Shunsaku Horiuchi - Academia.edu (original) (raw)

Papers by Shunsaku Horiuchi

Journal of Physics: Conference Series, 2008

To constrain the particle properties of dark matter using their annihilation products, robust pre... more To constrain the particle properties of dark matter using their annihilation products, robust predictions of the annihilation rates are critical. Since the annihilation rate scales as the dark matter density squared, a precise knowledge of the distribution of dark matter is required. Given the popular galactic center contains large uncertainties in the dark matter density, we focus on the signal

Physical Review D, 2013

The IceCube neutrino observatory has detected two cascade events with energies near 1 PeV . Witho... more The IceCube neutrino observatory has detected two cascade events with energies near 1 PeV . Without invoking new physics, we analyze the source of these neutrinos. We show that atmospheric conventional neutrinos and cosmogenic neutrinos (those produced in the propagation of ultra-highenergy cosmic rays) are strongly disfavored. For atmospheric prompt neutrinos or a diffuse background of neutrinos produced in astrophysical objects, the situation is less clear. We show that there are tensions with observed data, but that the details depend on the least-known aspects of the IceCube analysis. Very likely, prompt neutrinos are disfavored and astrophysical neutrinos are plausible. We demonstrate that the fastest way to reveal the origin of the observed PeV neutrinos is to search for neutrino cascades in the range below 1 PeV, for which dedicated analyses with high sensitivity have yet to appear, and where many more events could be found.

Physical Review D, 2014

The extragalactic dark matter (DM) annihilation signal depends on the product of the clumping fac... more The extragalactic dark matter (DM) annihilation signal depends on the product of the clumping factor, δ 2 , and the velocity-weighted annihilation cross section, σv. This "clumping factorσv" degeneracy can be broken by comparing DM annihilation signals from multiple sources. In particular, one can constrain the minimum DM halo mass, Mmin, which depends on the mass of the DM particles and the kinetic decoupling temperature, by comparing observations of individual DM sources to the diffuse DM annihilation signal. We demonstrate this with careful semi-analytic treatments of the DM contribution to the diffuse Isotropic Gamma-Ray Background (IGRB), and compare it with two recent hints of DM from the Galactic Center, namely, ∼ 130 GeV DM annihilating dominantly in the χχ → γγ channel, and (10 − 30) GeV DM annihilating in the χχ → bb or χχ → τ + τ − channels. We show that, even in the most conservative analysis, the Fermi IGRB measurement already provides interesting sensitivity. A more detailed analysis of the IGRB, with new Fermi IGRB measurements and modeling of astrophysical backgrounds, may be able to probe values of Mmin up to 1 M for the 130 GeV candidate and 10 −6 M for the light DM candidates. Increasing the substructure content of halos by a reasonable amount would further improve these constraints.

Physical Review D, 2015

ABSTRACT A sterile neutrino of ~keV mass is a well motivated dark matter candidate. Its decay gen... more ABSTRACT A sterile neutrino of ~keV mass is a well motivated dark matter candidate. Its decay generates a X-ray line which offers a unique target for X-ray telescopes. For the first time, we use the Gamma-ray Burst Monitor (GBM) onboard the Fermi Gamma-Ray Space Telescope to search for sterile neutrino decay lines; our analysis covers the energy range 10-25 keV (sterile neutrino mass 20-50 keV), which is inaccessible to X-ray and gamma-ray satellites such as Chandra, Suzaku, XMM-Newton, and INTEGRAL. The extremely wide field of view of the GBM enables a large fraction of the Milky Way dark matter halo to be probed. After implementing careful data cuts, we obtain ~53 days of full sky observational data. We search for sterile neutrino decay lines in the energy spectrum, and find no significant signal. From this, we obtain upper limits on the sterile neutrino mixing angle as a function of mass. In the sterile neutrino mass range 25-40 keV, we improve upon previous upper limits by approximately an order of magnitude. Better understanding of detector and astrophysical backgrounds, as well as detector response, will further improve the sensitivity of a search with the GBM.

