arXiv subjects

L. R. Cominsky

Publications and source records attributed to L. R. Cominsky.

9 recordsLinked to original sources

arXiv2015

The 1st Fermi Lat Supernova Remnant Catalog

To uniformly determine the properties of supernova remnants (SNRs) at high energies, we have developed the first systematic survey at energies from 1 to 100 GeV using data from the Fermi Large Area Telescope. Based on the spatial overlap of sources detected at GeV energies with SNRs known from radio surveys, we classify 30 sources as likely GeV SNRs. We also report 14 marginal associations and 245 flux upper limits. A mock catalog in which the positions of known remnants are scrambled in Galactic longitude, allows us to determine an upper limit of 22% on the number of GeV candidates falsely identified as SNRs. We have also developed a method to estimate spectral and spatial systematic errors arising from the diffuse interstellar emission model, a key component of all Galactic Fermi LAT analyses. By studying remnants uniformly in aggregate, we measure the GeV properties common to these objects and provide a crucial context for the detailed modeling of individual SNRs. Combining our GeV results with multiwavelength (MW) data, including radio, X-ray, and TeV, demonstrates the need for improvements to previously sufficient, simple models describing the GeV and radio emission from these objects. We model the GeV and MW emission from SNRs in aggregate to constrain their maximal contribution to observed Galactic cosmic rays.

astro-ph.HE↗

arXiv2013

The First Pulse of the Extremely Bright GRB 130427A: A Test Lab for Synchrotron Shocks

Gamma-ray burst (GRB) 130427A is one of the most energetic GRBs ever observed. The initial pulse up to 2.5 s is possibly the brightest well-isolated pulse observed to date. A fine time resolution spectral analysis shows power-law decays of the peak energy from the onset of the pulse, consistent with models of internal synchrotron shock pulses. However, a strongly correlated power-law behavior is observed between the luminosity and the spectral peak energy that is inconsistent with curvature effects arising in the relativistic outflow. It is difficult for any of the existing models to account for all of the observed spectral and temporal behaviors simultaneously.

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↗

arXiv2009

The Spectral Energy Distribution of Fermi bright blazars

(Abridged) We have conducted a detailed investigation of the broad-band spectral properties of the γ-ray selected blazars of the Fermi LAT Bright AGN Sample (LBAS). By combining our accurately estimated Fermi gamma-ray spectra with Swift, radio, infra-red, optical and other hard X-ray/gamma-ray data, collected within three months of the LBAS data taking period, we were able to assemble high-quality and quasi-simultaneous Spectral Energy Distributions (SED) for 48 LBAS blazars.The SED of these gamma-ray sources is similar to that of blazars discovered at other wavelengths, clearly showing, in the usual Log $ν$ - Log $ν$ F$_ν$ representation, the typical broad-band spectral signatures normally attributed to a combination of low-energy synchrotron radiation followed by inverse Compton emission of one or more components. We have used these SEDs to characterize the peak intensity of both the low and the high-energy components. The results have been used to derive empirical relationships that estimate the position of the two peaks from the broad-band colors (i.e. the radio to optical and optical to X-ray spectral slopes) and from the gamma-ray spectral index. Our data show that the synchrotron peak frequency $ν_p^S$ is positioned between 10$^{12.5}$ and 10$^{14.5}$ Hz in broad-lined FSRQs and between $10^{13}$ and $10^{17}$ Hz in featureless BL Lacertae objects.We find that the gamma-ray spectral slope is strongly correlated with the synchrotron peak energy and with the X-ray spectral index, as expected at first order in synchrotron - inverse Compton scenarios. However, simple homogeneous, one-zone, Synchrotron Self Compton (SSC) models cannot explain most of our SEDs, especially in the case of FSRQs and low energy peaked (LBL) BL Lacs. (...)

astro-ph.CO↗

arXiv2006

The shock break-out of GRB 060218/SN 2006aj

Although the link between long Gamma Ray Bursts (GRBs) and supernovae (SNe) has been established, hitherto there have been no observations of the beginning of a supernova explosion and its intimate link to a GRB. In particular, we do not know however how a GRB jet emerges from the star surface nor how a GRB progenitor explodes. Here we report on observations of the close GRB060218 and its connection to SN2006aj. In addition to the classical non-thermal emission, GRB060218 shows a thermal component in its X-ray spectrum, which cools and shifts into the optical/UV band as time passes. We interpret these features as arising from the break out of a shock driven by a mildly relativistic shell into the dense wind surrounding the progenitor. Our observations allow us for the first time to catch a SN in the act of exploding, to directly observe the shock break-out and to provide strong evidence that the GRB progenitor was a Wolf-Rayet star.

