2,231 research outputs found
Search for variable gamma-ray emission from the Galactic plane in the Fermi data
High-energy gamma-ray emission from the Galactic plane above ~100 MeV is
composed of three main contributions: diffuse emission from cosmic ray
interactions in the interstellar medium, emission from extended sources, such
as supernova remnants and pulsar wind nebulae, and emission from isolated
compact source populations. The diffuse emission and emission from the extended
sources provide the dominant contribution to the flux almost everywhere in the
inner Galaxy, preventing the detection of isolated compact sources. In spite of
this difficulty, compact sources in the Galactic plane can be singled out based
on the variability properties of their gamma-ray emission. Our aim is to find
sources in the Fermi data that show long-term variability. We performed a
systematic study of the emission variability from the Galactic plane, by
constructing the variability maps. We find that emission from several
directions along the Galactic plane is significantly variable on a time scale
of months. These directions include, in addition to known variable Galactic
sources and background blazars, the Galactic ridge region at positive Galactic
longitudes and several regions containing young pulsars. We argue that
variability on the time scale of months may be common to pulsars, originating
from the inner parts of pulsar wind nebulae, similarly to what is observed in
the Crab pulsar.Comment: 4 pages, 4 figures, accepted to Astronomy & Astrophysic
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
Extragalactic Very-High-Energy gamma-ray background
We study the origin of the extragalactic diffuse gamma-ray background using
the data from the Fermi telescope. To estimate the background level, we count
photons at high Galactic latitudes |b|>60 degrees. Subtracting photons
associated to known sources and the residual cosmic ray and Galactic diffuse
backgrounds, we estimate the Extragalactic Gamma-ray Background (EGB) flux. We
find that the spectrum of EGB in the very-high-energy (VHE) band above 30 GeV
follows the stacked spectrum of BL Lacs. LAT data reveal the positive (1+z)^k,
1<k<4 cosmological evolution of the BL Lac source population consistent with
that of their parent population, FR I radio galaxies. We show that EGB at E>30
GeV could be completely explained by emission from unresolved BL Lacs if k~3.Comment: 8 pages, 6 figures, accepted to Astrophysics Journa
Social entrepreneurship in the United Arab Emirates: challenges and recommendations
The purpose of this study is to explore current challenges facing social entrepreneurs in the United Arab Emirates, and provide recommendations that can contribute to the development of the growing and vibrant community of social entrepreneurs in the UAE. The UAE is a recognized regional and global leader in supporting innovation and entrepreneurship. Recognizing this, this study seeks to contribute thoughtful insights that can help the UAE embrace social entrepreneurship and further act as a catalyst to push social entrepreneurship forward within the wider region. Based on primary interviews with key informants, several key challenges were identified related to a lack of institutional support a lack of social and cultural awareness regarding social entrepreneurship within the UAE. Further challenges include a lack of clear benchmarks related to monitoring and measuring social impact. Overall, the challenges identified highlighting the need for more government and institutional support, as well as a stronger link between the corporate sector, academia, and the social enterprise sector
Acceleration of Ultra-High Energy Cosmic Rays in the Colliding Shells of Blazars and GRBs: Constraints from the Fermi Gamma ray Space Telescope
Fermi Gamma ray Space Telescope measurements of spectra, variability time
scale, and maximum photon energy give lower limits to the apparent jet powers
and, through gammagamma opacity arguments, the bulk Lorentz factors of
relativistic jets. The maximum cosmic-ray particle energy is limited by these
two quantities in Fermi acceleration scenarios. Recent data are used to
constrain the maximum energies of cosmic-ray protons and Fe nuclei accelerated
in colliding shells of GRBs and blazars. The Fermi results indicate that Fe
rather than protons are more likely to be accelerated to ultra-high energies in
AGNs, whereas powerful GRBs can accelerate both protons and Fe to >~ 10^{20}
eV. Emissivity of nonthermal radiation from radio galaxies and blazars is
estimated from the First Fermi AGN Catalog, and shown to favor BL Lac objects
and FR1 radio galaxies over flat spectrum radio quasars, FR2 radio galaxies,
and long-duration GRBs as the sources of UHECRs.Comment: 8 pages, 3 figures, ApJ, in pres
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
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
Impact of the orbital uncertainties on the timing of pulsars in binary systems
The detection of pulsations from an X-ray binary is an unambiguous signature
of the presence of a neutron star in the system. When the pulsations are missed
in the radio band, their detection at other wavelengths, like X-ray or
gamma-rays, requires orbital demodulation, since the length of the observations
are often comparable to, or longer than the system orbital period. The detailed
knowledge of the orbital parameters of binary systems plays a crucial role in
the detection of the spin period of pulsars, since any uncertainty in their
determination translates into a loss in the coherence of the signal during the
demodulation process. In this paper, we present an analytical study aimed at
unveiling how the uncertainties in the orbital parameters might impact on
periodicity searches. We find a correlation between the power of the signal in
the demodulated arrival time series and the uncertainty in each of the orbital
parameters. This correlation is also a function of the pulsar frequency. We
test our analytical results with numerical simulations, finding good agreement
between them. Finally, we apply our study to the cases of LS 5039 and LS I +61
303 and consider the current level of uncertainties in the orbital parameters
of these systems and their impact on a possible detection of a hosted pulsar.