The Astrophysical Journal, 2013

We investigate star formation rate (SFR) calibrations in light of recent developments in the mode... more We investigate star formation rate (SFR) calibrations in light of recent developments in the modeling of stellar rotation. Using new published non-rotating and rotating stellar tracks, we study the integrated properties of synthetic stellar populations and find that the UV to SFR calibration for the rotating stellar population is 30% smaller than for the non-rotating stellar population, and 40% smaller for the Hα to SFR calibration. These reductions translate to smaller SFR estimates made from observed UV and Hα luminosities. Using the UV and Hα fluxes of a sample of ∼ 300 local galaxies, we derive a total (i.e., sky-coverage corrected) SFR within 11 Mpc of 120-170 M ⊙ yr −1 and 80-130 M ⊙ yr −1 for the non-rotating and rotating estimators, respectively. Independently, the number of core-collapse supernovae discovered in the same volume requires a total SFR of 270 +110 −80 M ⊙ yr −1 , suggesting a tension with the SFR estimates made with rotating calibrations. More generally, when compared with the directly estimated SFR, the local supernova discoveries strongly constrain any physical effects that might increase the energy output of massive stars, including, but not limited to, stellar rotation. The cosmic SFR and cosmic supernova rate data on the other hand show the opposite trend, with the cosmic SFR higher than that inferred from cosmic supernovae, constraining a significant decrease in the energy output of massive stars. Together, these lines of evidence suggest that the true SFR calibration factors cannot be too far from their canonical values.

The Astrophysical Journal, 2014

The Astrophysical Journal, 2012

Heavy nuclei such as nickel-56 are synthesized in a wide range of core-collapse supernovae (CCSN)... more Heavy nuclei such as nickel-56 are synthesized in a wide range of core-collapse supernovae (CCSN), including energetic supernovae associated with gamma-ray bursts (GRBs). Recent studies suggest that jet-like outflows are a common feature of CCSN. These outflows may entrain synthesized nuclei at launch or during propagation, and provide interesting multi-messenger signals including heavy ultra-high-energy cosmic rays. Here, we investigate the destruction processes of nuclei during crossing from the stellar material into the jet material via a cocoon, and during propagation after being successfully loaded into the jet. We find that nuclei can survive for a range of jet parameters because collisional cooling is faster than spallation. While canonical high-luminosity GRB jets may contain nuclei, magnetic-dominated models or low-luminosity jets with small bulk Lorentz factors are more favorable for having a significant heavy nuclei component.

The Astrophysical Journal, 2013

Trans-relativistic shocks that accompany some supernovae (SNe) produce X-ray burst emissions as t... more Trans-relativistic shocks that accompany some supernovae (SNe) produce X-ray burst emissions as they break out in the dense circumstellar medium around the progenitors. This phenomenon is sometimes associated with peculiar low-luminosity gamma-ray bursts (LL GRBs). Here, we investigate the high energy neutrino and gamma-ray counterparts of such a class of SNe. Just beyond the shock breakout radius, particle acceleration in the collisionless shock starts to operate in the presence of breakout photons. We show that protons may be accelerated to sufficiently high energies and produce high energy neutrinos and gamma rays via the photomeson interaction. These neutrinos and gamma rays may be detectable from 10 Mpc away by IceCube/KM3Net as multi-TeV transients almost simultaneously with the X-ray breakout, and even from 100 Mpc away with follow-up observations by CTA using a wide-field sky monitor like Swift as a trigger. A statistical technique using a stacking approach could also be possible for the detection, with the aid of the SN optical/infrared counterparts. Such multi-messenger observations offer the possibility to probe the transition of trans-relativistic shocks from radiation-mediated to collisionless ones, and would also constrain the mechanisms of particle acceleration and emission in LL GRBs.

