Matthew D. Kistler

The Star Formation Rate in the Reionization Era as Indicated by Gamma-Ray Bursts

High-redshift gamma-ray bursts (GRBs) offer an extraordinary opportunity to study aspects of the early universe, including the cosmic star formation rate (SFR). Motivated by the two recent highest-z GRBs, GRB 080913 at z 6.7 and GRB 090423 at z 8.1, and more than four years of Swift observations, we first confirm that the GRB rate does not trace the SFR in an unbiased way. Correcting for this, we find that the implied SFR to beyond z = 8 is consistent with Lyman Break Galaxy-based measurements after accounting for unseen galaxies at the faint end of the UV luminosity function. We show that this provides support for the integrated star formation in the range 6 z 8 to have been alone sufficient to reionize the universe.

A SURVEY ABOUT NOTHING: MONITORING A MILLION SUPERGIANTS FOR FAILED SUPERNOVAE

Extragalactic transient searches have historically been limited to looking for the appearance of new sources such as supernovae. It is now possible to carry out a new kind of survey that will do the opposite, that is, search for the disappearance of massive stars. This will entail the systematic observation of galaxies within a distance of 10 Mpc in order to watch ~106 supergiants. Reaching this critical number ensures that something will occur yearly, since these massive stars must end their lives with a core collapse within ~106 yr. Using deep imaging and image subtraction, it is possible to determine the fates of these stars, whether they end with a bang (supernova) or a whimper (fall out of sight). Such a survey would place completely new limits on the total rate of all core collapses, which is critical for determining the validity of supernova models. It would also determine the properties of supernova progenitors, better characterize poorly understood optical transients (such as η Carina-like mass ejections), find and characterize large numbers of Cepheids, luminous blue variables, and eclipsing binaries, and allow the discovery of any new phenomena that inhabit this relatively unexplored parameter space.

A NEW CLASS OF LUMINOUS TRANSIENTS AND A FIRST CENSUS OF THEIR MASSIVE STELLAR PROGENITORS

The progenitors of SN 2008S and the 2008 luminous transient in NGC 300 were deeply dust-enshrouded massive stars, with extremely red mid-infrared (MIR) colors and relatively low bolometric luminosities (≈5 × 10 4 L� ). The transients were optically faint compared to normal core-collapse supernovae (ccSNe), with peak absolute visual magnitudes of −13 MV −15, and their spectra exhibit narrow Balmer and [Ca ii] emission lines. These events are unique among transient–progenitor pairs and hence constitute a new class. Additional members of this class may include the M85 transient, SN 1999bw, 2002bu, and others. Whether they are true supernovae or bright massive-star eruptions, we argue that their rate is of order ∼20% of the ccSN rate in star-forming galaxies. This fact is remarkable in light of the observation that a very small fraction of all massive stars in any one galaxy, at any moment, have the infrared colors of the progenitors of SN 2008S and the NGC 300 transient. We show this by extracting MIR and optical luminosity, color, and variability properties of massive stars in M33 using archival imaging. We find that the fraction of massive stars with colors consistent with the progenitors of SN 2008S and the NGC 300 transient is 10 −4 . In fact, only 10 similar objects exist in M33 (and perhaps 1)—all of which lie at the luminous red extremum of the asymptotic giant branch sequence. That these transients are simultaneously relativelycommon with respect to supernovae, while their progenitors are remarkably rare compared to the massive star population, implies that the dust-enshrouded phase is a short-lived phenomenon in the lives of many massive stars. This shrouded epoch can occur only in the last10 4 yr before explosion, be it death or merely eruption. We discuss the implications of this finding for the evolution and census of “low-mass” massive stars (i.e., ∼8–12 M� ), and we connect it with theoretical discussions of electron-capture supernovae (ecSNe) near this mass range. Other potential mechanisms, including the explosive birth of massive white dwarfs and massive star outbursts, are also discussed. A systematic census with (warm)Spitzer of galaxies in the local universe (D10 Mpc) for analogous progenitors would significantly improve our knowledge of this channel to massive stellar explosions, and potentially to others with obscured progenitors.

Revealing the High-Redshift Star Formation Rate with Gamma-Ray Bursts

While the high-z frontier of star formation rate (SFR) studies has advanced rapidly, direct measurements beyond -->z ~ 4 remain difficult, as shown by significant disagreements among different results. Gamma-ray bursts, owing to their brightness and association with massive stars, offer hope of clarifying this situation, provided that the GRB rate can be properly related to the SFR. The Swift GRB data reveal an increasing evolution in the GRB rate relative to the SFR at intermediate z; taking this into account, we use the highest-z GRB data to make a new determination of the SFR at -->z = 4–7. Our results exceed the lowest direct SFR measurements and imply that no steep drop exists in the SFR up to at least -->z ~ 6.5. We discuss the implications of our result for cosmic reionization, the efficiency of the universe in producing stellar-mass black holes, and GRB feedback in star-forming hosts.

