arXiv subjects

T. J. Brandt

Publications and source records attributed to T. J. Brandt.

35 records · Page 2Linked to original sources

arXiv2013

The 1st Fermi LAT SNR Catalog: Probing the Gamma-ray Population

While supernova remnants (SNRs) are widely thought to be powerful accelerators, evidence comes largely from a small number of well-studied cases. Here we systematically determine the gamma-ray emission from all known Galactic SNRs, disentangling them from the sea of Galactic cosmic rays. Using Fermi LAT data we have characterized the GeV emission in all regions containing SNRs, accounting for systematic uncertainties caused by source confusion, diffuse emission modeling, and instrumental response. More than a dozen remnants are identified through spatial extension or detection at TeV energies, with potential associations for >40 more. From this population study, two clear classes of gamma-ray-emitting SNRs emerge: young remnants and those interacting with a dense medium. This large statistical sample also reveals a possible correlation between GeV and radio flux. The growing number of identified SNRs will help to disentangle the effects of age and environment on the aggregate properties of SNRs at high energies.

astro-ph.HE↗

arXiv2013

The 1st Fermi LAT SNR Catalog: Constraining the Cosmic Ray Contribution

Despite tantalizing evidence that supernova remnants (SNRs) are the source of Galactic cosmic rays (CRs), including the recent detection of a spectral signature of hadronic gamma-ray emission from two SNRs, their origin in aggregate remains elusive. We address the long-standing question of Galactic CR nuclei origins using our statistically significant GeV SNR sample to estimate the contribution of SNRs to directly observed CRs. Interactions between CRs and ambient gas near the SNRs emit photons via pion decay at GeV energies, providing an in situ tracer for CRs otherwise measured directly with balloon-borne and satellite experiments near the Earth. To date, the Fermi LAT SNR Catalog has detected more than 50 SNRs and potential associations in classes with a variety of properties, yet all remain possible accelerators. We investigate the GeV and multiwavelength (MW) emission from SNRs to constrain their maximal contribution to observed Galactic CRs. Our work demonstrates the need for improvements to previously sufficient simple models describing the GeV and MW emission from these objects.

astro-ph.HE↗

arXiv2013

The 1st Fermi LAT SNR Catalog: the Impact of Interstellar Emission Modeling

Galactic interstellar emission contributes substantially to Fermi LAT observations in the Galactic plane, the location of the majority of supernova remnants (SNRs). To explore some systematic effects on SNRs' properties caused by interstellar emission modeling, we have developed a method comparing the official LAT interstellar emission model results to eight alternative models. We created the eight alternative Galactic interstellar models by varying a few input parameters to GALPROP, namely the height of the cosmic ray propagation halo, cosmic ray source distribution in the Galaxy, and atomic hydrogen spin temperature. We have analyzed eight representative SNRs chosen to encompass a range of Galactic locations, extensions, and spectral properties using the eight different interstellar emission models. We will present the results and method in detail and discuss the implications for studies such as the 1st Fermi LAT SNR Catalog.

astro-ph.HE↗

arXiv2013

Constraints on the Galactic Population of TEV Pulsar Wind Nebulae Using Fermi Large Area Telescope Observations

Pulsar wind nebulae (PWNe) have been established as the most populous class of TeV gamma-ray emitters. Since launch, the Fermi Large Area Telescope (LAT)identified five high-energy (100MeV <E< 100 GeV) gamma-ray sources as PWNe, and detected a large number of PWNe candidates, all powered by young and energetic pulsars. The wealth of multi-wavelength data available and the new results provided by Fermi-LAT give us an opportunity to find new PWNe and to explore the radiative processes taking place in known ones. The TeV gamma-ray unidentifiedsources (UNIDs) are the best candidates for finding new PWNe. Using 45 months of Fermi-LAT data for energies above 10 GeV, an analysis was performed near the position of 58TeV PWNe and UNIDs within 5deg of the Galactic Plane to establish new constraints on PWNe properties and find new clues on the nature of UNIDs. Of the 58 sources, 30 were detected, and this work provides their gamma-rayfluxes for energies above 10 GeV. The spectral energy distributions (SED) andupper limits, in the multi-wavelength context, also provide new information on the source nature and can help distinguish between emission scenarios, i.e. between classification as a pulsar candidate or as a PWN candidate. Six new GeV PWNe candidates are described in detail and compared with existing models. A population study of GeV PWNe candidates as a function of the pulsar/PWN system characteristics is presented.

