cover 28.8 deg$^2$ of the localization region in the $i$ and $z$ bands (containing 3% of the BAYESTAR localization probability), starting 10 hours after the event was announced and spanning four epochs at $2-24$ days after the GW detection. We achieve $5σ$ point-source limiting magnitudes of $i\approx21.7$ and $z\approx21.5$, with a scatter of $0.4$ mag, in our difference images. Given the two day delay, we search this area for a rapidly declining optical counterpart with $\gtrsim 3σ$ significance steady decline between the first and final observations. We recover four sources that pass our selection criteria, of which three are cataloged AGN. The fourth source is offset by $5.8$ arcsec from the center of a galaxy at a distance of 187 Mpc, exhibits a rapid decline by $0.5$ mag over $4$ days, and has a red color of $i-z\approx 0.3$ mag. These properties roughly match the expectations for a kilonova. However, this source was detected several times, starting $94$ days prior to GW151226, in the Pan-STARRS Survey for Transients (dubbed as PS15cdi) and is therefore unrelated to the GW event. Given its long-term behavior, PS15cdi is likely a Type IIP supernova that transitioned out of its plateau phase during our observations, mimicking a kilonova-like behavior. We comment on the implications of this detection for contamination in future optical follow-up observations.

astro-ph.HE↗

arXiv2016

Superluminous supernova 2015bn in the nebular phase: evidence for the engine-powered explosion of a stripped massive star

We present nebular-phase imaging and spectroscopy for the hydrogen-poor superluminous supernova SN 2015bn, at redshift z=0.1136, spanning +250-400 d after maximum light. The light curve exhibits a steepening in the decline rate from 1.4 mag/(100 d) to 1.7 mag/(100 d), suggestive of a significant decrease in the opacity. This change is accompanied by a transition from a blue continuum superposed with photospheric absorption lines to a nebular spectrum dominated by emission lines of oxygen, calcium and magnesium. There are no obvious signatures of circumstellar interaction or large nickel mass. We show that the spectrum at +400 d is virtually identical to a number of energetic Type Ic supernovae such as SN 1997dq, SN 2012au, and SN 1998bw, indicating similar core conditions and strengthening the link between `hypernovae'/long gamma-ray bursts and superluminous supernovae. A single explosion mechanism may unify these events that span absolute magnitudes of -22 < M_B < -17. Both the light curve and spectrum of SN 2015bn are consistent with an engine-driven explosion ejecting 7-30 M$_\odot$ of oxygen-dominated ejecta (for reasonable choices in temperature and opacity). A strong and relatively narrow O II $λ$7774 line, seen in a number of these energetic events but not in normal supernovae, may point to an inner shell that is the signature of a central engine.

astro-ph.HE↗