1,330 research outputs found
Detector and Telescope Development for ProtoEXIST and Fine Beam Measurements of Spectral Response of CZT Detectors
We outline our plan to develop ProtoEXIST, a balloon-borne prototype
experiment for the Energetic X-ray Imaging Survey Telescope (EXIST) for the
Black Hole Finder Probe. EXIST will consist of multiple wide-field hard X-ray
coded-aperture telescopes. The current design of the EXIST mission employs two
types of telescope systems: high energy telescopes (HETs) using CZT detectors,
and low energy telescopes (LETs) using Si detectors. With ProtoEXIST, we will
develop and demonstrate the technologies required for the EXIST HETs. As part
of our development efforts, we also present recent laboratory measurements of
the spectral response and efficiency variation of imaging CZT detectors on a
fine scale (~0.5 mm). The preliminary results confirm the need for multi-pixel
readouts and small inter-pixel gaps to achieve uniform spectral response and
high detection efficiency across detectors.Comment: 9 pages, 12 figures, 1 table, appears in SPIE 2005 proceedings (5898:
UV, X-ray, and Gamma-ray Space Instrumentation for Astronomy XIV
The 1 keV to 200 keV X-ray Spectrum of NGC 2992 and NGC 3081
The Seyfert 2 galaxies NGC 2992 and NGC 3081 have been observed by INTEGRAL
and Swift. We report about the results and the comparison of the spectrum above
10 keV based on INTEGRAL IBIS/ISGRI, Swift/BAT, and BeppoSAX/PDS. A spectrum
can be extracted in the X-ray energy band ranging from 1 keV up to 200 keV.
Although NGC 2992 shows a complex spectrum below 10 keV, the hard tail observed
by various missions exhibits a slope with photon index = 2, independent on the
flux level during the observation. No cut-off is detectable up to the detection
limit around 200 keV. In addition, NGC 3081 is detected in the INTEGRAL and
Swift observation and also shows an unbroken Gamma = 1.8 spectrum up to 150
keV. These two Seyfert galaxies give further evidence that a high-energy
cut-off in the hard X-ray spectra is often located at energies E_C >> 100 keV.
In NGC 2992 a constant spectral shape is observed over a hard X-ray luminosity
variation by a factor of 11. This might indicate that the physical conditions
of the emitting hot plasma are constant, while the amount of plasma varies, due
to long-term flaring activity.Comment: 8 pages, 4 figures, accepted for publication in Ap
Minimizing the stochasticity of halos in large-scale structure surveys
In recent work (Seljak, Hamaus and Desjacques 2009) it was found that
weighting central halo galaxies by halo mass can significantly suppress their
stochasticity relative to the dark matter, well below the Poisson model
expectation. In this paper we extend this study with the goal of finding the
optimal mass-dependent halo weighting and use -body simulations to perform a
general analysis of halo stochasticity and its dependence on halo mass. We
investigate the stochasticity matrix, defined as , where is the dark matter
overdensity in Fourier space, the halo overdensity of the -th
halo mass bin and the halo bias. In contrast to the Poisson model
predictions we detect nonvanishing correlations between different mass bins. We
also find the diagonal terms to be sub-Poissonian for the highest-mass halos.
The diagonalization of this matrix results in one large and one low eigenvalue,
with the remaining eigenvalues close to the Poisson prediction ,
where is the mean halo number density. The eigenmode with the lowest
eigenvalue contains most of the information and the corresponding eigenvector
provides an optimal weighting function to minimize the stochasticity between
halos and dark matter. We find this optimal weighting function to match linear
mass weighting at high masses, while at the low-mass end the weights approach a
constant whose value depends on the low-mass cut in the halo mass function.
