3,641 research outputs found
Chandra LETGS spectroscopy of the Quasar MR2251-178 and its warm absorber
We present an analysis of our Chandra Low Energy Transmission Grating
Spectrometer (LETGS) observation of the quasar MR2251-178. The warm absorber of
MR2251-178 is well described by a hydrogen column density, N_H~2x10^21 cm^-2,
and an ionization parameter log(xi)~0.6. We find in the spectrum weak evidence
for narrow absorption lines from Carbon and Nitrogen which indicate that the
ionized material is in outflow. We note changes (in time) of the absorption
structure in the band (0.6-1) keV (around the UTAs plus the OVII and OVIII
K-edges) at different periods of the observation. We measure a (0.1-2) keV flux
of 2.58x10^-11 ergs cm^-2 s^-1. This flux implies that the nuclear source of
MR2251-178 is in a relatively low state. No significant variability is seen in
the light curve. We do not find evidence for an extra cold material in the line
of sight, and set an upper limit of N_H~1.2x10^20 cm^-2. The X-ray spectrum
does not appear to show evidence for dusty material, though an upper limit in
the neutral carbon and oxygen column densities can only be set to N_CI~2x10^19
cm^-2 and N_OI~9x10^19 cm^-2, respectively.Comment: 42 pages, 12 figures, Accepted in Apj. Typo in abstract (ver2): "We
do not find evidence for an extra...
Comments on black holes I: The possibility of complementarity
We comment on a recent paper of Almheiri, Marolf, Polchinski and Sully who
argue against black hole complementarity based on the claim that an infalling
observer 'burns' as he approaches the horizon. We show that in fact
measurements made by an infalling observer outside the horizon are
statistically identical for the cases of vacuum at the horizon and radiation
emerging from a stretched horizon. This forces us to follow the dynamics all
the way to the horizon, where we need to know the details of Planck scale
physics. We note that in string theory the fuzzball structure of microstates
does not give any place to 'continue through' this Planck regime. AMPS argue
that interactions near the horizon preclude traditional complementarity. But
the conjecture of 'fuzzball complementarity' works in the opposite way: the
infalling quantum is absorbed by the fuzzball surface, and it is the resulting
dynamics that is conjectured to admit a complementary description.Comment: 34 pages, 6 figures, v3: clarifications & references adde
The information paradox: A pedagogical introduction
The black hole information paradox is a very poorly understood problem. It is
often believed that Hawking's argument is not precisely formulated, and a more
careful accounting of naturally occurring quantum corrections will allow the
radiation process to become unitary. We show that such is not the case, by
proving that small corrections to the leading order Hawking computation cannot
remove the entanglement between the radiation and the hole. We formulate
Hawking's argument as a `theorem': assuming `traditional' physics at the
horizon and usual assumptions of locality we will be forced into mixed states
or remnants. We also argue that one cannot explain away the problem by invoking
AdS/CFT duality. We conclude with recent results on the quantum physics of
black holes which show the the interior of black holes have a `fuzzball'
structure. This nontrivial structure of microstates resolves the information
paradox, and gives a qualitative picture of how classical intuition can break
down in black hole physics.Comment: 38 pages, 7 figures, Latex (Expanded form of lectures given at CERN
for the RTN Winter School, Feb 09), typo correcte
What can the information paradox tell us about the early Universe?
In recent years we have come to understand how the information paradox is
resolved in string theory. The huge entropy of black
holes is realized by an explicit set of horizon sized `fuzzball' wavefunctions.