Monthly Notices of the Royal Astronomical Society Letters

Mapping supernovae to their progenitors is fundamental to understanding the collapse of massive s... more Mapping supernovae to their progenitors is fundamental to understanding the collapse of massive stars. We investigate the red supergiant problem, which concerns why red supergiants with masses ∼16–30 M⊙ have not been identified as progenitors of Type IIP supernovae, and the supernova rate problem, which concerns why the observed cosmic supernova rate is smaller than the observed cosmic star formation rate. We find key physics to solving these in the compactness parameter, which characterizes the density structure of the progenitor. If massive stars with compactness above ξ2.5 ∼ 0.2 fail to produce canonical supernovae, (i) stars in the mass range 16–30 M⊙ populate an island of stars that have high ξ2.5 and do not produce canonical supernovae, and (ii) the fraction of such stars is consistent with the missing fraction of supernovae relative to star formation. We support this scenario with a series of two- and three-dimensional radiation hydrodynamics core-collapse simulations. Using ...

Powerful radio-loud active galactic nuclei (AGN) with large Mpc-scale jets have been theoreticall... more Powerful radio-loud active galactic nuclei (AGN) with large Mpc-scale jets have been theoretically motivated as emitters of high-energy cosmic rays. Recent radio observations have established a populous class of young radio-loud galaxies with compact ($< 1$ kpc) symmetric jets that are morphologically similar to large-scale AGNs. We show that these compact AGNs, so-called compact symmetric objects (CSOs), can accelerate protons

Highly relativistic jets are a key element of current gamma-ray burst (GRB) models, where the jet... more Highly relativistic jets are a key element of current gamma-ray burst (GRB) models, where the jet kinetic energy is converted to radiation energy at optically thin shocks. Mildly relativistic jets with smaller Lorentz factors are typically optically thick to gamma rays, and do not produce the spectacular GRB phenomenon. Jets which stall inside the progenitor similarly do not produce a

Recent measurements by the Pierre Auger Observatory suggest that the composition of ultra-high en... more Recent measurements by the Pierre Auger Observatory suggest that the composition of ultra-high energy cosmic rays (UHECRs) becomes dominated by heavy nuclei at high energies. However, until now there has been no astrophysical motivation for considering a source highly enriched in heavy elements. Here we demonstrate that the outflows from Gamma-Ray Bursts (GRBs) may indeed be composed primarily of nuclei

We identify a "supernova rate problem": the measured cosmic core-collapse supernova rat... more We identify a "supernova rate problem": the measured cosmic core-collapse supernova rate is a factor ~ 2 smaller (with significance ~ 2 sigma) than that predicted from the measured cosmic massive-star formation rate. This comparison is critical for topics from galaxy evolution and enrichment to the abundance of neutron stars and black holes. We systematically explore possible resolutions. The precision

Under the fossil field hypothesis of the origin of magnetar magnetic fields, the magnetar inherit... more Under the fossil field hypothesis of the origin of magnetar magnetic fields, the magnetar inherits its magnetic field from its progenitor. We show that during the supernova of such a progenitor, protons may be accelerated to �104 GeV as the supernova shock propagates in the magnetic stellar envelope. Inelastic nuclear collisions of these protons produce a flash of high-energy neutrinos

Monthly Notices of the Royal Astronomical Society, 2014

The BICEP 2 results, when interpreted as a gravitational wave signal and combined with other CMB ... more The BICEP 2 results, when interpreted as a gravitational wave signal and combined with other CMB data, suggest a roll-off in power towards small scales in the primordial matter power spectrum. Among the simplest possibilities is a running of the spectral index. Here we show that the preferred level of running alleviates small-scale issues within the ΛCDM model, more so even than viable WDM models. We use cosmological zoom-in simulations of a Milky Way-size halo along with full-box simulations to compare predictions among four separate cosmologies: a BICEP 2-inspired running index model (α s = −0.024), two fixed-tilt ΛCDM models motivated by Planck, and a 2.6 keV thermal WDM model. We find that the running BICEP 2 model reduces the central densities of large dwarf-size halos (V max ∼ 30 − 80 km s −1 ) and alleviates the too-big-to-fail problem significantly compared to our adopted Planck and WDM cases. Further, the BICEP 2 model suppresses the count of small subhalos by ∼ 50% relative to Planck models, and yields a significantly lower "boost" factor for dark matter annihilation signals. Our findings highlight the need to understand the shape of the primordial power spectrum in order to correctly interpret small-scale data.

