112 research outputs found
INTEGRAL and Swift/XRT observations of IGR~J16460+0849
IGR J16460+0849 is reported in the 3rd IBIS catalog with the shortest
exposure of about 10 ks among all the detected sources, which makes it the most
interesting target to be investigated with a deeper exposure. We analyze all
available observations carried out by the International Gamma-Ray Astrophysics
Laboratory (INTEGRAL) on the unidentified source IGR J16460+0849. The data
were processed by using the latest version OSA 7.0. In addition we analyze also
all the available Swift/XRT data on this source. We find that IGR J16460+0849
has a detection significance of ~ 4.4 sigma in the 20-100 keV band during the
observational period between March 2003 and September 2004, when it was exposed
by ~ 19 ks. Thereafter the source was not detected anymore, despite an
additional exposure of ~ 271 ks. This suggests a flux variability on a
timescale of years. The spectral analysis shows that the IBIS/ISGRI data are
well presented by a power-law shape, with a photon index obtained as
1.45+-0.86. So far, the source has been detected consistently by IBIS/ISGRI in
the subsequent observations and in the adjacent energy bands. We have analyzed
the Swift/XRT observations on IGR J16460+0849 as well, and found no source
inside the IBIS/ISGRI error circle. The non-detection during the Swift/XRT
observation is consistent with the source having a variable nature.Comment: published in A&
INTEGRAL and Swift/XRT observations of IGR J19405-3016
IGR J19405-3016 is reported in the 3rd IBIS catalog as being one of its
lowest significance sources (~4.6 sigma under an exposure of about 371 ks).
This leads to a caveat in multi-wavelength study although the source was
identified in optical as a Seyfert 1. The currently increased INTEGRAL data
stimulate us to investigate the reality of this source. We analyze all
available observations carried out by INTEGRAL and Swift on IGR J19405-3016. We
find that IGR J19405-3016 has a detection significance of ~ 9.4 sigma in the
20-60 keV band during the observational period between March 2003 and March
2008. Thus confirms a real source detection reported previously. The source
position and error location are therefore updated. The source is found to be
constant over years at the hard X-rays. Over the three XRT observations, the
source flux varied by up to 39% from the average, and the spectrum is generally
soft. The combined XRT/ISGRI spectrum is well fitted with a simple power law
model (photon index 2.11+-0.03). Such a photon index is well consistent with
the mean value 1.98 (dispersion 0.27) as obtained from Swift/BAT AGN samples at
14-195 keV. The spectral slope of IGR J19405-3016 is larger than the average
spectral slope found by Molina et al. (2009). A similar discrepancy is found
with the results of Beckmann et al. (2009) regarding Seyfert 1 AGNs. A possible
explanation of this simple spectral description may be that the low level of
the column density allows for the `true' spectrum to appear at soft X-rays as
well.Comment: Astronomy and Astrophysics, in pres
Deconvolving the pre-Himalayan Indian margin – tales of crustal growth and destruction
The metamorphic core of the Himalaya is composed of Indian cratonic rocks with two distinct crustal affinities that are defined by radiogenic isotopic geochemistry and detrital zircon age spectra. One is derived predominantly from the Paleoproterozoic and Archean rocks of the Indian cratonic interior and is either represented as metamorphosed sedimentary rocks of the Lesser Himalayan Sequence (LHS) or as slices of the distal cratonic margin. The other is the Greater Himalayan Sequence (GHS) whose provenance is less clear and has an enigmatic affinity. Here we present new detrital zircon Hf analyses from LHS and GHS samples spanning over 1000 kilometers along the orogen that respectively show a striking similarity in age spectra and Hf isotope ratios. Within the GHS, the zircon age populations at 2800–2500 Ma, 1800 Ma, 1000 Ma and 500 Ma can be ascribed to various Gondwanan source regions; however, a pervasive and dominant Tonian age population (∼860–800 Ma) with a variably enriched radiogenic Hf isotope signature (εHf = 10 to -20) has not been identified from Gondwana or peripheral accreted terranes. We suggest this