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NASA NTRS2013

First Results from NuSTAR Observations of Mkn 421

Mkn 421 is a nearby active galactic nucleus dominated at all wavelengths by a very broad nonthermal continuum thought to arise from a relativistic jet seen at a small angle to the line of sight. Its spectral energy distribution peaks in the X-ray and TeV gamma-ray bands, where the energy output is dominated by cooling of high-energy electrons in the jet. In order to study the electron distribution and its evolution, we carried out a dedicated multi-wavelength campaign, including extensive observations by the recently launched highly sensitive hard X-ray telescope NuSTAR, between December 2012 and May 2013. Here we present some intial results based on NuSTAR data from January through March 2013, as well as calibration observations conducted in 2012. Although the observations cover some of the faintest hard X-ray flux states ever observed for Mkn 421, the sensitivity is high enough to resolve intra-day spectral variability. We find that in this low state the dominant f ux variations are smooth on timescales of hours, with typical intra-hour variations of approximately less than 5%. We do not find evidence for either a cutoff in the hard X-ray spectrum, or a rise towards a high-energy component, but rather that at low flux the spectrum assumes a power law shape with a photon index of approximately 3. The spectrum is found to harden with increasing brightness.

active galactic nuclei (AGN)↗

NASA NTRS2013

A New Nearby Pulsar Wind Nebula Overlapping the RX J0852.0-4622 Supernova Remnant

Context. Energetic pulsars can be embedded in a nebula of relativistic leptons which is powered by the dissipation of the rotational energy of the pulsar. The object PSR J0855􀀀4644 is an energetic and fast-spinning pulsar (E' = 1.1 x 10(exp 36) erg/s, P=65 ms) discovered near the South-East rim of the supernova remnant (SNR) RXJ0852.0-4622 (aka Vela Jr) by the Parkes multibeam survey. The position of the pulsar is in spatial coincidence with an enhancement in X-rays and TeV gamma-rays, which could be due to its putative pulsar wind nebula (PWN). Aims. The purpose of this study is to search for di use non-thermal X-ray emission around PSR J0855-4644 to test for the presence of a PWN and to estimate the distance to the pulsar. Methods. An X-ray observation was carried out with the XMM-Newton satellite to constrain the properties of the pulsar and its nebula. The absorption column density derived in X-rays from the pulsar and from different regions of the rim of the SNR was compared with the absorption derived from the atomic (HI) and molecular ((12)CO) gas distribution along the corresponding lines of sight to estimate the distance to the pulsar and to the SNR. Results. The observation has revealed the X-ray counterpart of the pulsar together with surrounding extended emission thus confirming the existence of a PWN. The comparison of column densities provided an upper limit to the distance of the pulsar PSR J0855-4644 and the SNR RX J0852.0-4622 (d less than or equal to 900 pc). Although both objects are at compatible distances, we rule out that the pulsar and the SNR are associated. With this revised distance, PSR J0855-4644 is the second most energetic pulsar, after the Vela pulsar, within a radius of 1 kpc and could therefore contribute to the local cosmic-ray e-/e+ spectrum.

overlapping↗

NASA NTRS2010

Derivation of a Relation for the Steepening of TeV Selected Blazar Gamma-Ray Spectra with Energy and Redshift

We derive a relation for the steepening of blazar gamma-ray spectra between the multi-GeV Fermi energy range and the TeV energy range observed by atmospheric Cerenkov telescopes. The change in spectral index is produced by two effects: (1) an intrinsic steepening, independent of redshift, owing to the properties of emission and absorption in the source, and (2) a redshift-dependent steepening produced by intergalactic pair production interactions of blazar gamma-rays with low energy photons of the "intergalactic background light" (IBL). Given this relation, with good enough data on the mean gamma-ray SED of TeV Selected BL Lacs, the redshift evolution of the IBL can, in principle, be determined independently of stellar evolution models. We apply our relation to the results of new Fermi observations of TeV selected blazars.

Stecker, F.↗

NASA NTRS1991

Search for TeV emission from 4U 0115 + 63

While previous studies of the X-ray binary 4U 0115+63 suggest it to be a steady, TeV gamma-ray emitter, with pulses at the 3.6-sec neutron-star period, the present analysis of data obtained on 60 nights over a 4-yr period with an imaging gamma-ray telescope has uncovered no indication of TeV signals. The derived upper limit of 1 x 10 to the -11th/sq-cm sec, for gamma-rays of energy above 0.7 TeV, is noted to be significantly below the previously reported fluxes.

