348 research outputs found
EXIST: Mission Design Concept and Technology Program
The Energetic X-ray Imaging Survey Telescope (EXIST) is a proposed very large
area coded aperture telescope array, incorporating 8m^2 of pixellated Cd-Zn-Te
(CZT) detectors, to conduct a full-sky imaging and temporal hard x-ray (10-600
keV) survey each 95min orbit. With a sensitivity (5sigma, 1yr) of ~0.05mCrab
(10-150 keV), it will extend the ROSAT soft x-ray (0.5-2.5keV) and proposed
ROSITA medium x-ray (2-10 keV) surveys into the hard x-ray band and enable
identification and study of sources ~10-20X fainter than with the ~15-100keV
survey planned for the upcoming Swift mission. At ~100-600 keV, the ~1mCrab
sensitivity is 300X that achieved in the only previous (HEAO-A4, non-imaging)
all-sky survey. EXIST will address a broad range of key science objectives:
from obscured AGN and surveys for black holes on all scales, which constrain
the accretion history of the universe, to the highest sensitivity and
resolution studies of gamma-ray bursts it will conduct as the Next Generation
Gamma-Ray Burst mission. We summarize the science objectives and mission
drivers, and the results of a mission design study for implementation as a free
flyer mission, with Delta IV launch. Key issues affecting the telescope and
detector design are discussed, and a summary of some of the current design
concepts being studied in support of EXIST is presented for the wide-field but
high resolution coded aperture imaging and very large area array of imaging CZT
detectors. Overall mission design is summarized, and technology development
needs and a development program are outlined which would enable the launch of
EXIST by the end of the decade, as recommended by the NAS/NRC Decadal Survey.Comment: 14 pages, 8 figures, 2 tables. PDF file only. Presented at SPIE (Aug.
2002) and to appear in Proc. SPIE, vol. 485
High time resolution observation of the transient event of 5 March 1979
The detection of an intense gamma ray burst with the monitor proportional counter on the HEAO 2 spacecraft is discussed with particular emphasis on the measurement of the time of onset of the event. Based on the mean observed counting rate in the burst and assuming a sharp rise, the uncertainty in the burst onset is found to be + or - 220 microseconds. The time of occurrence was 57124.826908 + or - 0.000220 s UT on March 5, 1979, and the location of the HEAO 2 satellite at this time was latitude 22.15 deg, longitude -27.60 deg at an altitude of 525.0 km
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
Interstellar extinction and the distribution of stellar populations in the direction of the ultra-deep Chandra Galactic field
We studied the stellar population in the central 6.6x6.6arcmin,region of the
ultra-deep (1Msec) Chandra Galactic field - the "Chandra bulge field" (CBF)
approximately 1.5 degrees away from the Galactic Center - using the Hubble
Space Telescope ACS/WFC blue (F435W) and red (F625W) images. We mainly focus on
the behavior of red clump giants - a distinct stellar population, which is
known to have an essentially constant intrinsic luminosity and color. By
studying the variation in the position of the red clump giants on a spatially
resolved color-magnitude diagram, we confirm the anomalous total-to-selective
extinction ratio, as reported in previous work for other Galactic bulge fields.
We show that the interstellar extinction in this area is = 4 on
average, but varies significantly between ~3-5 on angular scales as small as 1
arcminute. Using the distribution of red clump giants in an
extinction-corrected color-magnitude diagram, we constrain the shape of a
stellar-mass distribution model in the direction of this ultra-deep Chandra
field, which will be used in a future analysis of the population of X-ray
sources. We also show that the adopted model for the stellar density
distribution predicts an infrared surface brightness in the direction of the
"Chandra bulge field" in good agreement (i.e. within ~15%) with the actual
measurements derived from the Spitzer/IRAC observations.Comment: 9 pages, 9 figures. Accepted for publication in A&
Transcribing Multi-Instrument Polyphonic Music With Hierarchical Eigeninstruments
This paper presents a general probabilistic model for transcribing single-channel music recordings containing multiple polyphonic instrument sources. The system requires no prior knowledge of the instruments present in the mixture (other than the number), although it can benefit from information about instrument type if available. In contrast to many existing polyphonic transcription systems, our approach explicitly models the individual instruments and is thereby able to assign detected notes to their respective sources. We use training instruments to learn a set of linear manifolds in model parameter space which are then used during transcription to constrain the properties of models fit to the target mixture. This leads to a hierarchical mixture-of-subspaces design which makes it possible to supply the system with prior knowledge at different levels of abstraction. The proposed technique is evaluated on both recorded and synthesized mixtures containing two, three, four, and five instruments each. We compare our approach in terms of transcription with (i.e., detected pitches must be associated with the correct instrument) and without source-assignment to another multi-instrument transcription system as well as a baseline non-negative matrix factorization (NMF) algorithm. For two-instrument mixtures evaluated with source-assignment, we obtain average frame-level F-measures of up to 0.52 in the completely blind transcription setting (i.e., no prior knowledge of the instruments in the mixture) and up to 0.67 if we assume knowledge of the basic instrument types. For transcription without source assignment, these numbers rise to 0.76 and 0.83, respectively
Thermal X-rays from Millisecond Pulsars: Constraining the Fundamental Properties of Neutron Stars
Abridged) We model the X-ray properties of millisecond pulsars (MSPs) by
considering hot spot emission from a weakly magnetized rotating neutron star
(NS) covered by an optically-thick hydrogen atmosphere. We investigate the
limitations of using the thermal X-ray pulse profiles of MSPs to constrain the
mass-to-radius () ratio of the underlying NS. The accuracy is strongly
dependent on the viewing angle and magnetic inclination. For certain systems,
the accuracy is ultimately limited only by photon statistics implying that
future X-ray observatories could, in principle, achieve constraints on
and hence the NS equation of state to better than 5%. We demonstrate that
valuable information regarding the basic properties of the NS can be extracted
even from X-ray data of fairly limited photon statistics through modeling of
archival spectroscopic and timing observations of the nearby isolated PSRs
J0030+0451 and J2124--3358. The X-ray emission from these pulsars is consistent
with the presence of a hydrogen atmosphere and a dipolar magnetic field
configuration, in agreement with previous findings for PSR J0437--4715. For
both MSPs, the favorable geometry allows us to place interesting limits on the
allowed of NSs. Assuming 1.4 M, the stellar radius is
constrained to be km and km (68% confidence) for PSRs
J0030+0451 and J2124--3358, respectively. We explore the prospects of using
future observatories such as \textit{Constellation-X} and \textit{XEUS} to
conduct blind X-ray timing searches for MSPs not detectable at radio
wavelengths due to unfavorable viewing geometry. Using the observational
constraints on the pulsar obliquities we are also able to place strong
constraints on the magnetic field evolution model proposed by Ruderman.Comment: 9 pages, 7 figures, published in the Astrophysical Journal (Volume
689, Issue 1, pp. 407-415
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
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