Finally, we employ the halo model to derive the stochasticity matrix and the
scale-dependent bias from an analytical perspective. It is remarkably
successful in reproducing our numerical results and predicts that the
stochasticity between halos and the dark matter can be reduced further when
going to halo masses lower than we can resolve in current simulations.Comment: 17 pages, 14 figures, matched the published version in Phys. Rev. D
including one new figur
Proposed Next Generation GRB Mission: EXIST
A next generation Gamma Ray Burst (GRB) mission to follow the upcoming Swift
mission is described. The proposed Energetic X-ray Imaging Survey Telescope,
EXIST, would yield the limiting (practical) GRB trigger sensitivity, broad-band
spectral and temporal response, and spatial resolution over a wide field. It
would provide high resolution spectra and locations for GRBs detected at GeV
energies with GLAST. Together with the next generation missions
Constellation-X, NGST and LISA and optical-survey (LSST) telescopes, EXIST
would enable GRBs to be used as probes of the early universe and the first
generation of stars. EXIST alone would give ~10-50" positions (long or short
GRBs), approximate redshifts from lags, and constrain physics of jets, orphan
afterglows, neutrinos and SGRs.Comment: 4 pages, 4 figures. Presented at Woods Hole GRB Conf. (2001); to
appear in AIP Conf. Pro
Observing GRBs with EXIST
We describe the Energetic X-ray Imaging Survey Telescope EXIST, designed to carry out a sensitive all-sky survey in the 10 keV – 600 keV band. The primary goal of EXIST is to find black holes in the local and distant universe. EXIST also traces cosmic star formation via gamma-ray bursts and gamma-ray lines from radioactive elements ejected by supernovae and novae
Swift BAT Survey of AGN
We present the results of the analysis of the first 9 months of data of the
Swift BAT survey of AGN in the 14-195 keV band. Using archival X-ray data or
follow-up Swift XRT observations, we have identified 129 (103 AGN) of 130
objects detected at |b|> 15 deg and with significance >4.8 sigma. One source
remains unidentified. These same X-ray data have allowed measurement of the
X-ray properties of the objects. We fit a power law to the log N - log S
distribution, and find the slope to be 1.42+/-0.14. Characterizing the
differential luminosity function data as a broken power law, we find a break
luminosity log L_*(erg/s) = 43.85+/-0.26, a low luminosity power law slope
a=0.84^{+0.16}_{-0.22}, and a high luminosity power law slope
b=2.55^{+0.43}_{-0.30}, similar to the values that have been reported based on
INTEGRAL data. We obtain a mean photon index 1.98 in the 14-195 keV band, with
an rms spread of 0.27. Integration of our luminosity function gives a local
volume density of AGN above 10^{41} erg/s of 2.4x10^{-3}/Mpc^3, which is about
10% of the total luminous local galaxy density above M_*=-19.75. We have
obtained X-ray spectra from the literature and from Swift XRT follow-up
observations. These show that the distribution of log n_H is essentially flat
from n_H=10^{20}/cm^{2} to 10^{24}/cm^2, with 50% of the objects having column
densities of less than 10^{22}/cm^{2}. BAT Seyfert galaxies have a median
redshift of 0.03, a maximum log luminosity of 45.1, and approximately half have
log n_H > 22.Comment: 15 pages, 15 figures, 2 tables; to appear in the Astrophysical
Journal, July 10, 2008, v. 68
Cosmic X-ray background and Earth albedo Spectra with Swift/BAT
We use Swift/BAT Earth occultation data at different geomagnetic latitudes to
derive a sensitive measurement of the Cosmic X-ray background (CXB) and of the
Earth albedo emission in the 15--200 keV band. We compare our CXB spectrum with
recent (INTEGRAL, BeppoSAX) and past results (HEAO-1) and find good agreement.
Using an independent measurement of the CXB spectrum we are able to confirm our
results. This study shows that the BAT CXB spectrum has a normalization
~8(+/-3)% larger than the HEAO-1 measurement. The BAT accurate Earth albedo
spectrum can be used to predict the level of photon background for satellites
in low Earth and mid inclination orbits.Comment: Accepted for publication in the Astrophysical Journal. 38 Pages, 16
Figures, 2 Table
XSS J00564+4548 and IGR J00234+6141 -- new cataclysmic variables from RXTE and INTEGRAL all sky surveys
We present the results of optical identification of two X-ray sources from
RXTE and INTEGRAL all sky surveys: XSS J00564+4548 and IGR J00234+6141. Using
the optical data from Russian-Turkish 1.5-m Telescope (RTT150) and SWIFT X-ray
observations, we show that these sources most probably are intermediate polars,
i.e. binary systems with accreting white dwarfs with not very strong magnetic
field (<~10 MG). Periodical oscillations of optical emission with periods 480 s
and 570 s were found. We argue that these periods most probably correspond to
the rotating periods of the white dwarfs in these systems. Further optical
observations scheduled at RTT150 will allow to study the parameters of these
systems in more detail.Comment: 5 pages, 8 figures, accepted for publication in Astronomy Letter
Characteristics of EGRET Blazars in the VLBA Imaging and Polarimetry Survey (VIPS)
We examine the radio properties of EGRET-detected blazars observed as part of
the VLBA Imaging and Polarimetry Survey (VIPS). VIPS has a flux limit roughly
an order of magnitude below the MOJAVE survey and most other samples that have
been used to study the properties of EGRET blazars. At lower flux levels, radio
flux density does not directly correlate with gamma-ray flux density. We do
find that the EGRET-detected blazars tend to have higher brightness
temperatures, greater core fractions, and possibly larger than average jet
opening angles. A weak correlation is also found with jet length and with
polarization. All of the well-established trends can be explained by
systematically larger Doppler factors in the gamma-ray loud blazars, consistent
with the measurements of higher apparent velocities found in monitoring
programs carried out at radio frequencies above 10 GHz.Comment: 20 pages, 7 figures, accepted to Ap
- …