298 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
An Intense Gamma-Ray Flare of PKS1622-297
We report the observation by the Compton Gamma Ray Observatory of a
spectacular flare of radio source PKS 1622-297. A peak flux of 17E-6 cm^-2 s^-1
(E > 100 MeV) was observed. The corresponding isotropic luminosity is 2.9E49
erg/s. We find that PKS 1622-297 exhibits gamma-ray intra-day variability. A
flux increase by a factor of at least 3.6 was observed to occur in less than
7.1 hours (with 99% confidence). Assuming an exponential rise, the
corresponding doubling time is less than 3.8 hours. A significant flux decrease
by a factor of ~2 in 9.7 hours was also observed. Without beaming, the rapid
flux change and large isotropic luminosity are inconsistent with the
Elliot-Shapiro condition (assuming that gas accretion is the immediate source
of power for the gamma-rays). This inconsistency suggests that the gamma-ray
emission is beamed. A minimum Doppler factor of 8.1 is implied by the observed
lack of pair-production opacity (assuming x-rays are emitted co-spatially with
the gamma-rays). Simultaneous observation by EGRET and OSSE finds a spectrum
adequately fit by a power law with photon index of -1.9. Although the
significance is not sufficient to establish this beyond doubt, the high-energy
gamma-ray spectrum appears to evolve from hard to soft as a flare progresses.Comment: 14 pages, 4 figures, 1 tabl
Fermi LAT observations of the Geminga pulsar
We report on the \textit{Fermi}-LAT observations of the Geminga pulsar, the
second brightest non-variable GeV source in the -ray sky and the first
example of a radio-quiet -ray pulsar. The observations cover one year,
from the launch of the satellite through 2009 June 15. A data sample of
over 60,000 photons enabled us to build a timing solution based solely on
rays. Timing analysis shows two prominent peaks, separated by = 0.497 0.004 in phase, which narrow with increasing energy. Pulsed
rays are observed beyond 18 GeV, precluding emission below 2.7 stellar
radii because of magnetic absorption. The phase-averaged spectrum was fitted
with a power law with exponential cut-off of spectral index = (1.30
0.01 0.04), cut-off energy = (2.46 0.04 0.17)
GeV and an integral photon flux above 0.1 GeV of (4.14 0.02 0.32)
10 cm s. The first uncertainties are statistical
and the second are systematic. The phase-resolved spectroscopy shows a clear
evolution of the spectral parameters, with the spectral index reaching a
minimum value just before the leading peak and the cut-off energy having maxima
around the peaks. Phase-resolved spectroscopy reveals that pulsar emission is
present at all rotational phases. The spectral shape, broad pulse profile, and
maximum photon energy favor the outer magnetospheric emission scenarios.Comment: 32 pages, 12 figures, 3 tables. Accepted for publication in The
Astrophysical Journal. Corresponding authors: Denis Dumora
([email protected]), Fabio Gargano ([email protected]),
Massimiliano Razzano ([email protected]
Fermi Gamma-ray Space Telescope Observations of Recent Gamma-ray Outbursts from 3C 454.3
The flat spectrum radio quasar 3C~454.3 underwent an extraordinary outburst
in December 2009 when it became the brightest gamma-ray source in the sky for
over one week. Its daily flux measured with the Fermi Large Area Telescope at
photon energies E>100 MeV reached F = 22+/-1 x 10^-6 ph cm^-2 s^-1,
representing the highest daily flux of any blazar ever recorded in high-energy
gamma-rays. It again became the brightest source in the sky in 2010 April,
triggering a pointed-mode observation by Fermi. The correlated gamma-ray
temporal and spectral properties during these exceptional events are presented
and discussed. The main results show flux variability over time scales less
than 3 h and very mild spectral variability with an indication of gradual
hardening preceding major flares. No consistent loop pattern emerged in the
gamma-ray spectral index vs flux plane. A minimum Doppler factor of ~ 15 is
derived, and the maximum energy of a photon from 3C 454.3 is ~ 20 GeV. The
spectral break at a few GeV is inconsistent with Klein-Nishina softening from
power-law electrons scattering Ly_alpha line radiation, and a break in the