The wavefunction of a collapsing shell spreads relatively quickly over this
large phase space of states, invalidating the classical black hole geometry the
shell would have created. We argue that a related effect may occur in the early
Universe. When matter is crushed to high densities we can access a similarly
large phase space of gravitational `fuzzball' solutions. While we cannot
estimate specific quantities at this point, a qualitative analysis suggests
that spreading over phase space creates an extra `push' expanding the Universe
to larger volumes.Comment: 6 pages, 3 figures (Essay awarded second prize in the Gravity
Research Foundation essay competition 2012
An X-ray view of Mrk 705: A borderline narrow-line Seyfert 1 galaxy
Mrk 705 exhibits optical properties of both narrow- and broad-line Seyfert 1
galaxies. We examine the X-ray properties of this borderline object utilising
proprietary and public data from Chandra, ASCA, ROSAT and RXTE, spanning more
than twelve years. Though long-term flux variability from the pointed
observations appears rather modest (about 3 times), we do find examples of rare
large amplitude outbursts in the RXTE monitoring data. There is very little
evidence of long-term spectral variability as the low- and high-energy spectra
appear constant with time. A 6.4 keV emission line is detected in the ASCA
spectra of Mrk 705, but not during the later, higher flux state Chandra
observation. However, the upper limit on the equivalent width of a line in the
Chandra spectrum is consistent with a constant-flux emission line and a
brighter continuum, suggesting that the line is emitted from distant material
such as the putative torus. Overall, the X-ray properties of Mrk 705 appear
typical of BLS1 activity.Comment: 7 pages, 7 figures. Accepted for publication in A&A (Research Note
Aspidella: un fósil de cuerpo blando ediacárico de la Arenisca de Jodhpur (Supergrupo de Marwar), área de Sursagar, Jodhpur, Rajasthan occidental, India
This paper describes well-preserved Aspidella remains, the Ediacaran body fossils from the Jodhpur Sandstone of the Marwar Supergroup in Sursagar area, Jodhpur, western Rajasthan, India. They show distinct morphological features previously described in other famous Ediacaran fossil sites, such as the Fermuse Formation of Newfoundland, South Australia and White Sea of RussiaSe describen en este trabajo los restos bien conservados de Aspidella, fósiles de cuerpo blando ediacáricos de la Arenisca de Jodhpur (Supergrupo de Marwar) el área de Sursagar, Jodhpur, al oeste de Rajasthan, India. Éstos muestran características morfológicas características descritas anteriormente en otros famosos yacimientos fósiles ediacáricos, como los de la Formación de Fermuse (Terranova), Australia meridional y el Mar Blanco de Rusia
Steel Industry-Wastes as New Materials for Road Construction
A large number of steel plants have been set up in India and they are producing several million tonnes of iron and steel. However, the production of iron and steel is always associated with the generation of waste materials like air cooled slag, steel slag, granulated slag and flyash etc. These materials are posing serious problems of disposal and causing environmental pollution. Road Construction is one such sector where these materials can be utilised in bulk. CRRI started a few projects sponsored by Steel Authority of India Limited (SAIL), and all these materials were tested on a big scale in CRRI laboratory. Based on the success of laboratory results, various combinations were tried in the field and finally a road was constructed. This road has been monitored regularly for surface unevenness and pavement structural strength. This paper gives the results of extensive laboratory investigations and performance of the road
Role of Shrubs in Rangeland Based Livestock Production System in Western Rajasthan, India
Rangeland covers ~ 51% of the terrestrial surface of the Earth, and supports half of the world‟s livestock. More than 80 % of forage consumed by domestic livestock is supplied by rangelands in Africa and Asia. Hot arid regions of India cover an area of 31.7 million hectares and out of which Rajasthan state constitutes ~ 61.8 % of the entire area. The crop production in this area is low, unstable and risky. Livestock sustains arid farming systems by virtue of their potential to reduce risk, alleviate poverty, important providers of nutrients and traction for growing crops in smallholder systems and the imparting sustainability of household.
The hot arid region of Rajasthan has about 29.08 million livestock. Pastures and grazing lands, residential area and wayside grazing lands, crop by-products and residues, fodder crops etc. are the major feed resources. For the small ruminants and camels, the rangelands are the major source of feed supply. Shrubs are vital component of rangeland vegetation and cover \u3e 70 % of desert landscapes from Aravalli to the international border of the Indian part of Thar desert. They are an important source of feed and assure feed availability in a drought situation. They have remarkable morpho-physiological adaptations to different land forms and survive under harsh edapho-climatic conditions. During recent past, a rapid decline population of browse shrubs is noticed due to their over exploitation, destruction of natural habitat, expending irrigation facilities, increasing agricultural mechanization and changes in land use pattern. In the present paper key species, fodder production and nutritive value of shrubs of hot arid rangelands are discussed
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