Journal of Cosmology and Astroparticle Physics, 2015

ABSTRACT We present a new extended gamma ray excess toward the Galactic Center that traces the 3.... more ABSTRACT We present a new extended gamma ray excess toward the Galactic Center that traces the 3.4 micron infrared emission morphology. Combined with its measured spectrum, this new extended source is consistent with inverse Compton emission from a high-energy electron-positron population with energies up to about 10 GeV. Previously detected emissions tracing the 20 cm radio, interpreted as bremsstrahlung radiation, and the Galactic Center Extended emission tracing a spherical distribution and peaking at 2 GeV, are also detected. We show that the inverse Compton and bremsstrahlung emissions are likely due to the same source of electrons and positrons. All three extended emissions may be explained within the framework of a model where the dark matter annihilates to leptons or a model with unresolved millisecond pulsars in the Galactic Center.

Physical Review D, 2009

The Diffuse Supernova Neutrino Background (DSNB) provides an immediate opportunity to study the e... more The Diffuse Supernova Neutrino Background (DSNB) provides an immediate opportunity to study the emission of MeV thermal neutrinos from core-collapse supernovae. The DSNB is a powerful probe of stellar and neutrino physics, provided that the core-collapse rate is large enough and that its uncertainty is small enough. To assess the important physics enabled by the DSNB, we start with the cosmic star formation history of and confirm its normalization and evolution by cross-checks with the supernova rate, extragalactic background light, and stellar mass density. We find a sufficient core-collapse rate with small uncertainties that translate into a variation of ±40% in the DSNB event spectrum. Considering thermal neutrino spectra with effective temperatures between 4-6 MeV, the predicted DSNB is within a factor 4-2 below the upper limit obtained by Super-Kamiokande in 2003. Furthermore, detection prospects would be dramatically improved with a gadolinium-enhanced Super-Kamiokande: the backgrounds would be significantly reduced, the fluxes and uncertainties converge at the lower threshold energy, and the predicted event rate is 1.2-5.6 events yr −1 in the energy range 10-26 MeV. These results demonstrate the imminent detection of the DSNB by Super-Kamiokande and its exciting prospects for studying stellar and neutrino physics. PACS numbers: 97.60.Bw, 95.85.Ry, 98.70.Vc

Physical Review D, 2014

We construct empirical models of the diffuse gamma-ray background toward the Galactic Center. Inc... more We construct empirical models of the diffuse gamma-ray background toward the Galactic Center. Including all known point sources and a template of emission associated with interactions of cosmic rays with molecular gas, we show that the extended emission observed previously in the Fermi Large Area Telescope data toward the Galactic Center is detected at high significance for all permutations of the diffuse model components. However, we find that the fluxes and spectra of the sources in our model change significantly depending on the background model. In particular, the spectrum of the central Sgr A * source is less steep than in previous works and the recovered spectrum of the extended emission has large systematic uncertainties, especially at lower energies. If the extended emission is interpreted to be due to dark matter annihilation, we find annihilation into pure b-quark and τ -lepton channels to be statistically equivalent goodness of fits. In the case of the pure b-quark channel, we find a dark matter mass of 39.4 +3.7 −2.9 stat. (±7.9 sys.) GeV, while a pure τ + τ − -channel case has an estimated dark matter mass of 9.43 +0.63 −0.52 stat. (±1.2 sys.) GeV. Alternatively, if the extended emission is interpreted to be astrophysical in origin such as due to unresolved millisecond pulsars, we obtain strong bounds on dark matter annihilation, although systematic uncertainties due to the dependence on the background models are significant.