Macomb, D. J.↗

NASA NTRS1993

The high-energy diffuse cosmic gamma-ray background radiation from blazars

We predict the spectrum and flux of the high-energy diffuse cosmic gamma-ray background radiation produced by blazars. Our calculations are based on the Compton Observatory EGRET observations of high-energy gamma-rays from these objects, the Whipple observation of TeV gamma-rays from Mrk 421, and radio population studies of flat-spectrum radio sources. Our statistical analysis gives a result consistent with a linear correlation. We then use the radio luminosity function and redshift evolution to calculate the gamma-ray background. We find that blazars cannot account for the flux level or steep spectrum of the background observed by SAS 2 in the sub-GeV range, but could likely provide the dominant background component in the multi-GeV energy range and above. We predict a differential photon spectral index for this component of about 2 up to about 10 GeV. Above that energy, the spectrum should gradually steepen owing to pair production by interactions of the gamma-rays with intergalactic infrared photons. Using the calculations of this effect by Stecker et al. (1992), we estimate a spectral index of about 3.5 for energies above a TeV.

Stecker, F. W.↗

NASA NTRS2003

ASCA and Bepposax Observations of the Newly Discovered SNR RX J0852-4622

This grant supports an investigation of the supernova remnant RX J0852-4622 (G266.1-1.2), a large-diameter, nearby SNR for which the X-ray emission is dominated by nonthermal processes. As only the third such SNR discovered, for which there is direct evidence of cosmic-ray acceleration dominating the SNR dynamics, this is an exceptionally important object. Our progress on this study to date has been good. We have published the results of a spectral mapping of the XNR as well as the identification of a compact X-ray source which may be the associated neutron star. Our final work involves modeling of the emission using limits from the radio flux in order to estimate the inverse-Compton flux that might be observable in the TeV gamma-ray regime. We have just completed similar modeling of the emission from G347.3-0.5 (Lazendic et al. - submitted to ApJ), and the results have drawn considerable interest in the gamma-ray community (Slane 2002; astro-ph/0212353). Extensive observations of G266.1--1.2 with the CANGAROO TeV gamma-ray telescope have been carried out, and our modeling of the broad-band emission characteristic are of importance for such studies.

Slane, Patrick↗

NASA NTRS2022

CALET Search for Electromagnetic Counterparts of Gravitational Waves During the LIGO/Virgo O3 Run

The CALorimetric Electron Telescope (CALET)on the International Space Station consists of a high-energy cosmic-ray CALorimeter (CAL)and a lower-energy CALET Gamma-ray Burst Monitor (CGBM). CAL is sensitive to electrons up to 20 TeV, cosmic-ray nuclei from Z=1 throughZ∼40, and gamma rays over the range1 GeV–10 TeV. CGBM observes gamma rays from 7 keV to 20 MeV. The combined CAL-CGBM instrument has conducted a search for gamma-ray bursts (GRBs)since 2015 October. We report here on the results of a search for X-ray/gamma-ray counterparts to gravitational-wave events reported during the LIGO/Virgo observing run O3. No events have been detected that pass all acceptance criteria. We describe the components, performance, and triggering algorithms of the CGBM—the two Hard X-ray Monitors consisting of LaBr3(Ce)scintillators sensitive to 7 keV–1 MeV gamma rays and a Soft Gamma-ray Monitor BGO scintillator sensitive to 40 keV–20 MeV—and the high-energy CAL consisting of a charge detection module, imaging calorimeter, and the fully active total absorption calorimeter. The analysis procedure is described and upper limits to the time-averaged fluxes are presented.

