280 research outputs found
A Survey for Transient Astronomical Radio Emission at 611 MHz
We have constructed and operated the Survey for Transient Astronomical Radio
Emission (STARE) to detect transient astronomical radio emission at 611 MHz
originating from the sky over the northeastern United States. The system is
sensitive to transient events on timescales of 0.125 s to a few minutes, with a
typical zenith flux density detection threshold of approximately 27 kJy. During
18 months of around-the-clock observing with three geographically separated
instruments, we detected a total of 4,318,486 radio bursts. 99.9% of these
events were rejected as locally generated interference, determined by requiring
the simultaneous observation of an event at all three sites for it to be
identified as having an astronomical origin. The remaining 3,898 events have
been found to be associated with 99 solar radio bursts. These results
demonstrate the remarkably effective RFI rejection achieved by a coincidence
technique using precision timing (such as GPS clocks) at geographically
separated sites. The non-detection of extra-solar bursting or flaring radio
sources has improved the flux density sensitivity and timescale sensitivity
limits set by several similar experiments in the 1970s. We discuss the
consequences of these limits for the immediate solar neighborhood and the
discovery of previously unknown classes of sources. We also discuss other
possible uses for the large collection of 611 MHz monitoring data assembled by
STARE.Comment: 24 pages, 6 figures; to appear in PAS
High-Energy Gamma-Ray Observations of Two Young, Energetic Radio Pulsars
We present results of Compton Gamma-Ray Observatory EGRET observations of the
unidentified high-energy gamma-ray sources 2EG J1049-5847 (GEV J1047-5840, 3EG
J1048-5840) and 2EG J1103-6106 (3EG J1102-6103). These sources are spatially
coincident with the young, energetic radio pulsars PSRs B1046-58 and
J1105-6107, respectively. We find evidence for an association between PSR
B1046-58 and 2EG J1049-5847. The gamma-ray pulse profile, obtained by folding
time-tagged photons having energies above 400 MeV using contemporaneous radio
ephemerides, has probability of arising by chance of 1.2E-4 according to the
binning-independent H-test. A spatial analysis of the on-pulse photons reveals
a point source of equivalent significance 10.2 sigma. Off-pulse, the
significance drops to 5.8 sigma. Archival ASCA data show that the only hard
X-ray point source in the 95% confidence error box of the gamma-ray source is
spatially coincident with the pulsar within the 1' uncertainty (Pivovaroff,
Kaspi & Gotthelf 1999). The double peaked gamma-ray pulse morphology and
leading radio pulse are similar to those seen for other gamma-ray pulsars and
are well-explained in models in which the gamma-ray emission is produced in
charge-depleted gaps in the outer magnetosphere. The inferred pulsed gamma-ray
flux above 400 MeV, (2.5 +/- 0.6) x 10E-10 erg/cm^2/s, represents 0.011 +/-
0.003 of the pulsar's spin-down luminosity, for a distance of 3 kpc and 1 sr
beaming. For PSR J1105-6107, light curves obtained by folding EGRET photons
using contemporaneous radio ephemerides show no significant features. We
conclude that this pulsar converts less than 0.014 of its spin-down luminosity
into E > 100 MeV gamma-rays beaming in our direction (99% confidence), assuming
a distance of 7 kpc, 1 sr beaming and a duty cycle of 0.5.Comment: Accepted for publication in the Astrophysical Journa
A High-Energy Study of the Geminga Pulsar
We present the results of deep X-ray and gamma-ray observations of the
Geminga pulsar obtained in the final years of the ASCA and CGRO missions, and
an upper limit from RXTE. A phase-connected ephemeris from the gamma-rays is
derived that spans the years 1973-2000, after allowing for a minor glitch in
frequency of Delta f/f = 6.2 x 10^-10 in late 1996. ASCA observations of the
hard X-ray pulse profile in 1994 and 1999 confirm this glitch. An improved
characterization of the hard X-ray pulse profile and spectrum from the long
ASCA observation of 1999 confirms that there is a non-thermal X-ray component
that is distinct from the gamma-ray spectrum as measured by EGRET. It can be
parameterized as a power-law of photon index Gamma = 1.72 +/- 0.10 with a flux
of 2.62 x 10^-13 ergs/cm^2/s in the 0.7-5 keV band and pulsed fraction 0.54 +/-
0.05, similar to, but more precise than values measured previously. An
extrapolation of this spectrum into the energy band observed by the RXTE PCA is