Journal of Physics: Conference Series, 2008

To constrain the particle properties of dark matter using their annihilation products, robust pre... more To constrain the particle properties of dark matter using their annihilation products, robust predictions of the annihilation rates are critical. Since the annihilation rate scales as the dark matter density squared, a precise knowledge of the distribution of dark matter is required. Given the popular galactic center contains large uncertainties in the dark matter density, we focus on the signal

Physical Review D, 2013

The IceCube neutrino observatory has detected two cascade events with energies near 1 PeV . Witho... more The IceCube neutrino observatory has detected two cascade events with energies near 1 PeV . Without invoking new physics, we analyze the source of these neutrinos. We show that atmospheric conventional neutrinos and cosmogenic neutrinos (those produced in the propagation of ultra-highenergy cosmic rays) are strongly disfavored. For atmospheric prompt neutrinos or a diffuse background of neutrinos produced in astrophysical objects, the situation is less clear. We show that there are tensions with observed data, but that the details depend on the least-known aspects of the IceCube analysis. Very likely, prompt neutrinos are disfavored and astrophysical neutrinos are plausible. We demonstrate that the fastest way to reveal the origin of the observed PeV neutrinos is to search for neutrino cascades in the range below 1 PeV, for which dedicated analyses with high sensitivity have yet to appear, and where many more events could be found.

Physical Review D, 2014

The extragalactic dark matter (DM) annihilation signal depends on the product of the clumping fac... more The extragalactic dark matter (DM) annihilation signal depends on the product of the clumping factor, δ 2 , and the velocity-weighted annihilation cross section, σv. This "clumping factorσv" degeneracy can be broken by comparing DM annihilation signals from multiple sources. In particular, one can constrain the minimum DM halo mass, Mmin, which depends on the mass of the DM particles and the kinetic decoupling temperature, by comparing observations of individual DM sources to the diffuse DM annihilation signal. We demonstrate this with careful semi-analytic treatments of the DM contribution to the diffuse Isotropic Gamma-Ray Background (IGRB), and compare it with two recent hints of DM from the Galactic Center, namely, ∼ 130 GeV DM annihilating dominantly in the χχ → γγ channel, and (10 − 30) GeV DM annihilating in the χχ → bb or χχ → τ + τ − channels. We show that, even in the most conservative analysis, the Fermi IGRB measurement already provides interesting sensitivity. A more detailed analysis of the IGRB, with new Fermi IGRB measurements and modeling of astrophysical backgrounds, may be able to probe values of Mmin up to 1 M for the 130 GeV candidate and 10 −6 M for the light DM candidates. Increasing the substructure content of halos by a reasonable amount would further improve these constraints.

Physical Review D, 2015

ABSTRACT A sterile neutrino of ~keV mass is a well motivated dark matter candidate. Its decay gen... more ABSTRACT A sterile neutrino of ~keV mass is a well motivated dark matter candidate. Its decay generates a X-ray line which offers a unique target for X-ray telescopes. For the first time, we use the Gamma-ray Burst Monitor (GBM) onboard the Fermi Gamma-Ray Space Telescope to search for sterile neutrino decay lines; our analysis covers the energy range 10-25 keV (sterile neutrino mass 20-50 keV), which is inaccessible to X-ray and gamma-ray satellites such as Chandra, Suzaku, XMM-Newton, and INTEGRAL. The extremely wide field of view of the GBM enables a large fraction of the Milky Way dark matter halo to be probed. After implementing careful data cuts, we obtain ~53 days of full sky observational data. We search for sterile neutrino decay lines in the energy spectrum, and find no significant signal. From this, we obtain upper limits on the sterile neutrino mixing angle as a function of mass. In the sterile neutrino mass range 25-40 keV, we improve upon previous upper limits by approximately an order of magnitude. Better understanding of detector and astrophysical backgrounds, as well as detector response, will further improve the sensitivity of a search with the GBM.