Gamma-ray astronomy↗

NASA NTRS2011

Fermi Large Area Telescope Observations of the Supernova Remnant GS.7-0.1

We present a detailed analysis of the GeV gamma-ray emission toward the supernova remnant (SNR) G8.7-0.1 with the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope. An investigation of the relationship among G8.7-0.l and the TeV unidentified source HESS J1804-216 provides us with an important clue on diffusion process of cosmic rays if particle acceleration operates in the SNR. The GeV gamma-ray emission is extended with most of the emission in positional coincidence with the SNR G8.7-0.l and a lesser part located outside the western boundary of G8.7-0.l. The region of the gamma-ray emission overlaps spatially-connected molecular clouds, implying a physical connection for the gamma-ray structure. The total gamma-ray spectrum measured with LAT from 200 MeV-100 GeV can be described by a broken power-law function with a break of 2.4 +/- 0.6 (stat) +/- 1.2 (sys) GeV, and photon indices of 2.10 +/- 0.06 (stat) +/- 0.10 (sys) below the break and 2.70 +/- 0.12 (stat) +/- 0.l4 (sys) above the break. Given the spatial association among the gamma rays, the radio emission of G8.7-0.1, and the molecular clouds, the decay of 1IoS produced by particles accelerated in the SNR and hitting the molecular clouds naturally explains the GeV gamma-ray spectrum. We also find that the GeV morphology is not well represented by the TeV emission from HESS J1804-216 and that the spectrum in the GeV band is not consistent with the extrapolation of the TeV gamma-ray spectrum. The spectral index of the TeV emission is consistent with the particle spectral index predicted by a theory that assumes energy-dependent diffusion of particles accelerated in an SNR. We discuss the possibility that the TeV spectrum originates from the interaction of particles accelerated in G8.7-0.1 with molecular clouds, and we constrain the diffusion coefficient of the particles.

Ajello, M.↗

NASA NTRS2012

Fermi Large Area Telescope Observations of the Supernova Remnant GS.7-0.1

We present a detailed analysis of the GeV gamma-ray emission toward the supernova remnant (SNR) G8.7-0.1 with the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope. An investigation of the relationship among G8.7-0.1 and the TeV unidentified source HESS J1804-216 provides us with an important clue on diffusion process of cosmic rays if particle acceleration operates in the SNR. The GeV gamma-ray emission is extended with most of the emission in positional coincidence with the SNR G8.7-0.1 and a lesser part located outside the western boundary of G8.7-0.1. The region of the gamma-ray emission overlaps spatially-connected molecular clouds, implying a physical connection for the gamma-ray structure. The total gamma-ray spectrum measured with LAT from 200 MeV-100 GeV can be described by a broken power-law function with a break of 2.4 +/- 0.6 (stat) +/- 1.2 (sys) GeV, and photon indices of2.10 +/- 0.06 (stat) +/- 0.10 (sys) below the break and 2.70 +/- 0.12 (stat) +/- 0.14 (sys) above the break. Given the spatial association among the gamma rays, the radio emission ofG8.7-0.1, and the molecular clouds, the decay of pions produced by particles accelerated in the SNR and hitting the molecular clouds naturally explains the GeV gamma-ray spectrum. We also find that the GeV morphology is not well represented by the TeV emission from HESS Jl804-2l6 and that the spectrum in the Ge V band is not consistent with the extrapolation of the TeV gamma-ray spectrum. The spectral index of the TeV emission is consistent with the particle spectral index predicted by a theory that assumes energy-dependent diffusion of particles accelerated in an SNR. We discuss the possibility that the TeV-spectrum originates from the interaction of particles accelerated in G8.7-0.l with molecular clouds, and we constrain the diffusion coefficient of the particles.

Ferrara, E. C.↗

NASA NTRS2006

Gamma-ray Observations with Swift and their Impact on the TeV Community

The Swift gamma-ray burst explorer was launched on Nov. 20, 2004 from Cape Canaveral, Florida. The first instrument onboard became fully operational less than a month later. Since that time the Burst Alert Telescope (BAT) on Swift has detected more than 150 gamma-ray bursts (GRBs), most of which have also been observed within two minutes by the Swift narrow-field instruments: the X-Ray Telescope (XRT) and the Ultra-Violet and Optical Telescope (UVOT). Swift trigger notices are distributed worldwide within seconds of the trigger through the Gamma-ray burst Coordinates Network (GCN) and a substantial fraction of GRBs have been followed up by ground and space-based telescopes, ranging in wavelength from radio to TeV. Results have included the first rapid localization of a short GRB and further validation of the theory that short and long bursts have different origins; detailed observations of the power-law decay of burst afterglows leading to an improved understanding of the fireball and afterglow models; and detection of the most distant GRB ever found. Swift is also a sensitive X-ray observatory with capabilities to monitor galactic and extragalactic transients on a daily basis, carry out the first all-sky hard X-ray survey since HEAO-1, and study in detail the spectra of X-ray transients. The talk will emphasize the connection between Swift/BAT GRB observations and source monitoring and TeV observations.

