Infused Ice can Multiply IceCube's Sensitivity

The IceCube Neutrino Observatory is the world's largest neutrino detector with a cubic-kilometer instrumented volume at the South Pole. It is preparing for a major upgrade that will significantly increase its sensitivity. A promising technological innovation investigated for this upgrade is wavelength shifting optics. Augmenting sensors with such optics could increase the photo-collection area of IceCube's digital optical modules, and shift the incoming photons' wavelength to where these modules are the most sensitive. Here we investigate the use of IceCube's drill holes themselves as wavelength shifting optics. We calculate the sensitivity enhancement due to increasing the ice's refractive index in the holes, and infusing wavelength-shifting substrate into the ice. We find that, with adequate wavelength-shifter infusion, every ~0.05 increase in the ice's refractive index will increase IceCube's photon sensitivity by 100%, opening the possibility for the substantial, cost effective expansion of IceCube's reach.Comment: 30 pages, 5 figure

Can a Single High-energy Neutrino from Gamma-ray Bursts be a Discovery?

Current emission models of GeV-PeV neutrinos from gamma-ray bursts (GRBs) predict a neutrino flux with $\ll 1$ detected neutrinos per GRB with kilometer-scale neutrino observatories. The detection of this flux will require the stacking of data from a large number of GRBs, leading to an increased background rate, decreasing the significance of a single neutrino detection. We show that utilizing the temporal correlation between the expected gamma-ray and neutrino fluxes, one can significantly improve the neutrino signal-to-noise ratio. We describe how this temporal correlation can be used. Using realistic GRB and atmospheric neutrino fluxes and incorporating temporal, spectral and directional information, we estimate the probability of a single detected GRB-neutrino being a 5$\sigma$ discovery.Comment: 5 pages, 2 figures. Accepted to PRD Rapid Communicatio

Beyond the Horizon Distance: LIGO-Virgo can Boost Gravitational Wave Detection Rates by Exploiting the Mass Distribution of Neutron Stars

The masses of neutron stars in neutron star binaries are observed to fall in a narrow mass range around $\sim 1.33$ M$_{\odot}$. We explore the advantage of focusing on this region of the parameter space in gravitational wave searches. We find that an all-sky (externally triggered) search with optimally reduced template bank is expected to detect $14\%$ ($61\%$) more binary mergers than without the reduction. A reduced template bank can also represent significant improvement in technical cost. We also develop a more detailed search method using binary mass distribution, and find similar sensitivity increase to that due to the reduced template bank

Catalog of Isolated Emission Episodes in Gamma-ray Bursts from Fermi, Swift and BATSE

We report a comprehensive catalog of emission episodes within long gamma-ray bursts (GRBs) that are separated by a quiescent period during which gamma-ray emission falls below the background level. We use a fully automated identification method for an unbiased, large scale and expandable search. We examine a comprehensive sample of long GRBs from the BATSE, Swift and Fermi missions, assembling a total searched set of 2710 GRBs, the largest catalog of isolated emission episodes so far. Our search extends out to [-1000s,750s] around the burst trigger, expanding the covered time interval beyond previous studies and far beyond the nominal durations (T90) of most bursts. We compare our results to previous works by identifying pre-peak emission (or precursors), defined as isolated emission periods prior to the episode with the highest peak luminosity of the burst. We also systematically search for similarly defined periods after the burst's peak emission. We find that the pre-peak and post-peak emission periods are statistically similar, possibly indicating a common origin. For the analyzed GRBs, we identify 24% to have more than one isolated emission episode, with 11% having at least one pre-peak event and 15% having at least one post-peak event. We identify GRB activity significantly beyond their T90, which can be important for understanding the central engine activity as well as, e.g., gravitational-wave searches

How Gravitational-wave Observations Can Shape the Gamma-ray Burst Paradigm

By reaching through shrouding blastwaves, efficiently discovering off-axis events, and probing the central engine at work, gravitational wave (GW) observations will soon revolutionize the study of gamma-ray bursts. Already, analyses of GW data targeting gamma-ray bursts have helped constrain the central engines of selected events. Advanced GW detectors with significantly improved sensitivities are under construction. After outlining the GW emission mechanisms from gamma-ray burst progenitors (binary coalescences, stellar core collapses, magnetars, and others) that may be detectable with advanced detectors, we review how GWs will improve our understanding of gamma-ray burst central engines, their astrophysical formation channels, and the prospects and methods for different search strategies. We place special emphasis on multimessenger searches. To achieve the most scientific benefit, GW, electromagnetic, and neutrino observations should be combined to provide greater discriminating power and science reach.Comment: 48 pages, 3 figure