consistent with the non-detection of pulsed emission from Geminga with that
instrument. These results are interpreted in the context of outer-gap models,
and motivations for future X-ray observations of Geminga are given.Comment: 22 pages including 7 figure
A luminosity constraint on the origin of unidentified high energy sources
The identification of point sources poses a great challenge for the high
energy community. We present a new approach to evaluate the likelihood of a set
of sources being a Galactic population based on the simple assumption that
galaxies similar to the Milky Way host comparable populations of gamma-ray
emitters. We propose a luminosity constraint on Galactic source populations
which complements existing approaches by constraining the abundance and spatial
distribution of any objects of Galactic origin, rather than focusing on the
properties of a specific candidate emitter. We use M31 as a proxy for the Milky
Way, and demonstrate this technique by applying it to the unidentified EGRET
sources. We find that it is highly improbable that the majority of the
unidentified EGRET sources are members of a Galactic halo population (e.g.,
dark matter subhalos), but that current observations do not provide any
constraints on all of these sources being Galactic objects if they reside
entirely in the disk and bulge. Applying this method to upcoming observations
by the Fermi Gamma-ray Space Telescope has the potential to exclude association
of an even larger number of unidentified sources with any Galactic source
class.Comment: 18 pages, 4 figures, to appear in JPhys
Photon Splitting and Pair Creation in Highly Magnetized Pulsars
The absence of radio pulsars with long periods has lead to the popular notion
of a high P ``death line.'' In the standard picture, beyond this boundary,
pulsars with low spin rates cannot accelerate particles above the stellar
surface to high enough energies to initiate pair cascades, and the pair
creation needed for radio emission is strongly suppressed. In this paper we
explore the possibility of another pulsar ``death line'' in the context of
polar cap models, corresponding to high magnetic fields B in the upper portion
of the period-period derivative diagram, a domain where few radio pulsars are
observed. The origin of this high B boundary, which may occur when B becomes
comparable to or exceeds Gauss, is also due
to the suppression of magnetic pair creation, but primarily because of
ineffective competition with magnetic photon splitting. Threshold pair creation
also plays a prominent role in the suppression of cascades. We present Monte
Carlo calculations of the pair yields in photon splitting/pair cascades which
show that, in the absence of scattering effects, pair production is effectively
suppressed, but only if all three modes of photon splitting allowed by QED are
operating in high fields. This paper describes the probable shape and position
of the new ``death line,'' above which pulsars are expected to be radio quiet,
but perhaps still X-ray and gamma-ray bright. The hypothesized existence of
radio-quiet sources finds dramatic support in the recent discovery of
ultra-strong fields in Soft Gamma-ray Repeaters and Anomalous X-ray Pulsars.
Guidelines for moderate to high B pulsar searches at radio wavelengths and also
in the soft and hard gamma-ray bands are presented.Comment: 19 pages, including 1 table and 9 figures, AASTeX apjgalley format,
To appear in The Astrophysical Journal, Vol 547, February 1, 2001 issu
Multifrequency monitoring of the blazar 0716+714 during the GASP-WEBT-AGILE campaign of 2007
Since the CGRO operation in 1991-2000, one of the primary unresolved
questions about the blazar gamma-ray emission has been its possible correlation
with the low-energy (in particular optical) emission. To help answer this
problem, the Whole Earth Blazar Telescope (WEBT) consortium has organized the
GLAST-AGILE Support Program (GASP) to provide the optical-to-radio monitoring
data to be compared with the gamma-ray detections by the AGILE and GLAST
satellites. This new WEBT project started in early September 2007, just before
a strong gamma-ray detection of 0716+714 by AGILE. We present the GASP-WEBT
optical and radio light curves of this blazar obtained in July-November 2007,
about various AGILE pointings at the source. We construct NIR-to-UV spectral
energy distributions (SEDs), by assembling GASP-WEBT data together with UV data
from the Swift ToO observations of late October. We observe a contemporaneous
optical-radio outburst, which is a rare and interesting phenomenon in blazars.