astro-ph.HE↗

arXiv2013

A View of Supernova Remnant CTB 37A with the Fermi Gamma-ray Space Telescope

Supernovae and their remnants have long been favored as cosmic ray ac- celerators. Recent data from the Fermi Gamma-ray Space Telescope has given us an improved window into such sources, including the remnant CTB 37A. Using the Fermi Large Area Telescope, we found significant gamma-ray emission coincident with the remnant, which also emits in radio, X-ray, and very high energy gamma-rays. We modeled the multiwavelength spectrum using a combination of hadronic and leptonic emission with reasonable parameter values and determined that CTB 37A is a potential cosmic ray accelerator commensurate with direct observations. By assembling statistically significant populations of such objects, we will be able to more fully illuminate the mystery of cosmic ray origins.

astro-ph.HE↗

arXiv2013

A Method for Exploring Systematics Due to Galactic Interstellar Emission Modeling: Application to the Fermi LAT SNR Catalog

Galactic interstellar emission contributes substantially to Fermi LAT observations in the Galactic plane, where the majority of Supernova Remnants (SNRs) are located. We have developed a method to explore some systematic effects on SNRs' properties caused by interstellar emission modeling. We created eight alternative Galactic interstellar models by varying a few input parameters to GALPROP, namely the height of the cosmic ray propagation halo, cosmic ray source distribution in the Galaxy, and atomic hydrogen spin temperature. We have analyzed eight representative candidate SNRs chosen to encompass a range of Galactic locations, extensions, and spectral properties using the eight different interstellar emission models. The models were fitted to the LAT data with free independent normalization coefficients for the various components of the model along the line of sight in each region of interest. We will discuss the results and compare them with those obtained with the official LAT interstellar emission model.

astro-ph.HE↗

arXiv2013

Detection of the Characteristic Pion-Decay Signature in Supernova Remnants

Cosmic rays are particles (mostly protons) accelerated to relativistic speeds. Despite wide agreement that supernova remnants (SNRs) are the sources of galactic cosmic rays, unequivocal evidence for the acceleration of protons in these objects is still lacking. When accelerated protons encounter interstellar material, they produce neutral pions, which in turn decay into gamma rays. This offers a compelling way to detect the acceleration sites of protons. The identification of pion-decay gamma rays has been difficult because high-energy electrons also produce gamma rays via bremsstrahlung and inverse Compton scattering. We detected the characteristic pion-decay feature in the gamma-ray spectra of two SNRs, IC 443 and W44, with the Fermi Large Area Telescope. This detection provides direct evidence that cosmic-ray protons are accelerated in SNRs.

astro-ph.HE↗

arXiv2013

Constraints on the Galactic Halo Dark Matter from Fermi-LAT Diffuse Measurements

We have performed an analysis of the diffuse gamma-ray emission with the Fermi Large Area Telescope in the Milky Way Halo region searching for a signal from dark matter annihilation or decay. In the absence of a robust dark matter signal, constraints are presented. We consider both gamma rays produced directly in the dark matter annihilation/decay and produced by inverse Compton scattering of the e+e- produced in the annihilation/decay. Conservative limits are derived requiring that the dark matter signal does not exceed the observed diffuse gamma-ray emission. A second set of more stringent limits is derived based on modeling the foreground astrophysical diffuse emission using the GALPROP code. Uncertainties in the height of the diffusive cosmic-ray halo, the distribution of the cosmic-ray sources in the Galaxy, the index of the injection cosmic-ray electron spectrum and the column density of the interstellar gas are taken into account using a profile likelihood formalism, while the parameters governing the cosmic-ray propagation have been derived from fits to local cosmic-ray data. The resulting limits impact the range of particle masses over which dark matter thermal production in the early Universe is possible, and challenge the interpretation of the PAMELA/Fermi-LAT cosmic ray anomalies as annihilation of dark matter.