detrital zircon age population was derived from a crustal province that was subsequently removed by tectonic erosion. Substantial geologic evidence exists from previous studies across the Himalaya supporting the Cambro-Ordovician Kurgiakh Orogeny. We propose the tectonic removal of Tonian lithosphere occurred prior to or during this Cambro-Ordovician episode of orogenesis in a similar scenario as is seen in the modern Andean and Indonesian orogenies, wherein tectonic processes have removed significant portions of the continental lithosphere in a relatively short amount of time. This model described herein of the pre-Himalayan northern margin of Greater India highlights the paucity of the geologic record associated with the growth of continental crust. Although the continental crust is the archive of Earth history, it is vital to recognize the ways in which preservation bias and destruction of continental crust informs geologic models
A complete sample of 21-cm absorbers at z~1.3: Giant Metrewave Radio Telescope Survey Using MgII Systems
We present the results of a systematic Giant Metrewave Radio Telescope (GMRT)
survey of 21-cm absorption in a representative and unbiased sample of 35 strong
MgII systems in the redshift range: zabs~1.10-1.45, 33 of which have W_r>1 \AA.
The survey using ~400hrs of telescope time has resulted in 9 new 21-cm
detections and stringent 21-cm optical depth upper limits (median 3-sigma
optical depth per 10 km/s of 0.017) for the remaining 26 systems. This is by
far the largest number of 21-cm detections from any single survey of
intervening absorbers. Prior to our survey no intervening 21-cm system was
known in the above redshift range and only one system was known in the redshift
range 0.7<z<1.5. We discuss the relation between the detectability of 21-cm
absorption and various properties of UV absorption lines. We show that if MgII
systems are selected with the following criteria, MgII doublet ratio <1.3 and
W_r(MgI)/W_r(MgII)>0.3, then a detection rate of 21-cm absorption up to 90% can
be achieved. We estimate n_{21}, the number per unit redshift of 21-cm
absorbers with W_r(Mg(II)>W_o and integrated optical depth Tau_{21}>Tau_o and
show that n_{21} decreases with increasing redshift. In particular, for W_o=1.0
\AA and Tau_o>0.3 km\s, n_{21} falls by a factor 4 from =0.5 to =1.3. The
evolution seems to be stronger for stronger MgII systems. Using a subsample of
systems for which high frequency VLBA images are available, we show that the
effect is not related to the structure of the background radio sources and is
most probably due to the evolution of the cold neutral medium filling factor in
MgII systems. We find no correlation between the velocity spread of the 21-cm
absorption feature and W_r(MgII) at z~1.3.Comment: 22 pages, 8 tables, 12 figures, accepted for publication in MNRA
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
The provenance of Taklamakan desert sand
Sand migration in the vast Taklamakan desert within the Tarim Basin (Xinjiang Uyghur Autonomous region, PR China) is governed by two competing transport agents: wind and water, which work in diametrically opposed directions. Net aeolian transport is from northeast to south, while fluvial transport occurs from the south to the north and then west to east at the northern rim, due to a gradual northward slope of the underlying topography. We here present the first comprehensive provenance study of Taklamakan desert sand with the aim to characterise the interplay of these two transport mechanisms and their roles in the formation of the sand sea, and to consider the potential of the Tarim Basin as a contributing source to the Chinese Loess Plateau (CLP). Our dataset comprises 39 aeolian and fluvial samples, which were characterised by detrital-zircon U–Pb geochronology, heavy-mineral, and bulk-petrography analyses. Although the inter-sample differences of all three datasets are subtle, a multivariate statistical analysis using multidimensional scaling (MDS) clearly shows that Tarim desert sand is most similar in composition to rivers draining the Kunlun Shan (south) and the Pamirs (west), and is distinctly different from sediment sources in the Tian Shan (north). A small set of samples from the Junggar Basin (north of the Tian Shan) yields different detrital