2,806 research outputs found
An adaptive-binning method for generating constant-uncertainty/constant-significance light curves with Fermi-LAT data
We present a method enabling the creation of
constant-uncertainty/constant-significance light curves with the data of the
Fermi-Large Area Telescope (LAT). The adaptive-binning method enables more
information to be encapsulated within the light curve than with the
fixed-binning method. Although primarily developed for blazar studies, it can
be applied to any sources. This method allows the starting and ending times of
each interval to be calculated in a simple and quick way during a first step.
The reported mean flux and spectral index (assuming the spectrum is a power-law
distribution) in the interval are calculated via the standard LAT analysis
during a second step. The absence of major caveats associated with this method
has been established by means of Monte-Carlo simulations. We present the
performance of this method in determining duty cycles as well as power-density
spectra relative to the traditional fixed-binning method.Comment: 17 pages, 13 figures, 5 tables. Submitted to A&
PSR J2030+3641: radio discovery and gamma-ray study of a middle-aged pulsar in the now identified Fermi-LAT source 1FGL J2030.0+3641
In a radio search with the Green Bank Telescope of three unidentified low
Galactic latitude Fermi-LAT sources, we have discovered the middle-aged pulsar
J2030+3641, associated with 1FGL J2030.0+3641 (2FGL J2030.0+3640). Following
the detection of gamma-ray pulsations using a radio ephemeris, we have obtained
a phase-coherent timing solution based on gamma-ray and radio pulse arrival
times that spans the entire Fermi mission. With a rotation period of 0.2 s,
spin-down luminosity of 3e34 erg/s, and characteristic age of 0.5 Myr, PSR
J2030+3641 is a middle-aged neutron star with spin parameters similar to those
of the exceedingly gamma-ray-bright and radio-undetected Geminga. Its gamma-ray
flux is 1% that of Geminga, primarily because of its much larger distance, as
suggested by the large integrated column density of free electrons, DM=246
pc/cc. We fit the gamma-ray light curve, along with limited radio polarimetric
constraints, to four geometrical models of magnetospheric emission, and while
none of the fits have high significance some are encouraging and suggest that
further refinements of these models may be worthwhile. We argue that not many
more non-millisecond radio pulsars may be detected along the Galactic plane
that are responsible for LAT sources, but that modified methods to search for
gamma-ray pulsations should be productive -- PSR J2030+3641 would have been
found blindly in gamma rays if only >0.8 GeV photons had been considered, owing
to its relatively flat spectrum and location in a region of high soft
background.Comment: Accepted for publication in ApJ, 9 pages, 6 figure
The bright unidentified gamma-ray source 1FGL J1227.9-4852: Can it be associated with an LMXB?
We present an analysis of high energy (HE; 0.1-300 GeV) gamma-ray
observations of 1FGL J1227.9-4852 with the Fermi Gamma-ray Space Telescope,
follow-up radio observations with the Australia Telescope Compact Array, Giant
Metrewave Radio Telescope and Parkes radio telescopes of the same field and
follow-up optical observations with the ESO VLT. We also examine archival
XMM-Newton and INTEGRAL X-ray observations of the region around this source.
The gamma-ray spectrum of 1FGL J1227.9-4852 is best fit with an exponentially
cutoff power-law, reminiscent of the population of pulsars observed by Fermi. A
previously unknown, compact radio source within the 99.7% error circle of 1FGL
J1227.9-4852 is discovered and has a morphology consistent either with an AGN
core/jet structure or with two roughly symmetric lobes of a distant radio
galaxy. A single bright X-ray source XSS J12270-4859, a low-mass X-ray binary,
also lies within the 1FGL J1227.9-4852 error circle and we report the first
detection of radio emission from this source. The potential association of 1FGL
J1227.9-4852 with each of these counterparts is discussed. Based upon the
available data we find the association of the gamma-ray source to the compact
double radio source unlikely and suggest that XSS J12270-4859 is a more likely
counterpart to the new HE source. We propose that XSS J12270-4859 may be a
millisecond binary pulsar and draw comparisons with PSR J1023+0038.Comment: Accepted for publication in MNRAS; 9 pages, 8 figures, 2 table
Gamma-Ray Emission from Two Blazars Behind the Galactic Plane: B2013+370 & B2023+336
B2013+370 and B2023+336 are two blazars at low-galactic latitude that were
previously proposed to be the counterparts for the EGRET unidentified sources,
3EG J2016+3657 and 3EG J2027+3429. Gamma-ray emission associated with the EGRET
sources has been detected by the Fermi Gamma-ray Space Telescope, and the two
sources, 1FGL J2015.7+3708 and 1FGL J2027.6+3335, have been classified as
unidentified in the 1-year catalog. This analysis of the Fermi-LAT data
collected during 31 months reveals that the 1FGL sources are spatially
compatible with the blazars, and are significantly variable, supporting the
hypothesis of extragalactic origin for the gamma-ray emission. The gamma-ray
light curves are compared with 15 GHz radio light curves from the 40-m
telescope at the Owens Valley Radio Observatory (OVRO). Simultaneous
variability is seen in both bands for the two blazar candidates. The study is
completed with the X-ray analysis of 1FGL J2015.7+3708 using Swift observations
that were triggered in August 2010 by a Fermi-detected flare. The resulting
spectral energy distribution shows a two-component structure typical of
blazars. We also identify a second source in the field of view of 1FGL
J2027.6+3335 with similar characteristics to the known LAT pulsars. This study
gives solid evidence favoring blazar counterparts for these two unidentified
EGRET and Fermi sources, supporting the hypothesis that a number of
unidentified gamma-ray sources at low galactic latitudes are indeed of
extragalactic origin.Comment: 10 pages, 7 figures, 6 tables, accepted for publication in The
Astrophysical Journa
Conservative upper limits on WIMP annihilation cross section from Fermi-LAT -rays
The spectrum of an isotropic extragalactic -ray background (EGB) has
been measured by the Fermi-LAT telescope at high latitudes. Two new models for
the EGB are derived from the subtraction of unresolved point sources and
extragalactic diffuse processes, which could explain from 30% to 70% of the
Fermi-LAT EGB. Within the hypothesis that the two residual EGBs are entirely
due to the annihilation of dark matter (DM) particles in the Galactic halo, we
obtain upper limits on their annihilation cross section \sigmav.