astro-ph.CO↗

arXiv2012

Binary Millisecond Pulsar Discovery via Gamma-Ray Pulsations

Millisecond pulsars, old neutron stars spun-up by accreting matter from a companion star, can reach high rotation rates of hundreds of revolutions per second. Until now, all such "recycled" rotation-powered pulsars have been detected by their spin-modulated radio emission. In a computing-intensive blind search of gamma-ray data from the Fermi Large Area Telescope (with partial constraints from optical data), we detected a 2.5-millisecond pulsar, PSR J1311-3430. This unambiguously explains a formerly unidentified gamma-ray source that had been a decade-long enigma, confirming previous conjectures. The pulsar is in a circular orbit with an orbital period of only 93 minutes, the shortest of any spin-powered pulsar binary ever found.

astro-ph.HE↗

arXiv2012

Gamma-ray observations of the Orion Molecular Clouds with the Fermi Large Area Telescope

We report on the gamma-ray observations of giant molecular clouds Orion A and B with the Large Area Telescope (LAT) on-board the Fermi Gamma-ray Space Telescope. The gamma-ray emission in the energy band between \sim100 MeV and \sim100 GeV is predicted to trace the gas mass distribution in the clouds through nuclear interactions between the Galactic cosmic rays (CRs) and interstellar gas. The gamma-ray production cross-section for the nuclear interaction is known to \sim10% precision which makes the LAT a powerful tool to measure the gas mass column density distribution of molecular clouds for a known CR intensity. We present here such distributions for Orion A and B, and correlate them with those of the velocity integrated CO intensity (WCO) at a 1° \times1° pixel level. The correlation is found to be linear over a WCO range of ~10 fold when divided in 3 regions, suggesting penetration of nuclear CRs to most of the cloud volumes. The Wco-to-mass conversion factor, Xco, is found to be \sim2.3\times10^20 cm-2(K km s-1)-1 for the high-longitude part of Orion A (l > 212°), \sim1.7 times higher than \sim1.3 \times 10^20 found for the rest of Orion A and B. We interpret the apparent high Xco in the high-longitude region of Orion A in the light of recent works proposing a non-linear relation between H2 and CO densities in the diffuse molecular gas. Wco decreases faster than the H2 column density in the region making the gas "darker" to Wco.

astro-ph.HE↗

arXiv2012

Anisotropies in the diffuse gamma-ray background measured by the Fermi LAT

The contribution of unresolved sources to the diffuse gamma-ray background could induce anisotropies in this emission on small angular scales. We analyze the angular power spectrum of the diffuse emission measured by the Fermi LAT at Galactic latitudes |b| > 30 deg in four energy bins spanning 1 to 50 GeV. At multipoles \ell \ge 155, corresponding to angular scales \lesssim 2 deg, angular power above the photon noise level is detected at >99.99% CL in the 1-2 GeV, 2-5 GeV, and 5-10 GeV energy bins, and at >99% CL at 10-50 GeV. Within each energy bin the measured angular power takes approximately the same value at all multipoles \ell \ge 155, suggesting that it originates from the contribution of one or more unclustered source populations. The amplitude of the angular power normalized to the mean intensity in each energy bin is consistent with a constant value at all energies, C_P/ ^2 = 9.05 +/- 0.84 x 10^{-6} sr, while the energy dependence of C_P is consistent with the anisotropy arising from one or more source populations with power-law photon spectra with spectral index Γ_s = 2.40 +/- 0.07. We discuss the implications of the measured angular power for gamma-ray source populations that may provide a contribution to the diffuse gamma-ray background.

astro-ph.HE↗

arXiv2012

Search for Dark Matter Satellites using the FERMI-LAT

Numerical simulations based on the Lambda-CDM model of cosmology predict a large number of as yet unobserved Galactic dark matter satellites. We report the results of a Large Area Telescope (LAT) search for these satellites via the gamma-ray emission expected from the annihilation of weakly interacting massive particle (WIMP) dark matter. Some dark matter satellites are expected to have hard gamma-ray spectra, finite angular extents, and a lack of counterparts at other wavelengths. We sought to identify LAT sources with these characteristics, focusing on gamma-ray spectra consistent with WIMP annihilation through the $b \bar b$ channel. We found no viable dark matter satellite candidates using one year of data, and we present a framework for interpreting this result in the context of numerical simulations to constrain the velocity-averaged annihilation cross section for a conventional 100 GeV WIMP annihilating through the $b \bar b$ channel.