The Astrophysical Journal, 2013

We investigate star formation rate (SFR) calibrations in light of recent developments in the mode... more We investigate star formation rate (SFR) calibrations in light of recent developments in the modeling of stellar rotation. Using new published non-rotating and rotating stellar tracks, we study the integrated properties of synthetic stellar populations and find that the UV to SFR calibration for the rotating stellar population is 30% smaller than for the non-rotating stellar population, and 40% smaller for the Hα to SFR calibration. These reductions translate to smaller SFR estimates made from observed UV and Hα luminosities. Using the UV and Hα fluxes of a sample of ∼ 300 local galaxies, we derive a total (i.e., sky-coverage corrected) SFR within 11 Mpc of 120-170 M ⊙ yr −1 and 80-130 M ⊙ yr −1 for the non-rotating and rotating estimators, respectively. Independently, the number of core-collapse supernovae discovered in the same volume requires a total SFR of 270 +110 −80 M ⊙ yr −1 , suggesting a tension with the SFR estimates made with rotating calibrations. More generally, when compared with the directly estimated SFR, the local supernova discoveries strongly constrain any physical effects that might increase the energy output of massive stars, including, but not limited to, stellar rotation. The cosmic SFR and cosmic supernova rate data on the other hand show the opposite trend, with the cosmic SFR higher than that inferred from cosmic supernovae, constraining a significant decrease in the energy output of massive stars. Together, these lines of evidence suggest that the true SFR calibration factors cannot be too far from their canonical values.

The Astrophysical Journal, 2014

The Astrophysical Journal, 2012

Heavy nuclei such as nickel-56 are synthesized in a wide range of core-collapse supernovae (CCSN)... more Heavy nuclei such as nickel-56 are synthesized in a wide range of core-collapse supernovae (CCSN), including energetic supernovae associated with gamma-ray bursts (GRBs). Recent studies suggest that jet-like outflows are a common feature of CCSN. These outflows may entrain synthesized nuclei at launch or during propagation, and provide interesting multi-messenger signals including heavy ultra-high-energy cosmic rays. Here, we investigate the destruction processes of nuclei during crossing from the stellar material into the jet material via a cocoon, and during propagation after being successfully loaded into the jet. We find that nuclei can survive for a range of jet parameters because collisional cooling is faster than spallation. While canonical high-luminosity GRB jets may contain nuclei, magnetic-dominated models or low-luminosity jets with small bulk Lorentz factors are more favorable for having a significant heavy nuclei component.

The Astrophysical Journal, 2013

Trans-relativistic shocks that accompany some supernovae (SNe) produce X-ray burst emissions as t... more Trans-relativistic shocks that accompany some supernovae (SNe) produce X-ray burst emissions as they break out in the dense circumstellar medium around the progenitors. This phenomenon is sometimes associated with peculiar low-luminosity gamma-ray bursts (LL GRBs). Here, we investigate the high energy neutrino and gamma-ray counterparts of such a class of SNe. Just beyond the shock breakout radius, particle acceleration in the collisionless shock starts to operate in the presence of breakout photons. We show that protons may be accelerated to sufficiently high energies and produce high energy neutrinos and gamma rays via the photomeson interaction. These neutrinos and gamma rays may be detectable from 10 Mpc away by IceCube/KM3Net as multi-TeV transients almost simultaneously with the X-ray breakout, and even from 100 Mpc away with follow-up observations by CTA using a wide-field sky monitor like Swift as a trigger. A statistical technique using a stacking approach could also be possible for the detection, with the aid of the SN optical/infrared counterparts. Such multi-messenger observations offer the possibility to probe the transition of trans-relativistic shocks from radiation-mediated to collisionless ones, and would also constrain the mechanisms of particle acceleration and emission in LL GRBs.

