Showing papers by "John F. Beacom published in 2010"

The Diffuse Supernova Neutrino Background

Abstract: The diffuse supernova neutrino background (DSNB) is the weak glow of megaelectronvolt neutrinos and antineutrinos from distant core-collapse supernovae. The DSNB has not been detected yet, but the Super-Kamiokande (SK) 2003 upper limit on the flux is close to predictions, now quite precise, that are based on astrophysical data. If SK is modified with dissolved gadolinium to reduce detector backgrounds and increase the energy range for analysis, then it should detect the DSNB at a rate of a few events per year, providing a new probe of supernova neutrino emission and the cosmic core-collapse rate. If the DSNB is not detected, then new physics will be required. Neutrino astronomy, although uniquely powerful, has proven extremely difficult—only the Sun and the nearby Supernova 1987A have been detected to date—so the promise of detecting new sources soon is exciting indeed.

Secondary photons and neutrinos from cosmic rays produced by distant blazars.

Abstract: Secondary photons and neutrinos produced in the interactions of cosmic ray protons emitted by distant active galactic nuclei (AGN) with the photon background along the line of sight can reveal a wealth of new information about the intergalactic magnetic fields, extragalactic background light, and the acceleration mechanisms of cosmic rays. The secondary photons may have already been observed by gamma-ray telescopes. We show that the secondary neutrinos improve the prospects of discovering distant blazars by IceCube, and we discuss the ramifications for the cosmic backgrounds, magnetic fields, and AGN models.

Role of line-of-sight cosmic ray interactions in forming the spectra of distant blazars in TeV gamma rays and high-energy neutrinos

Abstract: Active galactic nuclei (AGN) can produce both gamma rays and cosmic rays. The observed high-energy gamma-ray signals from distant blazars may be dominated by secondary gamma rays produced along the line of sight by the interactions of cosmic-ray protons with background photons. This explains the surprisingly low attenuation observed for distant blazars, because the production of secondary gamma rays occurs, on average, much closer to Earth than the distance to the source. Thus the observed spectrum in the TeV range does not depend on the intrinsic gamma-ray spectrum, while it depends on the output of the source in cosmic rays. We apply this hypothesis to a number of sources and, in every case, we obtain an excellent fit, strengthening the interpretation of the observed spectra as being due to secondary gamma rays. We explore the ramifications of this interpretation for limits on the extragalactic background light and for the production of cosmic rays in AGN. We also make predictions for the neutrino signals, which can help probe acceleration of cosmic rays in AGN.

Primordial Black Holes as Dark Matter: Almost All or Almost Nothing

Abstract: Primordial black holes (PBHs) are expected to accrete particle dark matter around them to form ultracompact minihalos (UCMHs), if the PBHs themselves are not most of the dark matter. We show that if most dark matter is a thermal relic, then the inner regions of UCMHs around PBHs are highly luminous sources of annihilation products. Flux constraints on gamma rays and neutrinos set strong abundance limits, improving previous limits by orders of magnitude. Assuming enough particle dark matter exists to form UCMHs, we find that PBH . 10 -4 (for mDMc 2 ≈ 100 GeV) for a vast range in PBH mass. We briefly discuss the unc ertainties on our limits, including those due to the evolution of the UCMH luminosity as it annihilates. Subject headings:dark matter — early universe — diffuse radiation — gamma rays: diffuse background