astro-ph.HE↗

arXiv2010

Energy cross-calibration from the first CREAM flight: transition radiation detector versus calorimeter

The Cosmic Ray Energetics And Mass (CREAM) balloon experiment had two successful flights in 2004/05 and 2005/06. It was designed to perform energy measurements from a few GeV up to 1000 TeV, taking advantage of different detection techniques. The first instrument, CREAM-1, combined a transition radiation detector with a calorimeter to provide independent energy measurements of cosmicraynuclei. Each detector was calibrated with particle beams in a limited range of energies. In order to assess the absolute energy scale of the instrument and to investigate the systematic effects of each technique, a cross-calibration was performed by comparing the two independent energy estimates on selected samples of oxygen and carbon nuclei.

astro-ph.IM↗

arXiv2010

Elemental energy spectra of cosmic rays measured by CREAM-II

We present new measurements of the energy spectra of cosmic-ray (CR) nuclei from the second flight of the balloon-borne experiment CREAM (Cosmic Ray Energetics And Mass). The instrument (CREAM-II) was comprised of detectors based on different techniques (Cherenkov light, specific ionization in scintillators and silicon sensors) to provide a redundant charge identification and a thin ionization calorimeter capable of measuring the energy of cosmic rays up to several hundreds of TeV. The data analysis is described and the individual energy spectra of C, O, Ne, Mg, Si and Fe are reported up to ~ 10^14 eV. The spectral shape looks nearly the same for all the primary elements and can be expressed as a power law in energy E^{-2.66+/-0.04}. The nitrogen absolute intensity in the energy range 100-800 GeV/n is also measured.

astro-ph.HE↗

arXiv2010

Measurements of cosmic-ray energy spectra with the 2nd CREAM flight

During its second Antarctic flight, the CREAM (Cosmic Ray Energetics And Mass) balloon experiment collected data for 28 days, measuring the charge and the energy of cosmic rays (CR) with a redundant system of particle identification and an imaging thin ionization calorimeter. Preliminary direct measurements of the absolute intensities of individual CR nuclei are reported in the elemental range from carbon to iron at very high energy.

astro-ph.HE↗

arXiv2009

Energy spectra of cosmic-ray nuclei at high energies

We present new measurements of the energy spectra of cosmic-ray (CR) nuclei from the second flight of the balloon-borne experiment Cosmic Ray Energetics And Mass (CREAM). The instrument included different particle detectors to provide redundant charge identification and measure the energy of CRs up to several hundred TeV. The measured individual energy spectra of C, O, Ne, Mg, Si, and Fe are presented up to $\sim 10^{14}$ eV. The spectral shape looks nearly the same for these primary elements and it can be fitted to an $E^{-2.66 \pm 0.04}$ power law in energy. Moreover, a new measurement of the absolute intensity of nitrogen in the 100-800 GeV/$n$ energy range with smaller errors than previous observations, clearly indicates a hardening of the spectrum at high energy. The relative abundance of N/O at the top of the atmosphere is measured to be $0.080 \pm 0.025 $(stat.)$ \pm 0.025 $(sys.) at $\sim $800 GeV/$n$, in good agreement with a recent result from the first CREAM flight.

astro-ph.HE↗

arXiv2008

Measurements of cosmic-ray secondary nuclei at high energies with the first flight of the CREAM balloon-borne experiment

We present new measurements of heavy cosmic-ray nuclei at high energies per- formed during the first flight of the balloon-borne cosmic-ray experiment CREAM (Cosmic-Ray Energetics And Mass). This instrument uses multiple charge detectors and a transition radiation detector to provide the first high accuracy measurements of the relative abundances of elements from boron to oxygen up to energies around 1 TeV/n. The data agree with previous measurements at lower energies and show a relatively steep decline (~E$^-0.6$ to E$^-0.5$) at high energies. They further show the source abundance of nitrogen relative to oxygen is ~10% in the TeV/n region.

astro-ph↗