Krimm, Hans↗

NASA NTRS1992

The expected high-energy to ultra-high-energy gamma-ray spectrum of the Crab Nebula

The inverse Compton scattering model for the unpulsed TeV emission from the Crab Nebula is reexamined using the magnetic field distribution derived from MHD flow models of the nebula. It is shown that the observed flux can be explained if the average nebular field is indeed about 0.0003, as is predicted by the spectral break between radio and optical. The brightness distribution of the TeV gamma-ray signal is expected to extend out to about 1.5 arcmin from the pulsar. The present estimates predict a steady flux of unpulsed ultrahigh-energy gamma-rays due to the inverse Compton scattering of soft photons by shock-accelerated electrons and/or positrons in the vicinity of the shock.

De Jager, O. C.↗

NASA NTRS1998

Radio to Gamma-Ray Emission from Shell-Type Supernova Remnants: Predictions from Non-Linear Shock Acceleration Models

Supernova remnants (SNRs) are widely believed to be the principal source of galactic cosmic rays, produced by diffusive shock acceleration in the environs of the remnant's expanding blast wave. Such energetic particles can produce gamma-rays and lower energy photons via interactions with the ambient plasma. The recently reported observation of TeV gamma-rays from SN1006 by the CANGAROO Collaboration, combined with the fact that several unidentified EGRET sources have been associated with known radio/optical/X-ray-emitting remnants, provides powerful motivation for studying gamma-ray emission from SNRs. In this paper, we present results from a Monte Carlo simulation of non-linear shock structure and acceleration coupled with photon emission in shell-like SNRs. These non-linearities are a by-product of the dynamical influence of the accelerated cosmic rays on the shocked plasma and result in distributions of cosmic rays which deviate from pure power-laws. Such deviations are crucial to acceleration efficiency considerations and impact photon intensities and spectral shapes at all energies, producing GeV/TeV intensity ratios that are quite different from test particle predictions.

Baring, Matthew G.↗

NASA NTRS2011

Fermi-LAT Detection of Gamma-Ray Emission in the Vicinity of the Star Forming Regions W43 and Westerlund 2

Particle acceleration in massive star forming regions can proceed via a large variety of possible emission scenarios, including high-energy gamma-ray production in the colliding wind zone of the massive Wolf-Rayet binary (here WR 20a and WR I2Ia), collective wind scenarios, diffusive shock acceleration at the boundaries of wind-blown bubbles in the stellar cluster, and outbreak phenomena from hot stellar winds into the interstellar medium. In view of the recent Fermi-LAT detection of HESS JI023-575 (in the vicinity of Westerlund 2), we examine another very high energy (VHE) gamma-ray source, HESS JI848-0145 (in the vicinity ofW43), possibly associated with a massive star cluster. Considering multi-wavelength data, in particular TeV gamma-rays, we examine the available evidence that the gamma-ray emission coincident with Westerlund 2 and W43 could originate in particles accelerated by the above-mentioned mechanisms in massive star clusters.

Lemoine-Goumard, M.↗

NASA NTRS2008

Implications for High Energy Blazar Spectra from Intergalactic Absorption Calculations

Given a knowledge of the density spectra intergalactic low energy photons as a function of redshift, one can derive the intrinsic gamma-ray spectra and luminosities of blazars over a range of redshifts and look for possible trends in blazar evolution. Stecker, Baring & Summerlin have found some evidence hinting that TeV blazars with harder spectra have higher intrinsic TeV gamma-ray luminosities and indicating that there may be a correlation of spectral hardness and luminosity with redshift. Further work along these lines, treating recent observations of the blazers lES02291+200 and 3C279 in the TeV and sub-TeV energy ranges, has recently been explored by Stecker & Scully. GLAST will observe and investigate many blazars in the GeV energy range and will be sensitive to blazers at higher redshifts. I examine the implications high redshift gamma-ray absorption for both theoretical and observational blazer studies.