We also discuss the possible appearance of a sideband ambiguity in real data.
The latter can occur when, due to the use of uncertain orbital parameters, the
power of a putative pulsar is distributed in frequencies lying nearby the
pulsar period. Even if the appearance of a sideband is already a signature of a
pulsar component, it may introduce an ambiguity in the determination of its
period. We present here a method to solve the sideband issue.Comment: Accepted 2012 September 08 by MNRAS. The paper contains 18 figures
and 5 table
PSRs J0248+6021 and J2240+5832: Young Pulsars in the Northern Galactic Plane. Discovery, Timing, and Gamma-ray observations
Pulsars PSR J0248+6021 (rotation period P=217 ms and spin-down power Edot =
2.13E35 erg/s) and PSR J2240+5832 (P=140 ms, Edot = 2.12E35 erg/s) were
discovered in 1997 with the Nancay radio telescope during a northern Galactic
plane survey, using the Navy-Berkeley Pulsar Processor (NBPP) filter bank. GeV
gamma-ray pulsations from both were discovered using the Fermi Large Area
Telescope. Twelve years of radio and polarization data allow detailed
investigations. The two pulsars resemble each other both in radio and in
gamma-ray data. Both are rare in having a single gamma-ray pulse offset far
from the radio peak. The high dispersion measure for PSR J0248+6021 (DM = 370
pc cm^-3) is most likely due to its being within the dense, giant HII region W5
in the Perseus arm at a distance of 2 kpc, not beyond the edge of the Galaxy as
obtained from models of average electron distributions. Its high transverse
velocity and the low magnetic field along the line-of-sight favor this small
distance. Neither gamma-ray, X-ray, nor optical data yield evidence for a
pulsar wind nebula surrounding PSR J0248+6021. The gamma-ray luminosity for PSR
J0248+6021 is L_ gamma = (1.4 \pm 0.3)\times 10^34 erg/s. For PSR J2240+5832,
we find either L_gamma = (7.9 \pm 5.2) \times 10^34 erg/s if the pulsar is in
the Outer arm, or L_gamma = (2.2 \pm 1.7) \times 10^34 erg/s for the Perseus
arm. These luminosities are consistent with an L_gamma ~ sqrt(Edot) rule.
Comparison of the gamma-ray pulse profiles with model predictions, including
the constraints obtained from radio polarization data, favor emission in the
far magnetosphere. These two pulsars differ mainly in their inclination angles
and acceleration gap widths, which in turn explains the observed differences in
the gamma-ray peak widths.Comment: 13 pages, Accepted to Astronomy & Astrophysic
The blazar-like radio structure of the TeV source IC310
Context. The radio galaxy IC310 in the Perseus cluster has recently been
detected in the gamma-ray regime at GeV and TeV energies. The TeV emission
shows time variability and an extraordinarily hard spectrum, even harder than
the spectrum of the similar nearby gamma-ray emitting radio galaxy M87.
Aims. High-resolution studies of the radio morphology help to constrain the
geometry of the jet on sub-pc scales and to find out where the high-energy
emission might come from.
Methods. We analyzed May 2011 VLBA data of IC310 at a wavelength of 3.6 cm,
revealing the parsec-scale radio structure of this source. We compared our
findings with more information available from contemporary single-dish flux
density measurements with the 100-m Effelsberg radio telescope.
Results. We have detected a one-sided core-jet structure with blazar-like,
beamed radio emission oriented along the same position angle as the kiloparsec
scale radio structure observed in the past by connected interferometers.
Doppler-boosting favoritism is consistent with an angle of theta < 38 degrees
between the jet axis and the line-of-sight, i.e., very likely within the
boundary dividing low-luminosity radio galaxies and BL Lac objects in unified
schemes.
Conclusions. The stability of the jet orientation from parsec to kiloparsec
scales in IC310 argues against its classification as a headtail radio galaxy;
i.e., there is no indication of an interaction with the intracluster medium
that would determine the direction of the tail. IC310 seems to represent a
low-luminosity FRI radio galaxy at a borderline angle to reveal its BL Lac-type
central engine.Comment: 5 pages, 3 figures (1 color); A&A, accepte
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