Radio Forensics Could Unmask Nearby Off-axis Gamma-ray Bursts

The multi-messenger observation of gamma-ray burst (GRB)\,170817A from the nearby binary neutron-star merger GW170817 demonstrated that low-energy $\gamma$-ray emission can be observed at relatively large angles from GRB jet axes. If such structured emission is typical, then the currently known sample of short GRBs with no distance measurements may contain multiple nearby off-axis events whose delayed afterglows could have gone undetected. These nearby neutron star mergers may produce telltale radio flares peaking years after the prompt GRB emission that could still be observable. Here, we show that several short GRBs observed by the Burst Alert Telescope (BAT) on the Neil Gehrels \textit{Swift} satellite, with no identified afterglow and no distance measurement, could potentially be associated with radio flares detectable by sensitive cm-wavelength radio facilities such as the Karl G. Jansky Very Large Array. We also examine optical follow-up observations that have been carried out for these events, and find that a nearby GW170817-like kilonova is ruled out for only a third of them.Comment: Accepted in MNRAS; 11 pages, 3 figure

James Webb Space Telescope can Detect Kilonovae in Gravitational Wave Follow-up Search

Kilonovae represent an important electromagnetic counterpart for compact binary mergers, which could become the most commonly detected gravitational wave (GW) source. Follow-up observations, triggered by GW events, of kilonovae are nevertheless difficult due to poor localization by GW detectors and due to their faint near-infrared peak emission that has limited observational capability. We show that the Near-Infrared Camera (NIRCam) on the James Webb Space Telescope (JWST) will be able to detect kilonovae within the relevant GW-detection range of $\sim$ 200 Mpc in short ($\lesssim$ 12-second) exposure times for a week following the merger. Despite this sensitivity, a kilonova search fully covering a fiducial localized area of $10$ \mbox{deg}^2 will not be viable with NIRCam due to its limited field of view. However, targeted surveys may be developed to optimize the likelihood of discovering kilonovae efficiently within limited observing time. We estimate that a survey of $10$ \mbox{deg}^2 focused on galaxies within 200 Mpc would require about 13 hours, dominated by overhead times; a survey further focused on galaxies exhibiting high star-formation rates would require $\sim$ 5 hours. The characteristic time may be reduced to as little as $\sim$4 hours, without compromising the likelihood of detecting kilonovae, by surveying sky areas associated with 50%, rather than 90%, confidence regions of 3 GW events, rather than a single event. On detection and identification of a kilonova, a limited number of NIRCam follow-up observations could constrain the properties of matter ejected by the binary and the equation of state of dense nuclear matter.Comment: 9 pages, 1 figur

G2 can Illuminate the Black Hole Population near the Galactic Center

Galactic nuclei are expected to be densely populated with stellar and intermediate mass black holes. Exploring this population will have important consequences for the observation prospects of gravitational waves as well as understanding galactic evolution. The gas cloud G2 currently approaching Sgr A* provides an unprecedented opportunity to probe the black hole and neutron star population of the Galactic nucleus. We examine the possibility of a G2-black hole encounter and its detectability with current X-ray satellites, such as Chandra and NuSTAR. We find that multiple encounters are likely to occur close to the pericenter, which may be detectable upon favorable circumstances. This opportunity provides an additional, important science case for leading X-ray observatories to closely follow G2 on its way to the nucleus.Comment: Accepted to PRL. 4 pages, 2 picture

Early Solar System rr-process Abundances Limit Collapsar Origin

Heavy elements produced exclusively through rapid neutron capture (the '$r$-process') originate from violent cosmic explosions. While neutron star mergers are the primary candidates, another plausible production site are 'collapsars'---collapsing massive stars that form a black hole with an accretion disk. Here we show that collapsars are too rare to be the prime origin of $r$-process elements in the Solar System. By comparing numerical simulations with the early Solar System abundances of actinides produced exclusively through the $r$-process, we exclude higher than 20% contribution from collapsars with 90% confidence. We additionally limit $r$-process ejecta masses from collapsars to less than 10% of the ejecta mass from neutron star mergers, about $10^{-2}$M$_\odot$.Comment: Accepted for publication on ApJ Letters; 5 pages, 3 figure

Detection Prospects for GeV Neutrinos from Collisionally Heated Gamma-ray Bursts with IceCube/DeepCore

Jet heating via nuclear collisions may be the main mechanism for gamma-ray burst (GRB) emission. Besides producing the observed gamma-rays, collisional heating must generate 10-100 GeV neutrinos, implying a close relation between the neutrino and gamma-ray luminosities. We exploit this theoretical relation to make predictions for possible GRB detections by IceCube+DeepCore. To estimate the expected neutrino signal, we use the largest sample of bursts observed by BATSE in 1991-2000. A GRB neutrino could have been detected if IceCube+DeepCore operated at that time. Detection of 10-100 GeV neutrinos would have significant implications, shedding light on the composition of GRB jets and their Lorentz factors. This could be an important target in designing future upgrades of the IceCube+DeepCore observatory.Comment: 4 pages, 2 figure