Neutrino background flux from sources of ultrahigh-energy cosmic-ray nuclei

Abstract: Motivated by Pierre Auger Observatory results favoring a heavy nuclear composition for ultrahigh-energy (UHE) cosmic rays, we investigate implications for the cumulative neutrino background. The requirement that nuclei not be photodisintegrated constrains their interactions in sources, therefore limiting neutrino production via photomeson interactions. Assuming a $d{N}_{\mathrm{CR}}/d{E}_{\mathrm{CR}}\ensuremath{\propto}{E}_{\mathrm{CR}}^{\ensuremath{-}2}$ injection spectrum and photodisintegration via the giant dipole resonance, the background flux of neutrinos is lower than ${E}_{\ensuremath{ u}}^{2}{\ensuremath{\Phi}}_{\ensuremath{ u}}\ensuremath{\sim}{10}^{\ensuremath{-}9}\text{ }\text{ }\mathrm{GeV}\text{ }{\mathrm{cm}}^{\ensuremath{-}2}\text{ }{\mathrm{s}}^{\ensuremath{-}1}\text{ }{\mathrm{sr}}^{\ensuremath{-}1}$ if UHE nuclei ubiquitously survive in their sources. This is smaller than the analogous Waxman-Bahcall flux for UHE protons by about 1 order of magnitude and is below the projected IceCube sensitivity. If IceCube detects a neutrino background, it could be due to other sources, e.g., hadronuclear interactions of lower-energy cosmic rays; if it does not, this supports our strong restrictions on the properties of sources of UHE nuclei.

Census of self-obscured massive stars in nearby galaxies with spitzer: implications for understanding the progenitors of sn 2008s-like transients

Abstract: A new link in the causal mapping between massive stars and potentially fatal explosive transients opened with the 2008 discovery of the dust-obscured progenitors of the luminous outbursts in NGC 6946 and NGC 300. Here, we carry out a systematic mid-IR photometric search for massive, luminous, and self-obscured stars in four nearby galaxies: M33, NGC 300, M81, and NGC 6946. For detection, we use only the 3.6 {mu}m and 4.5 {mu}m IRAC bands, as these can still be used for multi-epoch Spitzer surveys of nearby galaxies ({approx}<10 Mpc). We combine familiar point-spread function and aperture photometry with an innovative application of image subtraction to catalog the self-obscured massive stars in these galaxies. In particular, we verify that stars analogous to the progenitors of the NGC 6946 (SN 2008S) and NGC 300 transients are truly rare in all four galaxies: their number may be as low as {approx}1 per galaxy at any given moment. This result empirically supports the idea that the dust-enshrouded phase is a very short lived phenomenon in the lives of many massive stars and that these objects constitute a natural extension of the asymptotic giant branch sequence. We also provide mid-IR catalogs of sources in NGC 300,more » M81, and NGC 6946.« less

Revealing Type Ia supernova physics with cosmic rates and nuclear gamma rays

Abstract: Type Ia supernovae (SNe Ia) remain mysterious despite their central importance in cosmology and their rapidly increasing discovery rate. The progenitors of SNe Ia can be probed by the delay time between progenitor birth and explosion as SNe Ia. The explosions and progenitors of SNe Ia can be probed by MeV nuclear gamma rays emitted in the decays of radioactive nickel and cobalt into iron. We compare the cosmic star formation and SN Ia rates, finding that their different redshift evolution requires a large fraction of SNe Ia to have large delay times. A delay-time distribution of the form t –α with α = 1.0 ± 0.3 provides a good fit, implying that 50% of SNe Ia explode more than ~1 Gyr after progenitor birth. The extrapolation of the cosmic SN Ia rate to z = 0 agrees with the rate we deduce from catalogs of local SNe Ia. We investigate prospects for gamma-ray telescopes to exploit the facts that escaping gamma rays directly reveal the power source of SNe Ia and uniquely provide tomography of the expanding ejecta. We find large improvements relative to earlier studies by Gehrels et al. in 1987 and Timmes & Woosley in 1997 due to larger and more certain SN Ia rates and advances in gamma-ray detectors. The proposed Advanced Compton Telescope, with a narrow-line sensitivity ~60 times better than that of current satellites, would, on an annual basis, detect up to ~100 SNe Ia (3σ) and provide revolutionary model discrimination for SNe Ia within 20 Mpc, with gamma-ray light curves measured with ~10σ significance daily for ~100 days. Even more modest improvements in detector sensitivity would open a new and invaluable astronomy with frequent SN Ia gamma-ray detections.

