67 research outputs found
Advances in the RXTE Proportional Counter Array Calibration: Nearing the Statistical Limit
During its 16 years of service the Rossi X-ray Timing Explorer (RXTE) mission
has provided an extensive archive of data, which will serve as a primary source
of high cadence observations of variable X-ray sources for fast timing studies.
It is, therefore, very important to have the most reliable calibration of RXTE
instruments. The Proportional Counter Array (PCA) is the primary instrument
on-board RXTE which provides data in 3-50 keV energy range with sub-millisecond
time resolution in up to 256 energy channels. In 2009 the RXTE team revised the
response residual minimization method used to derive the parameters of the PCA
physical model. The procedure is based on the residual minimization between the
model spectrum for Crab nebula emission and a calibration data set consisting
of a number of spectra from the Crab and the on-board Am241 calibration source,
uniformly covering the whole RXTE mission operation period. The new method led
to a much more effective model convergence and allowed for better understanding
of the PCA energy-to-channel relationship. It greatly improved the response
matrix performance. We describe the new version of the RXTE/PCA response
generator PCARMF v11.7 (HEASOFT Release 6.7) along with the corresponding
energy-to-channel conversion table (verson e05v04) and their difference from
the previous releases of PCA calibration. The new PCA response adequately
represents the spectrum of the calibration sources and successfully predicts
the energy of the narrow iron emission line in Cas-A throughout the RXTE
mission.Comment: 8 pages, 4 figures, accepted for publication in Ap
The Cosmic X-ray Background -- IRAS galaxy Correlation and the Local X-ray Volume Emissivity
We have cross-correlated the galaxies from the IRAS 2 Jy redshift survey
sample and the 0.7 Jy projected sample with the all-sky cosmic X-ray background
(CXB) map obtained from the HEAO-1 A2 experiment. We have developed a
formulation to model the cross-correlation between CXB surface brightness and
galaxy counts. This includes the effects of source clustering and the X-ray --
far infrared luminosity correlation. Using an X-ray flux limited sample of
AGNs, which has IRAS 60 measurements, we have estimated the
contribution of the AGN component to the observed CXB -- IRAS galaxy count
correlations in order to see whether there is an excess component, i.e.
contribution from low X-ray luminosity sources. We have applied both the
analytical approach and Monte-Carlo simulations for the estimations. Our
estimate of the local X-ray volume emissivity in the 2 -- 10 keV band is
consistent with the value expected from the luminosity function of AGNs alone
and the contribution to the local volume emissivity from lower-luminosity
sources is small.Comment: 23 pages 4 PS figures upon request ([email protected]),
LaTeX (AAS Macros Included
Absolute Timing of the Crab Pulsar with RXTE
We have monitored the phase of the main X-ray pulse of the Crab pulsar with
the Rossi X-ray Timing Explorer (RXTE) for almost eight years, since the start
of the mission in January 1996. The absolute time of RXTE's clock is
sufficiently accurate to allow this phase to be compared directly with the
radio profile. Our monitoring observations of the pulsar took place bi-weekly
(during the periods when it was at least 30 degrees from the Sun) and we
correlated the data with radio timing ephemerides derived from observations
made at Jodrell Bank. We have determined the phase of the X-ray main pulse for
each observation with a typical error in the individual data points of 50 us.
