Person:

Fong, W

We present the Pan-STARRS1 discovery of PS1-10afx, a unique hydrogen-deficient superluminous supernova (SLSN) at redshift z = 1.388. The light curve peaked at z P1 = 21.7 mag, making PS1-10afx comparable to the most luminous known SNe, with Mu = –22.3 mag. Our extensive optical and near-infrared observations indicate that the bolometric light curve of PS1-10afx rose on the unusually fast timescale of ~12 days to the extraordinary peak luminosity of 4.1 × 1044 erg s–1 (M bol = –22.8 mag) and subsequently faded rapidly. Equally important, the spectral energy distribution is unusually red for an SLSN, with a color temperature of ~6800 K near maximum light, in contrast to previous hydrogen-poor SLSNe, which are bright in the ultraviolet (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–1 and evidence for line blanketing in the rest-frame UV. Despite the fast rise, these parameters imply a very large emitting radius (gsim 5 × 1015 cm). We demonstrate that no existing theoretical model can satisfactorily explain this combination of properties: (1) a nickel-powered light curve cannot match the combination of high peak luminosity with the fast timescale; (2) models powered by the spindown energy of a rapidly rotating magnetar predict significantly hotter and faster ejecta; and (3) 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 ☉ yr–1, and is fairly massive (~2 × 1010 M ☉), with a stellar population age of ~108 yr, also in contrast to the young dwarf hosts of known hydrogen-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.

Gamma-ray bursts (GRBs) are relativistic explosions which originate at cosmological distances, and are among the most luminous transients in the universe. Following the prompt gamma-ray emission, a fading synchrotron "afterglow" is detectable at lower energies. While long-duration GRBs (duration > 2 sec) are linked to the deaths of massive stars, the progenitors of short-duration GRBs (duration < 2 sec) have remained elusive. Theoretical predictions formulated over the past two decades have suggested that they are the mergers of two compact objects, involving either two neutron stars (NS-NS) or a neutron star and a black hole (NS-BH). Such merging systems are also important to understand because they are premier candidates for gravitational wave detections with upcoming facilities and are considered likely sites of heavy element nucleosynthesis. The launch of the Swift satellite in 2004, with its rapid multi-wavelength monitoring and localization capabilities, led to the first discoveries of short GRB afterglows and therefore robust associations to host galaxies. At a Swift detection rate of ~8 events per year, the growing number of well-localized short GRBs enables comprehensive population studies of their afterglows and environments for the first time. In this thesis, I undertake a multi-wavelength observational campaign to address testable predictions for the progenitors of short GRBs. From their local environments, I show that short GRBs explode in diffuse regions of their host galaxies and are weakly correlated with the distribution of stellar mass and star formation in their host galaxies. I study the host galaxy demographics for the entire population and find that ~20-40% of short GRBs originate from elliptical galaxies, implying an older stellar progenitor. From their afterglows, I present evidence that some short GRBs are collimated in narrow jets of ~5-10 degrees, directly affecting the true energy scale and event rate. Finally, taking advantage of a decade of broad-band afterglow observations at radio through X-ray wavelengths, I find that short GRBs have median isotropic-equivalent energies of ~10^51 erg and that their local environments have low densities, ~10^-3-10^-2 cm^-3. Taken together, this thesis comprises several lines of independent evidence to demonstrate that short GRBs originate from the mergers of two compact objects, and also provides the first constraints on the explosion properties for a large sample of events. With the direct detection of gravitational waves from compact object mergers on the horizon, these studies provide necessary inputs to inform the next decade of joint electromagnetic-gravitational wave search strategies.