The Diffuse Supernova Neutrino Background

Abstract: The Diffuse Supernova Neutrino Background (DSNB) is the weak glow of MeV neutrinos and antineutrinos from distant core-collapse supernovae. The DSNB has not been detected yet, but the Super-Kamiokande (SK) 2003 upper limit on the electron antineutrino flux is close to predictions, now quite precise, based on astrophysical data. If SK is modified with dissolved gadolinium to reduce detector backgrounds and increase the energy range for analysis, then it should detect the DSNB at a rate of a few events per year, providing a new probe of supernova neutrino emission and the cosmic core-collapse rate. If the DSNB is not detected, then new physics will be required. Neutrino astronomy, while uniquely powerful, has proven extremely difficult -- only the Sun and the nearby Supernova 1987A have been detected to date -- so the promise of detecting new sources soon is exciting indeed.

Synoptic Sky Surveys and the Diffuse Supernova Neutrino Background: Removing Astrophysical Uncertainties and Revealing Invisible Supernovae

Abstract: The cumulative (anti)neutrino production from all core-collapse supernovae within our cosmic horizon gives rise to the diffuse supernova neutrino background (DSNB), which is on the verge of detectability. The observed flux depends on supernova physics, but also on the cosmic history of supernova explosions; currently, the cosmic supernova rate introduces a substantial ($\ifmmode\pm\else\textpm\fi{}40%$) uncertainty, largely through its absolute normalization. However, a new class of wide-field, repeated-scan (synoptic) optical sky surveys is coming online, and will map the sky in the time domain with unprecedented depth, completeness, and dynamic range. We show that these surveys will obtain the cosmic supernova rate by direct counting, in an unbiased way and with high statistics, and thus will allow for precise predictions of the DSNB. Upcoming sky surveys will substantially reduce the uncertainties in the DSNB source history to an anticipated $\ifmmode\pm\else\textpm\fi{}5%$ that is dominated by systematics, so that the observed high-energy flux thus will test supernova neutrino physics. The portion of the universe ($z\ensuremath{\lesssim}1$) accessible to upcoming sky surveys includes the progenitors of a large fraction ($\ensuremath{\simeq}87%$) of the expected 10\char21{}26 MeV DSNB event rate. We show that precision determination of the (optically detected) cosmic supernova history will also make the DSNB into a strong probe of an extra flux of neutrinos from optically invisible supernovae, which may be unseen either due to unexpected large dust obscuration in host galaxies, or because some core-collapse events proceed directly to black hole formation and fail to give an optical outburst.

Determining the escape fraction of ionizing photons during reionization with the GRB-derived star formation rate

Abstract: The fraction of ionizing photons that escape their host galaxies and so are able to ionize hydrogen in the inter-galactic medium (IGM) is a critical parameter in the analyses of the reionization era and early galaxy formation. Studies of the reionization history normally suffer from a degeneracy between the unknown values for the efficiency with which high-redshift galaxies turn mass into stars and the escape fraction of ionizing photons. Recent gamma-ray burst (GRB) measurements of the star formation rate density during reionization provide the first opportunity to break this degeneracy. We confront a semi-analytic model for reionization with the GRB-derived star formation rate, as well as observations of the Lyα forest and the CMB. Assuming that UV photons produced in star-forming galaxies dominate the reionization process, we show that the escape fraction of ionizing photons from high-redshift galaxies is ∼5 per cent [log fesc = -1.35 ± 0.15 (68 per cent)] for our fiducial model. This relatively small value of escape fraction follows from the slow evolution in the star formation rate density at z ≳ 4 implied by the number counts of high-redshift GRBs. Our derived value is reasonably stable against uncertainties in the modelling, including the implementation of radiative feedback, the possibility of an evolving escape fraction and the unknown shape of the IMF, which in sum contribute ∼0.2 dex of additional systematic uncertainty on the value of escape fraction. © 2009 RAS.