astro-ph↗

arXiv2005

Swift and XMM observations of the dark GRB 050326

We present Swift and XMM observations of GRB 050326, detected by Swift-BAT. The fluence was 7.7x10^-6 erg cm^-2 (20-150 keV), and its spectrum was hard, with a power law photon index 1.25. The afterglow light curve did not show any break nor flares between ~1 hr and ~6 d after the burst, and decayed with a slope 1.70. The afterglow spectrum is well fitted by a power-law model, suffering absorption both in the Milky Way and in the host galaxy. The rest-frame Hydrogen column density is significant, N_H_z > 4x10^21 cm^-2, and the redshift of the absorber is z > 1.5. There was good agreement between the Swift-XRT and XMM results. By comparing the prompt and afterglow fluxes, we found that an early break occurred before the XRT observation. The properties of the GRB 050326 afterglow are well described by a spherical fireball expanding in a uniform external medium, so a further steepening is expected at later times. The lack of such a break constrains the jet angle to be >7 deg. Using the redshift constraints provided by the X-ray analysis, we also estimated that the beaming-corrected gamma-ray energy was >3x10^51 erg, at the high end of GRB energies. Despite the brightness in X rays, only deep limits could be placed by Swift-UVOT at optical/UV wavelengths. Thus, this GRB was "truly dark", with the optical-to-X-ray spectrum violating the synchrotron limit. The optical and X-ray observations are consistent either with an absorbed event or with a high-redshift one. To obey the Ghirlanda relation, a moderate/large redshift z>4.5 is required. (abridged)

astro-ph↗

arXiv2004

The Swift Gamma-Ray Burst Mission

The Swift mission, scheduled for launch in early 2004, is a multiwavelength observatory for gamma-ray burst (GRB) astronomy. It is the first-of-its-kind autonomous rapid-slewing satellite for transient astronomy and pioneers the way for future rapid-reaction and multiwavelength missions. It will be far more powerful than any previous GRB mission, observing more than 100 bursts per year and performing detailed X-ray and UV/optical afterglow observations spanning timescales from 1 minute to several days after the burst. The objectives are to determine the origin of GRBs; classify GRBs and search for new types; study the interaction of the ultra-relativistic outflows of GRBs with their surrounding medium; and use GRBs to study the early universe out to z>10. The mission is being developed by a NASA-led international collaboration. It will carry three instruments: a new-generation wide-field gamma-ray (15-150 keV) detector; a narrow-field X-ray telescope; and a narrow-field UV/optical telescope. Redshift determinations will be made for most bursts. In addition to the primary GRB science, the mission will perform a hard X-ray survey to a sensitivity of ~1 mCrab (~2x10^{-11} erg cm^{-2} s^{-1} in the 15-150 keV band), more than an order of magnitude better than HEAO A-4. A flexible data and operations system will allow rapid follow-up observations of all types of high-energy transients, with rapid data downlink and uplink available through the NASA TDRSS system. The mission is currently funded for 2 years of operations and the spacecraft will have a lifetime to orbital decay of ~8 years. [ABRIDGED]

astro-ph↗

arXiv1999

X-Ray/Gamma-Ray Observations of the PSR B1259-63/SS 2883 system near Apastron

We report the results from X-ray and hard X-ray observations of the PSR B1259-63/SS 2883 system with the ASCA and CGRO satellites, performed between 1995 February and 1996 January when the pulsar was near apastron. The system was clearly detected in each of the two ASCA observations with luminosity in the 1-10 keV band of Lx = (9 +- 3) x10^32 (d/2 kpc)^2 erg/s, while CGRO/OSSE did not detect significant hard X-rays from the system. X-ray spectra obtained with ASCA are well-fit by a single power-law spectrum with a photon index of 1.6 +- 0.3. No pulsations were detected in either the ASCA or the OSSE data. We combine all existing X-ray and hard X-ray observations and present the orbital modulation in the luminosity and photon index for the entire orbit. The results are in agreement with predictions based on a synchrotron emission model from relativistic particles in a shocked pulsar wind interacting with the gaseous outflow from the Be star.

astro-ph↗

arXiv1997

Complex outburst behaviour from the black-hole candidate 4U 1630-47

We present data from different epochs in 1978, 1987-1991 and 1996 from the black-hole candidate 4U1630-47. For the first time we present almost complete coverage of the outbursts which started in 1987, 1988 and 1996. We find that the outburst behaviour of 4U1630-47 is more complex than previously realized. The source shows outbursts with durations on the order of ~100-200 days and sometimes intervals of long-term X-ray activity. The relatively short outbursts which occurred in 1987 and 1996 exhibited different colour behaviour: the outburst in 1987 showed softening of the X-ray spectrum, whereas the outburst in 1996 showed hardening of the X-ray spectrum, as the outbursts proceeded. The outburst which started in 1977 may have lasted for up to ~10 months, whereas the outburst which started in 1988 showed activity for up to ~2.4 years. Such long-term activity is reminiscent of that seen in GRS1716-249 and in the galactic superluminal sources GRS1915+105 and GROJ1655-40. We refine the outburst ephemeris of 4U\1630-47 and find that the outburst recurrence time scale may have changed from ~600 days to ~690 days between the 1984 and 1987 outbursts. If the recurrence interval of ~690 days continues, the next outburst of 4U1630-47 is predicted to occur in 1998 January.

astro-ph↗