The shape of the SEDs during the outburst appears peculiarly wavy because of an
optical excess and a UV drop-and-rise. The optical light curve is well sampled
during the AGILE pointings, showing prominent and sharp flares. A future
cross-correlation analysis of the optical and AGILE data will shed light on the
expected relationship between these flares and the gamma-ray events.Comment: 5 pages, 5 figures, to be published in A&A (Letters); revised to
match the final version (changes in Fig. 5 and related text
AGILE detection of GeV gamma-ray emission from the SNR W28
Supernova remnants (SNRs) are believed to be the main sources of Galactic
cosmic rays. Molecular clouds associated with SNRs can produce gamma-ray
emission through the interaction of accelerated particles with the concentrated
gas. The middle aged SNR W28, for its associated system of dense molecular
clouds, provides an excellent opportunity to test this hypothesis. We present
the AGILE/GRID observations of SNR W28, and compare them with observations at
other wavelengths (TeV and 12CO J=1-->0 molecular line emission). The gamma-ray
flux detected by AGILE from the dominant source associated with W28 is (14 +-
5) 10^-8 ph cm^-2 s^-1 for E > 400 MeV. This source is positionally well
correlated with the TeV emission observed by the HESS telescope. The local
variations of the GeV to TeV flux ratio suggest a difference between the CR
spectra of the north-west and south molecular cloud complexes. A model based on
a hadronic-induced interaction and diffusion with two molecular clouds at
different distances from the W28 shell can explain both the morphological and
spectral features observed by AGILE in the MeV-GeV energy range and by the HESS
telescope in the TeV energy range. The combined set of AGILE and H.E.S.S. data
strongly support a hadronic model for the gamma-ray production in W28.Comment: Accepted for publication in Astronomy & Astrophysics Letter
The extraordinary gamma-ray flare of the blazar 3C 454.3
We present the gamma-ray data of the extraordinary flaring activity above 100
MeV from the flat spectrum radio quasar 3C 454.3 detected by AGILE during the
month of December 2009. 3C 454.3, that has been among the most active blazars
of the FSRQ type since 2007, was detected in the gamma-ray range with a
progressively rising flux since November 10, 2009. The gamma-ray flux reached a
value comparable with that of the Vela pulsar on December 2, 2009. Remarkably,
between December 2 and 3, 2009 the source more than doubled its gamma-ray
emission and became the brightest gamma-ray source in the sky with a peak flux
of F_{\gamma,p} = (2000 \pm 400) x 10^-8 ph cm^-2 s^-1 for a 1-day integration
above 100 MeV. The gamma-ray intensity decreased in the following days with the
source flux remaining at large values near F \simeq (1000 \pm 200) x 10^-8 ph
cm^-2 s^-1 for more than a week. This exceptional gamma-ray flare dissipated
among the largest ever detected intrinsic radiated power in gamma-rays above
100 MeV (L_{\gamma, source, peak} \simeq 3 x 10^46 erg s^-1, for a relativistic
Doppler factor of {\delta} \simeq 30). The total isotropic irradiated energy of
the month-long episode in the range 100 MeV - 3 GeV is E_{\gamma,iso} \simeq
10^56 erg. We report the intensity and spectral evolution of the gamma-ray
emission across the flaring episode. We briefly discuss the important
theoretical implications of our detection.Comment: 17 pages, 3 figures, ApJ accepte
First AGILE Catalog of High Confidence Gamma-Ray Sources
We present the first catalog of high-confidence gamma-ray sources detected by
the AGILE satellite during observations performed from July 9, 2007 to June 30,
2008. Catalogued sources are detected by merging all the available data over
the entire time period. AGILE, launched in April 2007, is an ASI mission
devoted to gamma-ray observations in the 30 MeV - 50 GeV energy range, with
simultaneous X-ray imaging capability in the 18-60 keV band. This catalog is
based on Gamma-Ray Imaging Detector (GRID) data for energies greater than 100
MeV. For the first AGILE catalog we adopted a conservative analysis, with a
high-quality event filter optimized to select gamma-ray events within the
central zone of the instrument Field of View (radius of 40 degrees). This is a
significance-limited (4 sigma) catalog, and it is not a complete flux-limited
sample due to the non-uniform first year AGILE sky coverage. The catalog
includes 47 sources, 21 of which are associated with confirmed or candidate
pulsars, 13 with Blazars (7 FSRQ, 4 BL Lacs, 2 unknown type), 2 with HMXRBs, 2
with SNRs, 1 with a colliding-wind binary system, 8 with unidentified sources.Comment: Revised version, 15 pages, 3 figures, 3 tables. To be published in
Astronomy and Astrophysics. Text improved and clarified. Refined analysis of
complex regions of the Galactic plane yields a new list of high-confidence
sources including 47 sources (compared with the 40 sources appearing in the
first version
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