32 research outputs found
The Glacier Complexes of the Mountain Massifs of the North-West of Inner Asia and their Dynamics
The subject of this paper is
the glaciation of the mountain massifs
Mongun-Taiga, Tavan-Boghd-Ola, Turgeni-
Nuru, and Harhira-Nuru. The glaciation is
represented mostly by small forms that
sometimes form a single complex of domeshaped
peaks. According to the authors,
the modern glaciated area of the mountain
massifs is 21.2 km2 (Tavan-Boghd-Ola),
20.3 km2 (Mongun-Taiga), 42 km2 (Turgeni-
Nuru), and 33.1 km2 (Harhira-Nuru).
The area of the glaciers has been shrinking
since the mid 1960’s. In 1995–2008, the rate
of reduction of the glaciers’ area has grown
considerably: valley glaciers were rapidly
degrading and splitting; accumulation
of morainic material in the lower parts
of the glaciers accelerated. Small glaciers
transformed into snowfields and rock
glaciers. There has been also a degradation
of the highest parts of the glaciers and the
collapse of the glacial complexes with a
single zone of accumulation into isolated
from each other glaciers. Reduced snow
cover area has led to a rise in the firn
line and the disintegration of a common
accumulation area of the glacial complex.
In the of the Mongun-Taiga massif, in 1995–
2008, the firn line rose by 200–300 m. The
reduction of the glaciers significantly lagged
behind the change in the position of the
accumulation area boundary. In the past two
years, there has been a significant recovery
of the glaciers that could eventually lead to
their slower degradation or stabilization of
the glaciers in the study area
Propagation of cosmic-ray nucleons in the Galaxy
We describe a method for the numerical computation of the propagation of
primary and secondary nucleons, primary electrons, and secondary positrons and
electrons. Fragmentation and energy losses are computed using realistic
distributions for the interstellar gas and radiation fields, and diffusive
reacceleration is also incorporated. The models are adjusted to agree with the
observed cosmic-ray B/C and 10Be/9Be ratios. Models with diffusion and
convection do not account well for the observed energy dependence of B/C, while
models with reacceleration reproduce this easily. The height of the halo
propagation region is determined, using recent 10Be/9Be measurements, as >4 kpc
for diffusion/convection models and 4-12 kpc for reacceleration models. For
convection models we set an upper limit on the velocity gradient of dV/dz < 7
km/s/kpc. The radial distribution of cosmic-ray sources required is broader
than current estimates of the SNR distribution for all halo sizes. Full details
of the numerical method used to solve the cosmic-ray propagation equation are
given.Comment: 15 pages including 23 ps-figures and 3 tables, latex2e, uses
emulateapj.sty (ver. of 11 May 1998, enclosed), apjfonts.sty, timesfonts.sty.
To be published in ApJ 1998, v.509 (December 10 issue). More details can be
found at http://www.gamma.mpe-garching.mpg.de/~aws/aws.html Some references
are correcte
Positrons from particle dark-matter annihilation in the Galactic halo: propagation Green's functions
We have made a calculation of the propagation of positrons from dark-matter
particle annihilation in the Galactic halo in different models of the dark
matter halo distribution using our 3D code, and present fits to our numerical
propagation Green's functions. We show that the Green's functions are not very
sensitive to the dark matter distribution for the same local dark matter energy
density. We compare our predictions with computed cosmic ray positron spectra
(``background'') for the ``conventional'' CR nucleon spectrum which matches the
local measurements, and a modified spectrum which respects the limits imposed
by measurements of diffuse Galactic gamma-rays, antiprotons, and positrons. We
conclude that significant detection of a dark matter signal requires favourable
conditions and precise measurements unless the dark matter is clumpy which
would produce a stronger signal. Although our conclusion qualitatively agrees
with that of previous authors, it is based on a more realistic model of
particle propagation and thus reduces the scope for future speculations.
