9,649 research outputs found
Discovery of GeV gamma-ray emission from PSR B1259-63/LS 2883
The binary system PSR B1259-63/LS 2883 consists of a 47.8 ms radio pulsar
that orbits the companion Be star with a period of 3.4 years in a highly
eccentric orbit. The system is well sampled in radio, X-rays, and TeV
gamma-rays, and shows orbital-phase-dependent variability in all observed
frequencies. Here we report on the discovery of >100 MeV gamma-rays from PSR
B1259-63/LS 2883 through the 2010 pariastron passage. Using data collected with
the Large Area Telescope aboard Fermi from 33 days before pariastron to 75 days
after pariastron, PSR B1259-63/LS 2883 is detected at a significance of 13.6
standard deviations. The gamma-ray light curve is highly variable over the
above period, with changing photon index that correlates with gamma-ray flux.
In particular, two major flares that occur after the pariastron passage were
observed. The onset of gamma-ray emission occurs close to, but not at the same
orbital phases as, the two disk passages that occur ~1 month before and ~1
month after the pariastron passage. The fact that the GeV orbital light curve
is different from that of the X-ray and TeV light curves strongly suggests that
GeV gamma-ray emission originates from a different component. We speculate that
the observed GeV flares may be resulting from Doppler boosting effects.Comment: ApJ Letters, in press 7 pages, 4 figures (Fig. 4 added), 1 tabl
Radio/gamma-ray time delay in the parsec-scale cores of active galactic nuclei
We report the detection of a non-zero time delay between radio emission
measured by the VLBA at 15.4 GHz and gamma-ray radiation (gamma-ray leads
radio) registered by the Large Area Telescope (LAT) on board the Fermi
Gamma-Ray Space Telescope for a sample of 183 radio and gamma-ray bright active
galactic nuclei (AGNs). For the correlation analysis we used 100 MeV - 100 GeV
gamma-ray photon fluxes, taken from monthly binned measurements from the first
Fermi LAT catalog, and 15.4 GHz radio flux densities from the MOJAVE VLBA
program. The correlation is most pronounced if the core flux density is used,
strongly indicating that the gamma-ray emission is generated within the compact
region of the 15 GHz VLBA core. Determining the Pearson's r and Kendall's tau
correlation coefficients for different time lags, we find that for the majority
of sources the radio/gamma-ray delay ranges from 1 to 8 months in the
observer's frame and peaks at about 1.2 months in the source's frame. We
interpret the primary source of the time delay to be synchrotron opacity in the
nuclear region.Comment: 5 pages, 2 figures, 2 tables, accepted by the Astrophysical Journal
Letters; minor corrections to the text are made; full table 1 in electronic
form can be extracted from the preprint sourc
Cosmic-Ray Induced Diffuse Emissions from the Milky Way and Local Group Galaxies
Cosmic rays fill up the entire volume of galaxies, providing an important
source of heating and ionisation of the interstellar medium, and may play a
significant role in the regulation of star formation and galactic evolution.
Diffuse emissions from radio to high-energy gamma rays (> 100 MeV) arising from
various interactions between cosmic rays and the interstellar medium,
interstellar radiation field, and magnetic field, are currently the best way to
trace the intensities and spectra of cosmic rays in the Milky Way and other
galaxies. In this contribution, I describe our recent work to model the full
spectral energy distribution of galaxies like the Milky Way from radio to
gamma-ray energies. The application to other galaxies, in particular the
Magellanic Clouds and M31 that are detected in high-energy gamma-rays by the
Fermi-LAT, is also discussed.Comment: Contribution to "The Spectral Energy Distribution of Galaxies"
Proceedings IAU Symposium No. 284, 2011, eds. R.J. Tuffs & C.C.Popescu. 4
pages with 4 figure
Pulsar Outer-gap Electrodynamics: Hardening of Spectral Shape in the Trailing Peak in Gamma-ray Light Curve
The spectral characteristics of the pulsed gamma-ray emission from
outer-magnetospheric particle accelerators are investigated. Either positrons
or electrons are accelerated outwards by the magnetic-field-aligned electric
field to emit gamma-rays via curvature process. Since the particles move along
relatively straight paths in the trailing side of a rotating magnetosphere,
they attain higher Lorentz factors to emit more energetic gamma-rays than those
in the leading side. It is first demonstrated that the cutoff energy of the
curvature radiation evolves with the rotation phase owing to the variation of
the curvature radii of the particle paths and maximizes at a slightly later
phase of the trailing peak in the gamma-ray light curve.Comment: 5 pages, 5 figures, ApJ Letters in pres
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
Evidence for Gamma-Ray Halos Around Active Galactic Nuclei and the First Measurement of Intergalactic Magnetic Fields
Intergalactic magnetic fields (IGMF) can cause the appearance of halos around
the gamma-ray images of distant objects because an electromagnetic cascade
initiated by a high-energy gamma-ray interaction with the photon background is
broadened by magnetic deflections. We report evidence of such gamma-ray halos