Monthly Notices of the Royal Astronomical Society Letters

Mapping supernovae to their progenitors is fundamental to understanding the collapse of massive s... more Mapping supernovae to their progenitors is fundamental to understanding the collapse of massive stars. We investigate the red supergiant problem, which concerns why red supergiants with masses ∼16–30 M⊙ have not been identified as progenitors of Type IIP supernovae, and the supernova rate problem, which concerns why the observed cosmic supernova rate is smaller than the observed cosmic star formation rate. We find key physics to solving these in the compactness parameter, which characterizes the density structure of the progenitor. If massive stars with compactness above ξ2.5 ∼ 0.2 fail to produce canonical supernovae, (i) stars in the mass range 16–30 M⊙ populate an island of stars that have high ξ2.5 and do not produce canonical supernovae, and (ii) the fraction of such stars is consistent with the missing fraction of supernovae relative to star formation. We support this scenario with a series of two- and three-dimensional radiation hydrodynamics core-collapse simulations. Using ...

Powerful radio-loud active galactic nuclei (AGN) with large Mpc-scale jets have been theoreticall... more Powerful radio-loud active galactic nuclei (AGN) with large Mpc-scale jets have been theoretically motivated as emitters of high-energy cosmic rays. Recent radio observations have established a populous class of young radio-loud galaxies with compact ($< 1$ kpc) symmetric jets that are morphologically similar to large-scale AGNs. We show that these compact AGNs, so-called compact symmetric objects (CSOs), can accelerate protons

Highly relativistic jets are a key element of current gamma-ray burst (GRB) models, where the jet... more Highly relativistic jets are a key element of current gamma-ray burst (GRB) models, where the jet kinetic energy is converted to radiation energy at optically thin shocks. Mildly relativistic jets with smaller Lorentz factors are typically optically thick to gamma rays, and do not produce the spectacular GRB phenomenon. Jets which stall inside the progenitor similarly do not produce a

Recent measurements by the Pierre Auger Observatory suggest that the composition of ultra-high en... more Recent measurements by the Pierre Auger Observatory suggest that the composition of ultra-high energy cosmic rays (UHECRs) becomes dominated by heavy nuclei at high energies. However, until now there has been no astrophysical motivation for considering a source highly enriched in heavy elements. Here we demonstrate that the outflows from Gamma-Ray Bursts (GRBs) may indeed be composed primarily of nuclei

We identify a "supernova rate problem": the measured cosmic core-collapse supernova rat... more We identify a "supernova rate problem": the measured cosmic core-collapse supernova rate is a factor ~ 2 smaller (with significance ~ 2 sigma) than that predicted from the measured cosmic massive-star formation rate. This comparison is critical for topics from galaxy evolution and enrichment to the abundance of neutron stars and black holes. We systematically explore possible resolutions. The precision

Under the fossil field hypothesis of the origin of magnetar magnetic fields, the magnetar inherit... more Under the fossil field hypothesis of the origin of magnetar magnetic fields, the magnetar inherits its magnetic field from its progenitor. We show that during the supernova of such a progenitor, protons may be accelerated to �104 GeV as the supernova shock propagates in the magnetic stellar envelope. Inelastic nuclear collisions of these protons produce a flash of high-energy neutrinos

Monthly Notices of the Royal Astronomical Society, 2014

The BICEP 2 results, when interpreted as a gravitational wave signal and combined with other CMB ... more The BICEP 2 results, when interpreted as a gravitational wave signal and combined with other CMB data, suggest a roll-off in power towards small scales in the primordial matter power spectrum. Among the simplest possibilities is a running of the spectral index. Here we show that the preferred level of running alleviates small-scale issues within the ΛCDM model, more so even than viable WDM models. We use cosmological zoom-in simulations of a Milky Way-size halo along with full-box simulations to compare predictions among four separate cosmologies: a BICEP 2-inspired running index model (α s = −0.024), two fixed-tilt ΛCDM models motivated by Planck, and a 2.6 keV thermal WDM model. We find that the running BICEP 2 model reduces the central densities of large dwarf-size halos (V max ∼ 30 − 80 km s −1 ) and alleviates the too-big-to-fail problem significantly compared to our adopted Planck and WDM cases. Further, the BICEP 2 model suppresses the count of small subhalos by ∼ 50% relative to Planck models, and yields a significantly lower "boost" factor for dark matter annihilation signals. Our findings highlight the need to understand the shape of the primordial power spectrum in order to correctly interpret small-scale data.