compositions and age spectra than anywhere in the Tarim Basin, indicating that aeolian sediment exchange between the two basins is minimal. Although river transport dominates delivery of sand into the Tarim Basin, wind remobilises and reworks the sediment in the central sand sea. Characteristic signatures of main rivers can be traced from entrance into the basin to the terminus of the Tarim River, and those crossing the desert from the south to north can seasonally bypass sediment through the sand sea. Smaller ephemeral rivers from the Kunlun Shan end in the desert and discharge their sediment there. Both river run-off and wind intensity are strongly seasonal, their respective transport strength and opposing directions maintain the Taklamakan in its position and topography
A search for damped Lyman-alpha systems towards radio-loud quasars I: The optical survey
We present the results from the optical component of a survey for damped
Lyman-alpha systems (DLAs) towards radio-loud quasars. Our quasar sample is
drawn from the Texas radio survey with the following primary selection
criteria: z_em > 2.4, optical magnitudes B < 22 and 365 MHz flux density S_365
> 400 mJy. We obtained spectra for a sample of 45 QSOs with the William
Herschel Telescope, Very Large Telescope and Gemini-North, resulting in a
survey redshift path Delta z = 38.79. We detect nine DLAs and one sub-DLA with
a mean absorption redshift = 2.44. The DLA number density is n(z) =
0.23^{+0.11}_{-0.07}, in good agreement with the value derived for DLAs
detected in the Sloan Digital Sky Survey at this redshift. The DLA number
density of our sample is also in good agreement with optically-complete
radio-selected samples, supporting previous claims that n(z) is not
significantly affected by dust obscuration bias. We present N(HI) column
density determinations and metal line equivalent width measurements for all our
DLAs. The low frequency flux density selection criterion used for the quasar
sample implies that all absorbers will be suitable for follow-up absorption
spectroscopy in the redshifted HI 21 cm line. A following paper (Kanekar et
al.) will present HI 21 cm absorption studies of, and spin temperature
determinations for, our DLA sample.Comment: Accepted for publication in the MNRA
GLAST: Understanding the High Energy Gamma-Ray Sky
We discuss the ability of the GLAST Large Area Telescope (LAT) to identify,
resolve, and study the high energy gamma-ray sky. Compared to previous
instruments the telescope will have greatly improved sensitivity and ability to
localize gamma-ray point sources. The ability to resolve the location and
identity of EGRET unidentified sources is described. We summarize the current
knowledge of the high energy gamma-ray sky and discuss the astrophysics of
known and some prospective classes of gamma-ray emitters. In addition, we also
describe the potential of GLAST to resolve old puzzles and to discover new
classes of sources.Comment: To appear in Cosmic Gamma Ray Sources, Kluwer ASSL Series, Edited by
K.S. Cheng and G.E. Romer
The Evolution of Compact Binary Star Systems
We review the formation and evolution of compact binary stars consisting of
white dwarfs (WDs), neutron stars (NSs), and black holes (BHs). Binary NSs and
BHs are thought to be the primary astrophysical sources of gravitational waves
(GWs) within the frequency band of ground-based detectors, while compact
binaries of WDs are important sources of GWs at lower frequencies to be covered
by space interferometers (LISA). Major uncertainties in the current
understanding of properties of NSs and BHs most relevant to the GW studies are
discussed, including the treatment of the natal kicks which compact stellar
remnants acquire during the core collapse of massive stars and the common
envelope phase of binary evolution. We discuss the coalescence rates of binary
NSs and BHs and prospects for their detections, the formation and evolution of
binary WDs and their observational manifestations. Special attention is given
to AM CVn-stars -- compact binaries in which the Roche lobe is filled by
another WD or a low-mass partially degenerate helium-star, as these stars are
thought to be the best LISA verification binary GW sources.Comment: 105 pages, 18 figure
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