in the stacked images of the 170 brightest active galactic nuclei (AGN) in the
11-month source catalog of the Fermi Gamma-Ray Space Telescope. Excess over
point spread function in the surface brightness profile is statistically
significant at 3.5\sigma (99.95% confidence level), for the nearby, hard
population of AGN. The halo size and brightness are consistent with IGMF,
B_{IGMF} ~ 10^{-15} G. The knowledge of IGMF will facilitate the future
gamma-ray and charged-particle astronomy. Furthermore, since IGMF are likely to
originate from the primordial seed fields created shortly after the Big Bang,
this potentially opens a new window on the origin of cosmological magnetic
fields, inflation, and the phase transitions in the early Universe.Comment: 14 pages, 4 figures; detailed discussion on instrumental effects;
conclusions unchanged; accepted to ApJ Letter
Two-Step Acceleration Model of Cosmic Rays at Middle-Aged SNR
Recent gamma-ray observations of middle-aged supernova remnants revealed a
mysterious broken power-law spectrum. Using three-dimensional
magnetohydrodynamics simulations, we show that the interaction between a
supernova blast wave and interstellar clouds formed by thermal instability
generates multiple reflected shocks. The typical Mach numbers of the reflected
shocks are shown to be M ~ 2 depending on the density contrast between the
diffuse intercloud gas and clouds. These secondary shocks can further energize
cosmic-ray particles originally accelerated at the blast-wave shock. This
"two-step" acceleration scenario reproduces the observed gamma-ray spectrum and
predicts the high-energy spectral index ranging approximately from 3 to 4.Comment: 4 pages, 2 figures, accepted by ApJ Letter
The Effect of Blazar Spectral Breaks on the Blazar Contribution to the Extragalactic Gamma-ray Background
The spectral shapes of the contributions of different classes of unresolved
gamma-ray emitters can provide insight into their relative contributions to the
extragalactic gamma-ray background (EGB) and the natures of their spectra at
GeV energies. We calculate the spectral shapes of the contributions to the EGB
arising from BL Lacertae type objects (BL Lacs) and flat-spectrum radio quasars
(FSRQs) assuming blazar spectra can be described as broken power laws. We fit
the resulting total blazar spectral shape to the Fermi Large Area Telescope
measurements of the EGB, finding that the best-fit shape reproduces well the
shape of the Fermi EGB for various break scenarios. We conclude that a scenario
in which the contribution of blazars is dominant cannot be excluded on spectral
grounds alone, even if spectral breaks are shown to be common among Fermi
blazars. We also find that while the observation of a featureless (within
uncertainties) power-law EGB spectrum by Fermi does not necessarily imply a
single class of contributing unresolved sources with featureless individual
spectra, such an observation and the collective spectra of the separate
contributing populations determine the ratios of their contributions. As such,
a comparison with studies including blazar gamma-ray luminosity functions could
have profound implications for the blazar contribution to the EGB, blazar
evolution, and blazar gamma-ray spectra and emission.Comment: 8 pages, emulateapj format; 5 figures; accepted for publication in
Ap
Fermi-LAT Detection of the Young SuperNova Remnant Tycho
After almost three years of data taking in sky survey mode, the
\emph{Fermi}-LAT has detected -ray emission toward the Tycho's
Supernova Remnant (SNR). The Tycho SNR is among the youngest remnants in the
Galaxy, originating from a Type Ia Supernova in AD 1572. The -ray
integral flux from 400 MeV up to 100 GeV has been measured to be
(3.5) cms with a
photon index of 2.3
The gamma-ray spectrum of Geminga and the inverse Compton model of pulsar high energy emission
We reanalyze the Fermi spectra of the Geminga and Vela pulsars. We find that
the spectrum of Geminga above the break is exceptionally well approximated by a
simple power law without the exponential cut-off, making Geminga's spectrum
similar to that of Crab. Vela's broadband gamma-ray spectrum is equally well
fit with both the exponential cut-off and the double power law shapes.
In the broadband double power-law fits, for a typical Fermi spectrum of a
bright \gamma-ray pulsar, most of the errors accumulate due to the arbitrary
parametrization of the spectral roll-off. In addition, a power law with an
exponential cut-off gives an acceptable fit for the underlying double power-law
spectrum for a very broad range of parameters, making such fitting procedures
insensitive to the underlying Fermi photon spectrum.
Our results have important implications for the mechanism of pulsar high
energy emission. A number of observed properties of \gamma-ray pulsars, i.e.,
the broken power law spectra without exponential cut-offs and stretching in
case of Crab beyond the maximal curvature limit, spectral breaks close to or
exceeding the maximal breaks due to curvature emission, a Crab patterns of
relative intensities of the leading and trailing pulses repeated in the X-ray
and \gamma-ray regions, all point to the inverse Compton origin of the high
energy emission from majority of pulsars
- …