494 research outputs found
High Energy Gamma-Ray Emission From Blazars: EGRET Observations
We will present a summary of the observations of blazars by the Energetic
Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory
(CGRO). EGRET has detected high energy gamma-ray emission at energies greater
than 100 MeV from more that 50 blazars. These sources show inferred isotropic
luminosities as large as ergs s. One of the most
remarkable characteristics of the EGRET observations is that the gamma-ray
luminosity often dominates the bolometric power of the blazar. A few of the
blazars are seen to exhibit variability on very short time-scales of one day or
less. The combination of high luminosities and time variations seen in the
gamma-ray data indicate that gamma-rays are an important component of the
relativistic jet thought to characterize blazars. Currently most models for
blazars involve a beaming scenario. In leptonic models, where electrons are the
primary accelerated particles, gamma-ray emission is believed to be due to
inverse Compton scattering of low energy photons, although opinions differ as
to the source of the soft photons. Hardronic models involve secondary
production or photomeson production followed by pair cascades, and predict
associated neutrino production.Comment: 16 pages, 7 figures, style files included. Invited review paper in
"Observational Evidence for Black Holes in the Universe," 1999, ed. S. K.
Chakrabarti (Dordrecht: Kluwer), 215-23
Evidence for a Binary Companion to the Central Compact Object 1E 1207.4-5209
Unique among neutron stars, 1E 1207.4-5209 is an X-ray pulsar with a spin
period of 424 ms that contains at least two strong absorption features in its
energy spectrum. This neutron star has been identified as a member of the
radio-quiet compact central objects in supernova remnants. It has been found
that 1E 1207.4-5209 is not spinning down monotonically suggesting that this
neutron star undergoes strong, frequent glitches, contains a fall-back disk, or
possess a binary companion. Here, we report on a sequence of seven XMM-Newton
observations of 1E 1207.4-5209 performed during a 40 day window in June/July
2005. Due to unanticipated variance in the phase measurements beyond the
statistical uncertainties, we could not identify a unique phase-coherent timing
solution. The three most probable timing solutions give frequency time
derivatives of +0.9, -2.6, and +1.6 X 10^(-12) Hz/s (listed in descending order
of significance). We conclude that the local frequency derivative during our
XMM-Newton observing campaign differs from the long-term spin-down rate by more
than an order of magnitude, effectively ruling out glitch models for 1E
1207.4-5209. If the long-term spin frequency variations are caused by timing
noise, the strength of the timing noise in 1E 1207.4-5209 is much stronger than
in other pulsars with similar period derivatives. Therefore, it is highly
unlikely that the spin variations are caused by the same physical process that
causes timing noise in other isolated pulsars. The most plausible scenario for
the observed spin irregularities is the presence of a binary companion to 1E
1207.4-5209. We identified a family of orbital solutions that are consistent
with our phase-connected timing solution, archival frequency measurements, and
constraints on the companions mass imposed by deep IR and optical observations.Comment: 8 pages, 4 figures. To be published in the proceedings of "Isolated
Neutron Stars: from the Interior to the Surface" (April 24-28, 2006) - eds.
D. Page, R. Turolla & S. Zan
Assembly and photocarrier dynamics of heterostructured nanocomposite photoanodes from multicomponent colloidal nanocrystals
Multicomponent oxides and their heterostructures are rapidly emerging as promising light absorbers to drive oxidative chem. To fully exploit their functionality, precise tuning of their compn. and structure is crucial. Here, we report a novel soln.-based route to nanostructured bismuth vanadate (BiVO4) that facilitates the assembly of BiVO4/metal oxide (TiO2, WO3, and Al2O3) nanocomposites in which the morphol. of the metal oxide building blocks is finely tailored. The combination of transient absorption spectroscopy-spanning from picoseconds to second time scales-and photoelectrochem. measurements reveals that the achieved structural tunability is key to understanding and directing charge sepn., transport, and efficiency in these complex oxide heterostructured films
Might some gamma ray bursts be an observable signature of natural wormholes?
The extragalactic microlensing scenario for natural wormholes is examined. It
is shown that the main features of wormhole lensing events upon the light of
distant Active Galactic Nuclei (AGNs) are similar to some types of already
observed Gamma Ray Bursts (GRBs). Using recent satellite data on GRBs, an upper
limit to the negative mass density -- g cm --
under the form of wormhole-like objects is presented.Comment: extended version, additions on GRB physics, background sources and
cosmological consequences. Two ps figures. Accpeted for publication in Phys.
Rev.