Journal of Cosmology and Astroparticle Physics, 2015

ABSTRACT We present a new extended gamma ray excess toward the Galactic Center that traces the 3.... more ABSTRACT We present a new extended gamma ray excess toward the Galactic Center that traces the 3.4 micron infrared emission morphology. Combined with its measured spectrum, this new extended source is consistent with inverse Compton emission from a high-energy electron-positron population with energies up to about 10 GeV. Previously detected emissions tracing the 20 cm radio, interpreted as bremsstrahlung radiation, and the Galactic Center Extended emission tracing a spherical distribution and peaking at 2 GeV, are also detected. We show that the inverse Compton and bremsstrahlung emissions are likely due to the same source of electrons and positrons. All three extended emissions may be explained within the framework of a model where the dark matter annihilates to leptons or a model with unresolved millisecond pulsars in the Galactic Center.

Physical Review D, 2009

The Diffuse Supernova Neutrino Background (DSNB) provides an immediate opportunity to study the e... more The Diffuse Supernova Neutrino Background (DSNB) provides an immediate opportunity to study the emission of MeV thermal neutrinos from core-collapse supernovae. The DSNB is a powerful probe of stellar and neutrino physics, provided that the core-collapse rate is large enough and that its uncertainty is small enough. To assess the important physics enabled by the DSNB, we start with the cosmic star formation history of and confirm its normalization and evolution by cross-checks with the supernova rate, extragalactic background light, and stellar mass density. We find a sufficient core-collapse rate with small uncertainties that translate into a variation of ±40% in the DSNB event spectrum. Considering thermal neutrino spectra with effective temperatures between 4-6 MeV, the predicted DSNB is within a factor 4-2 below the upper limit obtained by Super-Kamiokande in 2003. Furthermore, detection prospects would be dramatically improved with a gadolinium-enhanced Super-Kamiokande: the backgrounds would be significantly reduced, the fluxes and uncertainties converge at the lower threshold energy, and the predicted event rate is 1.2-5.6 events yr −1 in the energy range 10-26 MeV. These results demonstrate the imminent detection of the DSNB by Super-Kamiokande and its exciting prospects for studying stellar and neutrino physics. PACS numbers: 97.60.Bw, 95.85.Ry, 98.70.Vc

Physical Review D, 2014

We construct empirical models of the diffuse gamma-ray background toward the Galactic Center. Inc... more We construct empirical models of the diffuse gamma-ray background toward the Galactic Center. Including all known point sources and a template of emission associated with interactions of cosmic rays with molecular gas, we show that the extended emission observed previously in the Fermi Large Area Telescope data toward the Galactic Center is detected at high significance for all permutations of the diffuse model components. However, we find that the fluxes and spectra of the sources in our model change significantly depending on the background model. In particular, the spectrum of the central Sgr A * source is less steep than in previous works and the recovered spectrum of the extended emission has large systematic uncertainties, especially at lower energies. If the extended emission is interpreted to be due to dark matter annihilation, we find annihilation into pure b-quark and τ -lepton channels to be statistically equivalent goodness of fits. In the case of the pure b-quark channel, we find a dark matter mass of 39.4 +3.7 −2.9 stat. (±7.9 sys.) GeV, while a pure τ + τ − -channel case has an estimated dark matter mass of 9.43 +0.63 −0.52 stat. (±1.2 sys.) GeV. Alternatively, if the extended emission is interpreted to be astrophysical in origin such as due to unresolved millisecond pulsars, we obtain strong bounds on dark matter annihilation, although systematic uncertainties due to the dependence on the background models are significant.