We present observations of the afterglows and host galaxies of three short-duration gamma-ray bursts (GRBs): 100625A, 101219A and 110112A. We find that GRB 100625A occurred in a z = 0.452 early-type galaxy with a stellar mass of ≈ 4.6×109 M⊙ and a stellar population age of ≈ 0.7 Gyr, and GRB 101219A originated in a starforming galaxy at z = 0.718 with a stellar mass of ≈ 1.4×109M⊙, a star formation rate of ≈ 16 M⊙ yr−1, and a stellar population age of ≈ 50 Myr. We also report the discovery of the optical afterglow of GRB 110112A, which lacks a coincident host galaxy to i & 26 mag and we cannot conclusively identify any field galaxy as a possible host. From afterglow modeling, the bursts have inferred circumburst densities of ≈ 10−4 − 1 cm−3, and isotropic-equivalent gamma-ray and kinetic energies of ≈ 1050 − 1051 erg. These three events highlight the diversity of galactic environments that host short GRBs. To quantify this diversity, we use the sample of 36 Swift short GRBs with robust associations to an environment (∼ 1/2 of 68 short bursts detected by Swift to May 2012) and classify bursts originating from four types of environments: late-type (≈ 50%), early-type (≈ 15%), inconclusive (≈ 20%), and “host-less” (lacking a coincident host galaxy to limits of & 26 mag; ≈ 15%). To find likely ranges for the true late- and early-type fractions, we assign each of the host-less bursts to either the late- or early-type category using probabilistic arguments, and consider the scenario that all hosts in the inconclusive category are early-type galaxies to set an upper bound on the early-type fraction. We calculate most likely ranges for the late- and early-type fractions of ≈ 60 − 80% and ≈ 20 − 40%, respectively. We find no clear trend between gamma-ray duration and host type. We also find no change to the fractions when excluding events recently claimed as possible contaminants from the long GRB/collapsar population. Our reported demographics are consistent with a short GRB rate driven by both stellar mass and star formation.

We present a detailed investigation of Hubble Space Telescope rest-frame UV/optical observations of 22 short gamma-ray burst (GRB) host galaxies and sub-galactic environments. Utilizing the high angular resolution and depth of HST we characterize the host galaxy morphologies, measure precise projected physical and hostnormalized offsets between the bursts and host centers, and calculate the locations of the bursts with respect to their host light distributions (rest-frame UV and optical). We calculate a median short GRB projected physical offset of 4.5 kpc, about 3.5 times larger than that for long GRBs, and find that ≈ 25% of short GRBs have offsets of & 10 kpc. When compared to their host sizes, the median offset is 1.5 half-light radii (re), about 1.5 times larger than the values for long GRBs, core-collapse supernovae, and Type Ia supernovae. In addition, ≈ 20% of short GRBs having offsets of & 5re, and only ≈ 25% are located within 1re. We further find that short GRBs severely under-represent their hosts’ rest-frame optical and UV light, with ≈ 30 − 45% of the bursts located in regions of their host galaxies that have no detectable stellar light, and ≈ 55% in the regions with no UV light. Therefore, short GRBs do not occur in regions of star formation or even stellar mass. This demonstrates that the progenitor systems of short GRBs must migrate from their birth sites to their eventual explosion sites, a signature of kicks in compact object binary systems. Utilizing the full sample of offsets, we estimate natal kick velocities of ≈ 20 − 140 km s−1. These independent lines of evidence provide the strongest support to date that short GRBs result from the merger of compact object binaries (NS-NS/NS-BH).

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.

In the past decade, several rapidly evolving transients have been discovered whose timescales and luminosities are not easily explained by traditional supernovae (SNe) models. The sample size of these objects has remained small due, at least in part, to the challenges of detecting short timescale transients with traditional survey cadences. Here we present the results from a search within the Pan-STARRS1 Medium Deep Survey (PS1-MDS) for rapidly evolving and luminous transients. We identify 10 new transients with a time above half-maximum (t 1/2) of less than 12 days and –16.5 > M > –20 mag. This increases the number of known events in this region of SN phase space by roughly a factor of three. The median redshift of the PS1-MDS sample is z = 0.275 and they all exploded in star-forming galaxies. In general, the transients possess faster rise than decline timescale and blue colors at maximum light (g P1 – r P1 lesssim –0.2). Best-fit blackbodies reveal photospheric temperatures/radii that expand/cool with time and explosion spectra taken near maximum light are dominated by a blue continuum, consistent with a hot, optically thick, ejecta. We find it difficult to reconcile the short timescale, high peak luminosity (L > 1043 erg s–1), and lack of UV line blanketing observed in many of these transients with an explosion powered mainly by the radioactive decay of 56Ni. Rather, we find that many are consistent with either (1) cooling envelope emission from the explosion of a star with a low-mass extended envelope that ejected very little (<0.03 M ☉) radioactive material, or (2) a shock breakout within a dense, optically thick, wind surrounding the progenitor star. After calculating the detection efficiency for objects with rapid timescales in the PS1-MDS we find a volumetric rate of 4800-8000 events yr–1 Gpc–3 (4%-7% of the core-collapse SN rate at z = 0.2).