Very-high-energy gamma-ray signal from nuclear photodisintegration as a probe of extragalactic sources of ultrahigh-energy nuclei

Abstract: It is crucial to identify the ultrahigh-energy cosmic-ray sources and probe their unknown properties. Recent results from the Pierre Auger Observatory favor a heavy nuclear composition for the ultrahigh-energy cosmic rays. Under the requirement that heavy nuclei survive in these sources, using gamma-ray bursts as an example, we predict a diagnostic gamma-ray signal, unique to nuclei---the emission of deexcitation gamma rays following photodisintegration. These gamma rays, boosted from MeV to TeV-PeV energies, may be detectable by gamma-ray telescopes such as VERITAS, HESS, and MAGIC, and especially the next-generation CTA and AGIS. They are a promising messenger to identify and study individual ultrahigh-energy nuclei accelerators.

Pre-discovery and Follow-up Observations of the Nearby SN 2009nr: Implications for Prompt Type Ia SNe

Abstract: We present photometric and spectroscopic observations of the Type Ia supernova SN 2009nr in UGC 8255 (z=0.0122). Following the discovery announcement at what turned out to be ten days after peak, we detected it at V ~15.7 mag in data collected by the All Sky Automated Survey (ASAS) North telescope 2 weeks prior to the peak, and then followed it up with telescopes ranging in aperture from 10-cm to 6.5-m. Using early photometric data available only from ASAS, we find that the SN is similar to the over-luminous Type Ia SN 1991T, with a peak at Mv=-19.6 mag, and a slow decline rate of Dm_15(B)=0.95 mag. The early post-maximum spectra closely resemble those of SN 1991T, while the late time spectra are more similar to those of normal Type Ia SNe. Interestingly, SN 2009nr has a projected distance of 13.0 kpc (~4.3 disk scale lengths) from the nucleus of the small star-forming host galaxy UGC 8255. This indicates that the progenitor of SN 2009nr is not associated with a young stellar population, calling into question the conventional association of luminous SNe Ia with the "prompt" component directly correlated with current star formation. The pre-discovery observation of SN 2009nr using ASAS demonstrates the science utility of high cadence all sky surveys conducted using small telescopes for the discovery of nearby (d=<50 Mpc) supernovae.

Primordial Black Holes as Dark Matter: Almost All or Almost Nothing

Abstract: Primordial black holes (PBHs) are expected to accrete particle dark matter around them to form ultracompact minihalos (UCMHs), if the PBHs themselves are not most of the dark matter. We show that if most dark matter is a thermal relic, then the inner regions of UCMHs around PBHs are highly luminous sources of annihilation products. Flux constraints on gamma rays and neutrinos set strong abundance limits, improving previous limits by orders of magnitude. Assuming enough particle dark matter exists to form UCMHs, we find that Omega_PBH <~ 10^-4 (for m_DM c^2 ~ 100 GeV) for a vast range in PBH mass. We briefly discuss the uncertainties on our limits, including those due to the evolution of the UCMH luminosity as it annihilates.

The SN 2008S Progenitor Star: Gone or Again Self-Obscured?

Abstract: We obtained late-time optical and near-IR imaging of SN 2008S with the Large Binocular Telescope (LBT) We find that (1) it is again invisible at optical (UBVR) wavelengths to magnitude limits of approximately 25 mag, and (2) while detected in the near-IR (HK) at approximately 20 mag, it is fading rapidly The near-IR detections in March and May 2010 are consistent with dust emission at a blackbody temperature of T ~ 900 K and a total luminosity of L ~ 40000 Lsun, comparable to the luminosity of the obscured progenitor star If it is a supernova, the near-IR emission is likely due to shock heated dust since the elapsed time from peak is too long to support a near-IR dust echo and the decline in luminosity is shallower than the 56Co slope If it is reprocessed emission from a surviving progenitor, a dust photosphere must have reestablished itself closer to the star than before the transient (~40 AU rather than 150 AU), unless there is a second, cooler dust component that dominates at mid-IR wavelengths The continued rapid fading at roughly constant temperature favors transient emission, but the SED peaks in the mid-IR and future Spitzer observations will be needed to close the case