Reliable background evaluation requires new accurate positron measurements and
further developments in modelling production and propagation of cosmic ray
species in the Galaxy.Comment: 8 pages, 6 ps-figures, 3 tables, uses revtex. Accepted for
publication in Physical Review D. More details can be found at
http://www.gamma.mpe-garching.mpg.de/~aws/aws.htm
Diffuse continuum gamma rays from the Galaxy
A new study of the diffuse Galactic gamma-ray continuum radiation is
presented, using a cosmic-ray propagation model which includes nucleons,
antiprotons, electrons, positrons, and synchrotron radiation. Our treatment of
the inverse Compton (IC) scattering includes the effect of anisotropic
scattering in the Galactic interstellar radiation field (ISRF) and a new
evaluation of the ISRF itself. Models based on locally measured electron and
nucleon spectra and synchrotron constraints are consistent with gamma-ray
measurements in the 30-500 MeV range, but outside this range excesses are
apparent. A harder nucleon spectrum is considered but fitting to gamma rays
causes it to violate limits from positrons and antiprotons. A harder
interstellar electron spectrum allows the gamma-ray spectrum to be fitted above
1 GeV as well, and this can be further improved when combined with a modified
nucleon spectrum which still respects the limits imposed by antiprotons and
positrons. A large electron/IC halo is proposed which reproduces well the
high-latitude variation of gamma-ray emission. The halo contribution of
Galactic emission to the high-latitude gamma-ray intensity is large, with
implications for the study of the diffuse extragalactic component and
signatures of dark matter. The constraints provided by the radio synchrotron
spectral index do not allow all of the <30 MeV gamma-ray emission to be
explained in terms of a steep electron spectrum unless this takes the form of a
sharp upturn below 200 MeV. This leads us to prefer a source population as the
origin of the excess low-energy gamma rays.Comment: Final version accepted for publication in The Astrophysical Journal
(vol. 537, July 10, 2000 issue); Many Updates; 20 pages including 49
ps-figures, uses emulateapj.sty. More details can be found at
http://www.gamma.mpe-garching.mpg.de/~aws/aws.htm
Secondary antiprotons and propagation of cosmic rays in the Galaxy and heliosphere
High-energy collisions of cosmic-ray nuclei with interstellar gas are
believed to be the mechanism producing the majority of cosmic ray antiprotons.
Due to the kinematics of the process they are created with a nonzero momentum;
the characteristic spectral shape with a maximum at ~2 GeV and a sharp decrease
towards lower energies makes antiprotons a unique probe of models for particle
propagation in the Galaxy and modulation in the heliosphere. On the other hand,
accurate calculation of the secondary antiproton flux provides a ``background''
for searches for exotic signals from the annihilation of supersymmetric
particles and primordial black hole evaporation. Recently new data with large
statistics on both low and high energy antiproton fluxes have become available
which allow such tests to be performed. We use our propagation code GALPROP to
calculate interstellar cosmic-ray propagation for a variety of models. We show
that there is no simple model capable of accurately describing the whole
variety of data: boron/carbon and sub-iron/iron ratios, spectra of protons,
helium, antiprotons, positrons, electrons, and diffuse gamma rays. We find that
only a model with a break in the diffusion coefficient plus convection can
reproduce measurements of cosmic-ray species, and the reproduction of primaries
(p, He) can be further improved by introducing a break in the primary injection
spectra. For our best-fit model we make predictions of proton and antiproton
fluxes near the Earth for different modulation levels and magnetic polarity
using a steady-state drift model of propagation in the heliosphere.Comment: Many Updates, 20 pages, 15 ps-figures, emulateapj5.sty. To be
published in ApJ v.564 January 10, 2002 issue. More details can be found at
http://www.gamma.mpe-garching.mpg.de/~aws/aws.htm
A combined model for the X-ray to gamma-ray emission of Cyg X-1
We use recent data obtained by three (OSSE, BATSE, and COMPTEL) of four
instruments on board the Compton Gamma Ray Observatory, to construct a model of
Cyg X-1 which describes its emission in a broad energy range from soft X-rays
to MeV gamma-rays self-consistently. The gamma-ray emission is interpreted to
be the result of Comptonization, bremsstrahlung, and positron annihilation in a
hot optically thin and spatially extended region surrounding the whole
accretion disk. For the X-ray emission a standard corona-disk model is applied.