We present the discovery of the optical afterglow and early-type host galaxy of the short-duration GRB 100117A. The faint afterglow is detected 8.3 hr after the burst with rAB = 25.46 ± 0.20 mag. Follow-up optical and near-IR observations uncover a coincident compact red galaxy, identified as an early-type galaxy at a photometric redshift of z ≈ 0.6 − 0.9 (2σ) with a mass of ∼ 3 × 1010 M⊙, an age of ∼ 1 Gyr, and a luminosity of LB ≃ 0.5L∗. Spectroscopic observations of the host reveal a notable break corresponding to the Balmer/4000Å break at z ≈ 0.9, and stellar population spectral evolution template fits indicate z ≈ 0.915, which we adopt as the redshift of the host, with stellar population ages of ∼ 1−3 Gyr. From a possible weak detection of [O II]λ3727 emission at z = 0.915 we infer an upper bound on the star formation rate of ∼ 0.1 M⊙ yr−1, leading to a specific star formation rate of . 0.004 Gyr−1. Thus, GRB 100117A is only the second short burst to date with a secure early-type host (the other being GRB 050724 at z = 0.257) and it has one of the highest short GRB redshifts. The offset between the host center and the burst position, 470 ± 310 pc, is the smallest to date. Combined with the old stellar population age, this indicates that the burst likely originated from a progenitor with no significant kick velocity. However, from the brightness of the optical afterglow we infer a relatively low density of n ≈ 3×10−4 ǫ −3 e,−1 ǫ−1.75 B,−1 cm−3. The combination of an optically faint afterglow and host suggest that previous such events may have been missed, thereby potentially biasing the known short GRB host population against z & 1 early-type hosts.

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 ground-based optical and Hubble Space Telescope optical and near-IR observations of the shorthard GRB 130603B at z = 0.356, which demonstrate the presence of excess near-IR emission matching the expected brightness and color of an r-process powered transient (a “kilonova”). The early afterglow fades rapidly with α . −2.6 at t ≈ 8 − 32 hr post-burst and has a spectral index of β ≈ −1.5 (Fν ∝ tανβ), leading to an expected near-IR brightness at the time of the first HST observation of mF160W(t = 9.4d) & 29.3 AB mag. Instead, the detected source has mF160W = 25.8±0.2 AB mag, corresponding to a rest-frame absolute magnitude of MJ ≈ −15.2 mag. The upper limit in the HST optical observations is mF606W & 27.7 AB mag (3σ), indicating an unusually red color of V − H & 1.9 mag. Comparing the observed near-IR luminosity to theoretical models of kilonovae produced by ejecta from the merger of an NS-NS or NS-BH binary, we infer an ejecta mass of Mej ≈ 0.03 − 0.08 M⊙ for vej ≈ 0.1 − 0.3c. The inferred mass matches the expectations from numerical merger simulations. The presence of a kilonova provides the strongest evidence to date that short GRBs are produced by compact object mergers, and provides initial insight on the ejected mass and the primary role that compact object merger may play in the r-process. Equally important, it demonstrates that gravitational wave sources detected by Advanced LIGO/Virgo will be accompanied by optical/near-IR counterparts with unusually red colors, detectable by existing and upcoming large wide-field facilities (e.g., Pan-STARRS, DECam, Subaru, LSST).

We present Hubble Space Telescope (HST) rest-frame ultraviolet imaging of the host galaxies of 16 hydrogen-poor superluminous supernovae (SLSNe), including 11 events from the Pan-STARRS Medium Deep Survey. Taking advantage of the superb angular resolution of HST, we characterize the galaxies' morphological properties, sizes, and star formation rate (SFR) densities. We determine the supernova (SN) locations within the host galaxies through precise astrometric matching and measure physical and host-normalized offsets as well as the SN positions within the cumulative distribution of UV light pixel brightness. We find that the host galaxies of H-poor SLSNe are irregular, compact dwarf galaxies, with a median half-light radius of just 0.9 kpc. The UV-derived SFR densities are high ($\langle {{{\Sigma }}{{\rm SFR}}}\rangle \simeq 0.1{{M}{\odot }};{\rm y}{{{\rm r}}^{-1}};{\rm kp}{{{\rm c}}^{-2}}$), suggesting that SLSNe form in overdense environments. Their locations trace the UV light of their host galaxies, with a distribution intermediate between that of long-duration gamma-ray bursts (LGRBs; which are strongly clustered on the brightest regions of their hosts) and a uniform distribution (characteristic of normal core-collapse SNe), though cannot be statistically distinguished from either with the current sample size. Taken together, this strengthens the picture that SLSN progenitors require different conditions than those of ordinary core-collapse SNe to form and that they explode in broadly similar galaxies as do LGRBs. If the tendency for SLSNe to be less clustered on the brightest regions than are LGRBs is confirmed by a larger sample, this would indicate a different, potentially lower-mass progenitor for SLSNe than LRGBs.