Optical and radio variability of the BL Lac object AO 0235+16: a possible 5-6 year periodicity
New optical and radio data on the BL Lacertae object AO 0235+16 have been
collected in the last four years by a wide international collaboration, which
confirm the intense activity of this source. The optical data also include the
results of the Whole Earth Blazar Telescope (WEBT) first-light campaign
organized in November 1997. The optical spectrum is observed to basically
steepen when the source gets fainter. We have investigated the existence of
typical variability time scales and of possible correlations between the
optical and radio emissions by means of visual inspection, Discrete Correlation
Function analysis, and Discrete Fourier Transform technique. The major radio
outbursts are found to repeat quasi-regularly with a periodicity of about 5.7
years; this period is also in agreement with the occurrence of some of the
major optical outbursts, but not all of them.Comment: to be published in A&
Multifrequency Strategies for the Identification of Gamma-Ray Sources
More than half the sources in the Third EGRET (3EG) catalog have no firmly
established counterparts at other wavelengths and are unidentified. Some of
these unidentified sources have remained a mystery since the first surveys of
the gamma-ray sky with the COS-B satellite. The unidentified sources generally
have large error circles, and finding counterparts has often been a challenging
job. A multiwavelength approach, using X-ray, optical, and radio data, is often
needed to understand the nature of these sources. This chapter reviews the
technique of identification of EGRET sources using multiwavelength studies of
the gamma-ray fields.Comment: 35 pages, 22 figures. Chapter prepared for the book "Cosmic Gamma-ray
Sources", edited by K.S. Cheng and G.E. Romero, to be published by Kluwer
Academic Press, 2004. For complete article and higher resolution figures, go
to: http://www.astro.columbia.edu/~muk/mukherjee_multiwave.pd
Growth of CuCl thin films by magnetron sputtering for ultraviolet optoelectronic applications
Copper (I) chloride (CuCl) is a potential candidate for ultraviolet (UV) optoelectronics due to its close lattice match with Si (mismatch less than 0.4%) and a high UV excitonic emission at room temperature. CuCl thin films were deposited using radio frequency magnetron sputtering technique. The influence of target to substrate distance (dts) and sputtering pressure on the composition, microstructure, and UV emission properties of the films were analyzed. The films deposited with shorter target to substrate spacing (dts=3 cm) were found to be nonstoichiometric, and the film stoichiometry improves when the substrate is moved away from the target (dts=4.5 and 6 cm). A further increase in the spacing results in poor crystalline quality. The grain interface area increases when the sputtering pressure is increased from 1.1Ă10âÂł to 1Ă10âÂČ mbar at dts=6 cm. Room temperature cathodoluminescence spectrum shows an intense and sharp UV exciton (Zâ) emission at ~385 nm with a full width at half maximum of 16 nm for the films deposited at the optimum dts of 6 cm and a pressure of 1.1Ă10âÂł mbar. A broad deep level emission in the green region (~515 nm) is also observed. The relative intensity of the UV to green emission peaks decreased when the sputtering pressure was increased, consistent with an increase in grain boundary area. The variation in the stoichiometry and the crystallinity are attributed to the change in the intensity and energy of the flux of materials from the target due to the interaction with the background gas molecules
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
On the Hadronic Beam Model for Gamma-ray Production in Blazars
We consider, herein, a model for gamma-ray production in blazars in which a
relativistic, highly-collimated electron-proton beam interacts with a dense,
compact cloud as the jet propagates through the broad and perhaps narrow line
regions (BLR and NLR) of active galactic nuclei (AGN). During the propagation
of the beam through the cloud, the process of excitation of plasma waves
becomes an important energy loss mechanism, especially for mildly relativistic
proton beams. We compute the expected spectra of gamma-rays from the decay of
neutral pions produced in hadronic collisions of the beam with the cloud,
taking into account collisionless losses of the electron-proton beam. This
model may explain the X-ray and TeV gamma-ray (both low and high emission
states) of Mrk 421 as a result of synchrotron emission of secondary pairs from
the decay of charged pions and gamma-ray emission from the decay of neutral
pions for the plausible cloud parameters. However clouds can not be too hot and
too dense. Otherwise the TeV gamma-rays can be attenuated by the bremsstrahlung
radiation in the cloud and the secondary pairs are not able to efficiently
produce synchrotron flares because of the dominant role of inverse Compton
scattering.
The non-variable -ray emission observed from Mrk 421 in the EGRET
energy range cannot be described by the -rays from decay of neutral
pions provided that the spectrum of protons in the beam is well described by a
simple power law. These -rays might only be produced by secondary pairs
scattering the soft non-variable X-rays which might originate in the inner part
of the accretion disk.Comment: 14 pages,3 figures, latex, submitted to Ap
GRB 991216 Joins the Jet Set: Discovery and Monitoring of its Optical Afterglow
The optical light curve of the energetic gamma-ray burst GRB 991216 is
consistent with jet-like behavior in which a power-law decay steepens from
t**(-1.22 +/- 0.04) at early times to t**(-1.53 +/- 0.05) in a gradual
transition at around 2 d. The derivation of the late-time decay slope takes
into account the constant contribution of a host or intervening galaxy which
was measured 110 d after the event at R = 24.56 +/- 0.14, although the light
curve deviates from a single power law whether or not a constant term is
included. The early-time spectral energy distribution of the afterglow can be
described as F_nu ~ nu**(-0.74 +/- 0.05) or flatter between optical and X-ray,
which, together with the slow initial decay, is characteristic of standard
adiabatic evolution in a uniformly dense medium. Assuming that a reported
absorption-line redshift of 1.02 is correct, the apparent isotropic energy of
6.7 x 10**53 erg is reduced by a factor of ~ 200 in the jet model, and the
initial half-opening angle is ~ 6 deg. GRB 991216 is the third good example of
a jet-like afterglow (following GRB 990123 and GRB 990510), supporting a trend
in which the apparently most energetic gamma-ray events have the narrowest
collimation and a uniform ISM environment. This, plus the absence of evidence
for supernovae associated with jet-like afterglows, suggests that these events
may originate from a progenitor in which angular momentum plays an important
role but a massive stellar envelope or wind does not, e.g., the coalescence of
a compact binary.Comment: 19 pages, accepted by The Astrophysical Journa
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