Severe bounds on a possible Sommerfeld enhancement of the annihilation cross
section are set as well. Finally, would {\sigmav} be inversely proportional to
the WIMP velocity, very severe limits are derived for the velocity-independent
part of the annihilation cross section.Comment: Proceedings of XII Taup Conference, Munich, September 201
The detectability of dark matter annihilation with Fermi using the anisotropy energy spectrum of the gamma-ray background
The energy-dependence of the anisotropy (the anisotropy energy spectrum) of
the large-scale diffuse gamma-ray background can reveal the presence of
multiple source populations. Annihilating dark matter in the substructure of
the Milky Way halo could give rise to a modulation in the anisotropy energy
spectrum of the diffuse gamma-ray emission measured by Fermi, enabling the
detection of a dark matter signal. We determine the detectability of a
dark-matter-induced modulation for scenarios in which unresolved blazars are
the primary contributor to the measured emission above ~1 GeV and find that in
some scenarios pair-annihilation cross sections of order the value expected for
thermal relic dark matter can produce a detectable feature. We anticipate that
the sensitivity of this technique to specific dark matter models could be
improved by tailored likelihood analysis methods.Comment: 9 pages, 7 figures; figures updated, other minor revisions, published
in Ap
Spectral Lags Obtained by CCF of Smoothed Lightcurves
We present a new technique to calculate the spectral lags of gamma-ray bursts
(GRBs). Unlike previous processing methods, we first smooth the light curves of
gamma-ray bursts in high and low energy bands using the "Loess" filter, then,
we directly define the spectral lags as such to maximize the cross-correlation
function (CCF) between two smoothed light curves. This method is suitable for
various shapes of CCF; it effectively avoids the errors caused by manual
selections for the fitting function and fitting interval. Using the method, we
have carefully measured the spectral lags of individual pulses contained in
BAT/Swift gamma-ray bursts with known redshifts, and confirmed the
anti-correlation between the spectral lag and the isotropy luminosity. The
distribution of spectral lags can be well fitted by four Gaussian components,
with the centroids at 0.03 s, 0.09 s, 0.15 s, and 0.21 s, respectively. We find
that some spectral lags of the multi-peak GRBs seem to evolve with time
Polarization and photometric observations of the gamma-ray blazar PG 1553+113
We present the results of an observational photo-polarimetry campaign of the
blazar PG 1553+113 at optical wavelengths. The blazar was recently detected at
very high energies (> 100 GeV) by the H.E.S.S and MAGIC gamma-ray Cherenkov
telescopes.
Our high-temporal resolution data show significant variations in the linear
polarization percentage and position angle at inter-night time-scales, while at
shorter (intra-night) time-scales both parameters varied less significantly, if
at all. Changes in the polarization angle seem to be common in gamma-ray
emitting blazars. Simultaneous differential photometry (through the B and R
bands) shows no significant variability in the total optical flux. We provide B
and R magnitudes, along with a finding chart, for a set of field stars suitable
for differential photometry.Comment: 4 pages, 3 figures. To be published by Astronomy and Astrophysic
On leptonic models for blazars in the Fermi era
Some questions raised by Fermi-LAT data about blazars are summarized, along
with attempts at solutions within the context of leptonic models. These include
both spectral and statistical questions, including the origin of the GeV breaks
in low-synchrotron peaked blazars, the location of the gamma-ray emission
sites, the correlations in the spectral energy distributions with luminosity,
and the difficulty of synchrotron/SSC models to fit the spectra of some TeV
blazars.Comment: 9 pages, 1 figure, in "Beamed and Unbeamed Gamma Rays from Galaxies,"
Muonio, Finland, 11-15 April, 2011, ed. R. Wagner, L. Maraschi, A. Sillanpaa,
to appear in Journal of Physics: Conference Serie
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