The total ensemble is consistent with a phase that is constant over the
monitoring period, with the X-ray pulse leading the radio pulse by
0.0102+/-0.0012 period in phase, or 344+/-40 us in time. The error estimate is
dominated by a systematic error of 40 us in the radio data, arising from
uncertainties in the variable amount of pulse delay due to interstellar
scattering and instrumental calibration. The statistical error is 0.00015
period, or 5 us. The separation of the main pulse and interpulse appears to be
unchanging at time scales of a year or less, with an average value of
0.4001+/-0.0002 period. There is no apparent variation in these values with
energy over the 2-30 keV range. The lag between the radio and X-ray pulses may
be constant in phase (rotational) or constant in time (linear pathlength). We
are not (yet) able to distinguish between these two interpretations.Comment: 11 pages, 2 figure
An Optimized Photoelectron Track Reconstruction Method for Photoelectric X-Ray Polarimeters
We present a data processing algorithm for angular reconstruction and event selection applied to 2-D photoelectron track images from X-ray polarimeters. The method reconstructs the initial emission angle of a photoelectron from the initial portion of the track, which is obtained by continuously cutting a track until the image moments or number of pixels fall below tunable thresholds. In addition, event selection which rejects round tracks quantified with eccentricity and circularity is performed so that polarimetry sensitivity considering a trade-off between the modulation factor and signal acceptance is maximized. The modulation factors with applying track selection are 26.6 0.4, 46.1 0.4, 62.3 0.4, and 61.8 0.3% at 2.7, 4.5, 6.4, and 8.0 keV, respectively, using the same data previously analyzed by Iwakiri et al. (2016), where the corresponding numbers are 26.90.4, 43.40.4, 54.40.3, and 59.1 0.3%. The method improves polarimeter sensitivity by 5%10% at the high energy end of the band previously presented (Iwakiri et al. 2016)
X-ray and gamma-ray spectra and variability of the black-hole candidate GX 339-4
We analyse five observations of the X-ray binary GX 339-4 by the soft
gamma-ray OSSE detector on board CGRO simultaneous with either Ginga or RXTE
observations. The source was bright during four of them, with the luminosity of
L ~ 10^{37} erg/s and the spectrum typical for hard states of accreting black
holes, and it was in an off state during the fifth one, with L ~ 10^{35} erg/s.
Our broad-band spectral fits show the mean electron energy of electrons in the
Comptonizing plasma decreasing with increasing luminosity within the hard
(bright) state. For the observation with the best statistics at soft
gamma-rays, approximately 1/4 of energy in the Comptonizing plasma is probably
carried by non-thermal electrons. Then, considering the efficiency of
Comptonized hybrid synchrotron emission allows us to obtain an upper limit on
the strength of the magnetic field in the X-ray source. Furthermore, this
synchrotron emission is capable of producing the optical spectrum observed in
an optically-high state of GX 339-4. In the off state, the hard X-ray spectrum
is consistent with being dominated by bremsstrahlung. The unusually strong Fe K
alpha line observed by the PCA during that state is found not to be intrinsic
to the source but to originate mostly in the Galactic diffuse emission.Comment: 12 pages, 7 figures (2 in colour). Accepted for publication in MNRA
An Efficient, FPGA-Based, Cluster Detection Algorithm Implementation for a Strip Detector Readout System in a Time Projection Chamber Polarimeter
A fundamental challenge in a spaceborne application of a gas-based Time Projection Chamber (TPC) for observation of X-ray polarization is handling the large amount of data collected. The TPC polarimeter described uses the APV-25 Application Specific Integrated Circuit (ASIC) to readout a strip detector. Two dimensional photoelectron track images are created with a time projection technique and used to determine the polarization of the incident X-rays. The detector produces a 128x30 pixel image per photon interaction with each pixel registering 12 bits of collected charge. This creates challenging requirements for data storage and downlink bandwidth with only a modest incidence of photons and can have a significant impact on the overall mission cost. An approach is described for locating and isolating the photoelectron track within the detector image, yielding a much smaller data product, typically between 8x8 pixels and 20x20 pixels. This approach is implemented using a Microsemi RT-ProASIC3-3000 Field-Programmable Gate Array (FPGA), clocked at 20 MHz and utilizing 10.7k logic gates (14% of FPGA), 20 Block RAMs (17% of FPGA), and no external RAM. Results will be presented, demonstrating successful photoelectron track cluster detection with minimal impact to detector dead-time
The Segmented Aperture Interferometric Nulling Testbed (SAINT) III: Control Systems Analysis and Preliminary Results
This work presents a detailed current performance analysis for the telescope, pointing, and coronagraph com- ponent subsystems of the Segmented Aperture Interferometric Nulling Testbed (SAINT). The project pairs an active segmented mirror with the Visible Nulling Coronagraph (VNC) towards demonstrating capabilities for the future space observatories needed to directly detect and characterize Earth-sized worlds around nearby stars. We describe approaches to optimize subsystem wavefront sensing and control parameters, summarizing relevant scal- ing relations between these parameters, residual errors, and observed contrast measurements. Preliminary results from diagnostic testing under various control states are presented along with intermediate contrast measurements towards demonstrating the full system
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