Stecker, F↗

NASA NTRS2002

Broadband Observations and Modeling of the Shell-Type Supernova Remnant G347.3-0.5

The supernova remnant G347.3-0.5 emits a featureless power law in X-rays, thought to indicate shock acceleration of electrons to high energies. We here produce a broadband spectrum of the bright northwest limb of this source by combining radio observations from the Australia Telescope Compact Array (ATCA), X-ray observations from the Advanced Satellite for Cosmology and Astrophysics (ASCA), and TeV gamma-ray observations from the CANGAROO imaging Cerenkov telescope. We assume that this emission is produced by an electron population generated by diffusive shock acceleration at the remnant forward shock. The nonlinear aspects of the particle acceleration force a connection between the widely different wavelength bands and between the electrons and the unseen ions, presumably accelerated simultaneously with the electrons. This allows us to infer the relativistic proton spectrum and estimate ambient parameters such as the supernova explosion energy, magnetic field, matter density in the emission region, and efficiency of the shock acceleration process. We find convincing evidence that the shock acceleration is efficient, placing greater than 25% of the shock kinetic energy flux into relativistic ions. Despite this high efficiency, the maximum electron and proton energies, while depending somewhat on assumptions for the compression of the magnetic field in the shock, are well below the observed 'knee' at 10(exp 15) eV in the Galactic cosmic-ray spectrum.

Ellison, Donald C.↗

NASA NTRS1991

A search for the 2.223 MeV neutron capture gamma-ray line from the directions of Cygnus and the Galactic center

Free neutrons produced by nuclear reactions in high-temperature plasmas may undergo proton capture in ambient material, giving rise to gamma-ray emisison in a line at 2.223 MeV. Plausible settings for this process include accretion disks around black holes, material accreting onto neutron stars, and the binary companions of TeV gamma-ray sources. Data accumulated by the Solar Maximum Mission Gamma Ray Spectrometer (GRS) between 1980 and 1989 have been searched for evidence of this line, during periods when the black hole candidates Cygnus X-1 and the Galactic center were in transit across the GRS aperture. During these periods the neutron-star X-ray binaries Cygnus X-3 and Scorpius X-1 were also in the respective GRS fields of view. A 3-sigma upper limit of 0.0001/sq cm s has been placed on the steady emission in the 2.223 MeV line from the Galactic center and Scorpius X-1. Upper limits in the range 0.00012-0.00022 gamma/sq cm s have been set for Cygnus X-1 according to different models of the origin of the emission. The 3-sigma upper limit to the phase-averaged steady emission from Cygnus X-3 was found to be 0.00012 gamma/sq cm s.

Harris, Michael J.↗

NASA NTRS1997

Multiwavelength Blazar Studies

The discovery of a multitude of strongly gamma-ray emitting AGNs by EGRET and the detection of at least two AGNs at even higher energies by the Whipple Collaboration generated much interest in the study of those objects and extensive observations were conducted in the following years. The Michigan group concentrated its effort on continued observations of the two TeV gamma-ray sources, Markarian 421 and Markarian 501, to monitor variability in the high energy emission as well as to provide simultaneous measurements during multiwavelength observations of these objects.

Source record↗

NASA NTRS1998

300 GeV Observations of Unidentified EGRET Sources. A Search for TeV Counterparts to BATSE Gamma-ray Bursts

There are few things more intriguing in high energy astrophysics than the study of the highest energy particles in the universe. Where and how these particles achieve their extreme energies is of interest not only to the astrophysicist but also to the particle physicist. At GeV and TeV energies the problem is manageable since the physics is known and the acceleration processes feasible. But the energy spectrum extends to 10(exp 20)Ev and there the problem of their origin is both more difficult and interesting; in fact at these high energies we do not even know what the particles are. The study of the origin and distribution of relativistic particles in the universe has been a challenge for more than 80 years but it is only in recent years that the technology has become available to really address the question. Although something can be learned from studies of composition and energy spectrum, the origins (and thence the acceleration mechanisms) can only come from the direct study of the neutral particle component (in this respect the highest energy particles are effectively neutral since they are virtually undeflected). The feasible channels of investigation are therefore the study of the arrival directions of: (1) TeV photons (covered by the following U.S. experiments: STACEE, Whipple/VERITAS, MILAGRO and, to some extent, by EGRET/GLAST); (2) neutrinos of TeV energy and above (AMANDA/KM3); (3) the highest energy cosmic rays (HiRes, Auger). While these studies represent a form of astronomy they are the astronomy of the extraordinary universe, the universe populated by the most dynamic and physically exciting objects, the universe of the high energy astrophysicist whose cosmic laboratories represent conditions beyond anything that can be duplicated in a terrestrial laboratory. This extraordinary astronomy may say little about the normal evolution of stars and galaxies but it opens windows into cosmic particle acceleration where new and strange physical processes take place.

Weekes, Trevor C.↗