Discovery of the Dust-Enshrouded Progenitor of SN 2008S with Spitzer

We report the discovery of the progenitor of the recent Type IIn SN 2008S in the nearby galaxy NGC 6946. Surprisingly, it was not found in deep, preexplosion optical images of its host galaxy taken with the Large Binocular Telescope, but only through examination of archival Spitzer mid-IR data. A source coincident with the SN 2008S position is clearly detected in the 4.5, 5.8, and 8.0 ?m IRAC bands, showing no evident variability in the 3 years prior to the explosion, yet is undetected at 3.6 and 24 ?m. The distinct presence of ~440 K dust, along with stringent LBT limits on the optical fluxes, suggests that the progenitor of SN 2008S was engulfed in a shroud of its own dust. The inferred luminosity of 3.5 ? 104 L? implies a modest mass of ~10 M -->?. We conclude that objects like SN 2008S are not exclusively associated with the deaths or outbursts of very massive ? Carinae-like objects. This conclusion holds based solely on the optical flux limits even if our identification of the progenitor with the mid-IR source is incorrect.

An Unexpectedly Swift Rise in the Gamma-Ray Burst Rate

The association of long gamma-ray bursts with supernovae naturally suggests that the cosmic GRB rate should trace the star formation history. Finding otherwise would provide important clues concerning these rare, curious phenomena. Using a new estimate of Swift GRB energetics to construct a sample of 36 luminous GRBs with redshifts in the range z = 0-4, we find evidence of enhanced evolution in the GRB rate, with ~4 times as many GRBs observed at -->z ? 4 than expected from star formation measurements. This direct and empirical demonstration of needed additional evolution is a new result. It is consistent with theoretical expectations from metallicity effects, but other causes remain possible, and we consider them systematically.

Guaranteed and prospective Galactic TeV neutrino sources

Recent observations, particularly from the HESS Collaboration, have revealed rich Galactic populations of TeV gamma-ray sources, including a collection unseen in other wavelengths. Many of these gamma-ray spectra are well measured up to {approx}10 TeV, where low statistics make observations by air Cerenkov telescopes difficult. To understand these mysterious sources, especially at much higher energies--where a cutoff should eventually appear--new techniques are needed. We point out the following: (1) For a number of sources, it is very likely that pions, and hence TeV neutrinos, are produced; (2) As a general point, neutrinos should be a better probe of the highest energies than gamma rays, due to increasing detector efficiency; and (3) For several specific sources, the detection prospects for km{sup 3} neutrino telescopes are very good, {approx}1-10 events/year, with low atmospheric neutrino background rates above reasonable energy thresholds. Such signal rates, as small as they may seem, will allow neutrino telescopes to powerfully discriminate between models for the Galactic TeV sources, with important consequences for our understanding of cosmic-ray production.

Circumscribing late dark matter decays model-independently

A number of theories, spanning a wide range of mass scales, predict dark matter candidates that have lifetimes much longer than the age of the Universe, yet may produce a significant flux of gamma rays in their decays today. We constrain such late-decaying dark matter scenarios model-independently by utilizing gamma-ray line emission limits from the Galactic Center region obtained with the SPI spectrometer on INTEGRAL, and the determination of the isotropic diffuse photon background by SPI, COMPTEL, and EGRET observations. We show that no more than ∼5% of the unexplained MeV background can be produced by late dark matter decays either in the Galactic halo or cosmological sources.

Enhanced cosmological GRB rates and implications for cosmogenic neutrinos

Gamma-ray bursts, which are among the most violent events in the Universe, are one of the few viable candidates to produce ultra high-energy cosmic rays. Recently, observations have revealed that GRBs generally originate from metal-poor, low-luminosity galaxies and do not directly trace cosmic star formation, as might have been assumed from their association with core-collapse supernovae. Several implications follow from these findings. The redshift distribution of observed GRBs is expected to peak at higher redshift (compared to cosmic star formation), which is supported by the mean redshift of the Swift GRB sample, {approx}3. If GRBs are, in fact, the source of the observed UHECR, then cosmic-ray production would evolve with redshift in a stronger fashion than has been previously suggested. This necessarily leads, through the GZK process, to an enhancement in the flux of cosmogenic neutrinos, providing a near-term approach for testing the gamma-ray burst-cosmic-ray connection with ongoing and proposed UHE neutrino experiments.

Core-collapse astrophysics with a five-megaton neutrino detector

The legacy of solar neutrinos suggests that large neutrino detectors should be sited underground. However, to instead go underwater bypasses the need to move mountains, allowing much larger water Cerenkov detectors. We show that reaching a detector mass scale of ~5 Megatons, the size of the proposed Deep-TITAND, would permit observations of neutrino “mini-bursts” from supernovae in nearby galaxies on a roughly yearly basis, and we develop the immediate qualitative and quantitative consequences. Importantly, these mini-bursts would be detected over backgrounds without the need for optical evidence of the supernova, guaranteeing the beginning of time-domain MeV neutrino astronomy. The ability to identify, to the second, every core collapse in the local Universe would allow a continuous “death watch” of all stars within ~5  Mpc, making practical many previously-impossible tasks in probing rare outcomes and refining coordination of multiwavelength/multiparticle observations and analysis. These include the abilities to promptly detect otherwise-invisible prompt black hole formation, provide advance warning for supernova shock-breakout searches, define tight time windows for gravitational-wave searches, and identify “supernova impostors” by the nondetection of neutrinos. Observations of many supernovae, even with low numbers of detected neutrinos, will help answer questions about supernovae that cannot be resolved with a single high-statistics event in the Milky Way.