DUAL Gamma-Ray Mission

Abstract: Gamma-ray astronomy presents an extraordinary scientific potential for the study of the most powerful sources and the most violent events in the Universe. In order to take full advantage of this potential, the next generation of instrumentation for this domain will have to achieve an improvement in sensitivity over present technologies of at least an order of magnitude. The DUAL mission concept takes up this challenge in two complementary ways: a very long observation of the entire sky, combined with a large collection area for simultaneous observations of Type Ia SNe. While the Wide-Field Compton Telescope (WCT) accumulates data from the full gamma-ray sky (0.1-10 MeV) over the entire mission lifetime, the Laue-Lens Telescope (LLT) focuses on 56Co emission from SNe Ia (0.8-0.9 MeV), collecting gamma-rays from its large area crystal lens onto the WCT. Two separated spacecraft flying in formation will maintain the DUAL payloads at the lens' focal distance.

Primordial Black Holes as Dark Matter: All or Nothing

Abstract: Primordial black holes (PBHs) are expected to accrete particle dark matter around them to form ultracompact minihalos (UCMHs), if the PBHs themselves are not most of the dark matter. We show that if most dark matter is a thermal relic, then the inner regions of UCMHs around PBHs are highly luminous sources of annihilation products. Flux constraints on gamma rays and neutrinos set strong abundance limits, improving previous limits by orders of magnitude. Assuming enough particle dark matter exists to form UCMHs, we find that Omega_PBH <~ 10^-4 (for m_DM c^2 ~ 100 GeV) for a vast range in PBH mass. We briefly discuss the uncertainties on our limits, including those due to the evolution of the UCMH luminosity as it annihilates.

DUAL Gamma-Ray Mission

Abstract: Gamma-ray astronomy presents an extraordinary scientific potential for the study of the most powerful sources and the most violent events in the Universe. In order to take full advantage of this potential, the next generation of instrumentation for this domain will have to achieve an improvement in sensitivity over present technologies of at least an order of magnitude. The DUAL mission concept takes up this challenge in two complementary ways: a very long observation of the entire sky, combined with a large collection area for simultaneous observations of Type Ia SNe. While the Wide-Field Compton Telescope (WCT) accumulates data from the full gamma-ray sky (0.1-10 MeV) over the entire mission lifetime, the Laue-Lens Telescope (LLT) focuses on 56Co emission from SNe Ia (0.8-0.9 MeV), collecting gamma-rays from its large area crystal lens onto the WCT. Two separated spacecraft flying in formation will maintain the DUAL payloads at the lens' focal distance.

Revealing Type Ia supernova physics with cosmic rates and nuclear gamma rays

Abstract: Type Ia supernovae (SNIa) remain mysterious despite their central importance in cosmology and their rapidly increasing discovery rate. The progenitors of SNIa can be probed by the delay time between progenitor birth and explosion as SNIa. The explosions and progenitors of SNIa can be probed by MeV nuclear gamma rays emitted in the decays of radioactive nickel and cobalt into iron. We compare the cosmic star formation and SNIa rates, finding that their different redshift evolution requires a large fraction of SNIa to have large delay times. A delay time distribution of the form t^{-1.0 +/- 0.3} provides a good fit, implying 50% of SNIa explode more than ~ 1 Gyr after progenitor birth. The extrapolation of the cosmic SNIa rate to z = 0 agrees with the rate we deduce from catalogs of local SNIa. We investigate prospects for gamma-ray telescopes to exploit the facts that escaping gamma rays directly reveal the power source of SNIa and uniquely provide tomography of the expanding ejecta. We find large improvements relative to earlier studies by Gehrels et al. in 1987 and Timmes & Woosley in 1997 due to larger and more certain SNIa rates and advances in gamma-ray detectors. The proposed Advanced Compton Telescope, with a narrow-line sensitivity ~ 60 times better than that of current satellites, would, on an annual basis, detect up to ~ 100 SNIa (3 sigma) and provide revolutionary model discrimination for SNIa within 20 Mpc, with gamma-ray light curves measured with ~ 10 sigma significance daily for ~ 100 days. Even more modest improvements in detector sensitivity would open a new and invaluable astronomy with frequent SNIa gamma-ray detections.