underlying electron spectrum in blazar leptonic models is implied.Comment: submitted to the Astrophysical Journa
Rapid Synchrotron Flares from BL Lacertae Detected by ASCA and RXTE
We report the variable X-ray emission from BL Lacertae detected in the ASCA
ToO observation conducted during the EGRET and RXTE pointings, coincident with
the 1997 July outburst. The source showed a historically high state of X-ray,
optical, and gamma-ray emission, with its 2-10 keV flux peaking at ~3.3 E-11
erg/cm2/s. This is more than 3 times higher than the value measured by ASCA in
1995. We detected two rapid flares which occured only in the soft X-ray band,
while the hard X-ray flux also increased, but decayed with a much longer time
scale. Together with the requirement of a very steep and varying power law
dominating the soft X-ray band in addition to the hard power law, we suggest
that both the high energy end of the synchrotron spectrum and the hard inverse
Compton spectrum were visible in this source during the outburst. We discuss
the possible origins of the observed variability time scales, and interpret the
short time scales of the soft X-ray variability as reflecting the size of the
emission region.Comment: 20 pages, 4 figures, accepted for publication in the Astrophysical
Journa
Early Fermi Gamma-ray Space Telescope Observations of the Quasar 3C 454.3
This is the first report of Fermi Gamma-ray Space Telescope observations of
the quasar 3C 454.3, which has been undergoing pronounced long-term outbursts
since 2000. The data from the Large Area Telescope (LAT), covering 2008 July 7
- October 6, indicate strong, highly variable gamma-ray emission with an
average flux of ~3 x 10^{-6} photons cm^{-2} s^{-1}, for energies above 100
MeV. The gamma-ray flux is variable, with strong, distinct,
symmetrically-shaped flares for which the flux increases by a factor of several
on a time scale of about three days. This variability indicates a compact
emission region, and the requirement that the source is optically thin to
pair-production implies relativistic beaming with Doppler factor delta > 8,
consistent with the values inferred from VLBI observations of superluminal
expansion (delta ~ 25). The observed gamma-ray spectrum is not consistent with
a simple power-law, but instead steepens strongly above ~2 GeV, and is well
described by a broken power-law with photon indices of ~2.3 and ~3.5 below and
above the break, respectively. This is the first direct observation of a break
in the spectrum of a high luminosity blazar above 100 MeV, and it is likely
direct evidence for an intrinsic break in the energy distribution of the
radiating particles. Alternatively, the spectral softening above 2 GeV could be
due to gamma-ray absorption via photon-photon pair production on the soft X-ray
photon field of the host AGN, but such an interpretation would require the
dissipation region to be located very close (less than 100 gravitational radii)
to the black hole, which would be inconsistent with the X-ray spectrum of the
source.Comment: Accepted by the Astrophysical Journal; corresponding authors: Greg
Madejski ([email protected]) and Benoit Lott ([email protected]
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
Fermi Large Area Telescope Observations of Misaligned AGN
Analysis is presented on 15 months of data taken with the Large Area
Telescope (LAT) on the Fermi Gamma-ray Space Telescope for 11 non-blazar AGNs,
including 7 FRI radio galaxies and 4 FRII radio sources consisting of 2 FRII
radio galaxies and 2 steep spectrum radio quasars. The broad line FRI radio
galaxy 3C 120 is reported here as a gamma-ray source for the first time. The
analysis is based on directional associations of LAT sources with radio sources
in the 3CR, 3CRR and MS4 (collectively referred to as 3C-MS) catalogs. Seven of
the eleven LAT sources associated with 3C-MS radio sources have spectral
indices larger than 2.3 and, except for the FRI radio galaxy NGC 1275 that
shows possible spectral curvature, are well described by a power law. No
evidence for time variability is found for any sources other than NGC 1275. The
gamma-ray luminosities of FRI radio galaxies are significantly smaller than