We show that the Cyg X-1 spectrum accumulated by the CGRO instruments during a
~4 year time period between 1991 and 1995, as well as the HEAO-3 gamma1 and
gamma2 spectra can be well represented by our model. The derived parameters
match the observational results obtained from X-ray measurements.Comment: 11 pages including 6 ps-figures and 2 tables, latex2e, uses
emulateapj.sty (ver. of 18 Sep 96, enclosed), epsfig.sty, times.sty. To
appear in July 20, 1998 issue of ApJ (v.502
Fermi Large Area Telescope Constraints on the Gamma-ray Opacity of the Universe
The Extragalactic Background Light (EBL) includes photons with wavelengths
from ultraviolet to infrared, which are effective at attenuating gamma rays
with energy above ~10 GeV during propagation from sources at cosmological
distances. This results in a redshift- and energy-dependent attenuation of the
gamma-ray flux of extragalactic sources such as blazars and Gamma-Ray Bursts
(GRBs). The Large Area Telescope onboard Fermi detects a sample of gamma-ray
blazars with redshift up to z~3, and GRBs with redshift up to z~4.3. Using
photons above 10 GeV collected by Fermi over more than one year of observations
for these sources, we investigate the effect of gamma-ray flux attenuation by
the EBL. We place upper limits on the gamma-ray opacity of the Universe at
various energies and redshifts, and compare this with predictions from
well-known EBL models. We find that an EBL intensity in the optical-ultraviolet
wavelengths as great as predicted by the "baseline" model of Stecker et al.
(2006) can be ruled out with high confidence.Comment: 42 pages, 12 figures, accepted version (24 Aug.2010) for publication
in ApJ; Contact authors: A. Bouvier, A. Chen, S. Raino, S. Razzaque, A.
Reimer, L.C. Reye
Detection of High-Energy Gamma-Ray Emission from the Globular Cluster 47 Tucanae with Fermi
Gamma-Ray Pulsar Bonanza
Most of the pulsars we know about were detected through their radio emission; a few are known to pulse gamma rays but were first detected at other wavelengths (see the Perspective by
Halpern
). Using the Fermi Gamma-Ray Space Telescope,
Abdo
et al.
(p.
840
, published online 2 July; see the cover) report the detection of 16 previously unknown pulsars based on their gamma-ray emission alone. Thirteen of these coincide with previously unidentified gamma-ray sources, solving the 30-year-old mystery of their identities. Pulsars are fast-rotating neutron stars. With time they slow down and cease to radiate; however, if they are in a binary system, they can have their spin rates increased by mass transfer from their companion stars, starting a new life as millisecond pulsars. In another study,
Abdo
et al.
(p.
845
) report the detection of gamma-ray emission from the globular cluster 47 Tucanae, which is coming from an ensemble of millisecond pulsars in the cluster's core. The data imply that there are up to 60 millisecond pulsars in 47 Tucanae, twice as many as predicted by radio observations. In a further companion study,
Abdo
et al.
(p.
848
, published online 2 July) searched Fermi Large Area Telescope data for pulsations from all known millisecond pulsars outside of stellar clusters, finding gamma-ray pulsations for eight of them. Their properties resemble those of other gamma-ray pulsars, suggesting that they share the same basic emission mechanism. Indeed, both sets of pulsars favor emission models in which the gamma rays are produced in the outer magnetosphere of the neutron star
Gamma-ray and radio properties of six pulsars detected by the fermi large area telescope
We report the detection of pulsed Îł-rays for PSRs J0631+1036, J0659+1414, J0742-2822, J1420-6048, J1509-5850, and J1718-3825 using the Large Area Telescope on board the Fermi Gamma-ray Space Telescope (formerly known as GLAST). Although these six pulsars are diverse in terms of their spin parameters, they share an important feature: their Îł-ray light curves are (at least given the current count statistics) single peaked. For two pulsars, there are hints for a double-peaked structure in the light curves. The shapes of the observed light curves of this group of pulsars are discussed in the light of models for which the emission originates from high up in the magnetosphere. The observed phases of the Îł-ray light curves are, in general, consistent with those predicted by high-altitude models, although we speculate that the Îł-ray emission of PSR J0659+1414, possibly featuring the softest spectrum of all Fermi pulsars coupled with a very low efficiency, arises from relatively low down in the magnetosphere. High-quality radio polarization data are available showing that all but one have a high degree of linear polarization. This allows us to place some constraints on the viewing geometry and aids the comparison of the Îł-ray light curves with high-energy beam models
ERRATUM: "FERMI DETECTION OF Îł-RAY EMISSION FROM THE M2 SOFT X-RAY FLARE ON 2010 JUNE 12" (2012, ApJ, 745, 144)
Due to an error at the publisher, the times given for the major tick marks in the X-axis in Figure 1 of the published article are incorrect. The correctly labeled times should be "00:52:00," "00:54:00," ... , and "01:04:00." The correct version of Figure 1 and its caption is shown below. IOP Publishing sincerely regrets this error