those of BL Lac objects detected by the LAT, whereas the gamma-ray luminosities
of FRII sources are quite similar to those of FSRQs, which could reflect
different beaming factors for the gamma-ray emission. A core dominance study of
the 3CRR sample indicate that sources closer to the jet axis are preferentially
detected with the Fermi-LAT, insofar as the gamma-ray--detected misaligned AGNs
have larger core dominance at a given average radio flux. The results are
discussed in view of the AGN unification scenario.Comment: 28 pages, 8 figures, 2 tables. Accepted for publication in The
Astrophysical Journa
Bright AGN Source List from the First Three Months of the Fermi Large Area Telescope All-Sky Survey
The first three months of sky-survey operation with the Fermi Gamma Ray Space
Telescope (Fermi) Large Area Telescope (LAT) reveals 132 bright sources at
|b|>10 deg with test statistic greater than 100 (corresponding to about 10
sigma). Two methods, based on the CGRaBS, CRATES and BZCat catalogs, indicate
high-confidence associations of 106 of these sources with known AGNs. This
sample is referred to as the LAT Bright AGN Sample (LBAS). It contains two
radio galaxies, namely Centaurus A and NGC 1275, and 104 blazars consisting of
57 flat spectrum radio quasars (FSRQs), 42 BL Lac objects, and 5 blazars with
uncertain classification. Four new blazars were discovered on the basis of the
LAT detections. Remarkably, the LBAS includes 10 high-energy peaked BL Lacs
(HBLs), sources which were so far hard to detect in the GeV range. Another 10
lower-confidence associations are found. Only thirty three of the sources, plus
two at |b|>10 deg, were previously detected with EGRET, probably due to the
variable nature of these sources. The analysis of the gamma-ray properties of
the LBAS sources reveals that the average GeV spectra of BL Lac objects are
significantly harder than the spectra of FSRQs. No significant correlation
between radio and peak gamma-ray fluxes is observed. Blazar log N - log S and
luminosity functions are constructed to investigate the evolution of the
different blazar classes, with positive evolution indicated for FSRQs but none
for BLLacs. The contribution of LAT-blazars to the total extragalactic
gamma-ray intensity is estimated.Comment: Submitted to ApJ. Not yet refereed. 61 pages, 26 figure
Para que servem os inventĂĄrios de fauna?
InventĂĄrios de fauna acessam diretamente a diversidade de uma localidade, em um determinado espaço e tempo. Os dados primĂĄrios gerados pelos inventĂĄrios compĂ”em uma das ferramentas mais importantes na tomada de decisĂ”es a respeito do manejo de ĂĄreas naturais. Entretanto, vĂĄrios problemas tĂȘm sido observados em diversos nĂveis relacionados aos inventĂĄrios de fauna no Brasil e vĂŁo desde a formação de recursos humanos atĂ© a ausĂȘncia de padronização, de desenho experimental e de seleção de mĂ©todos inadequados. SĂŁo apresentados estudos de caso com mamĂferos, rĂ©pteis, anfĂbios e peixes, nos quais sĂŁo discutidos problemas como variabilidade temporal e mĂ©todos para detecção de fauna terrestre, sugerindo que tanto os inventĂĄrios quanto os programas de monitoramento devam se estender por prazos maiores e que os inventĂĄrios devem incluir diferentes metodologias para que os seus objetivos sejam plenamente alcançados.Inventories of fauna directly access the diversity of a locality in a certain period of time. The primary data generated by these inventories comprise one of the most important steps in decisions making regarding the management of natural areas. However, several problems have been observed at different levels related to inventories of fauna in Brazil, and range from the training of humans to the lack of standardization of experimental design and selection of inappropriate methods. We present case studies of mammals, reptiles, amphibians and fishes, where they discussed issues such temporal variability and methods for detection of terrestrial fauna, suggesting that both inventories and monitoring programs should be extended for longer terms and that inventories should include different methodologies to ensure that their goals are fully achieved
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