[Show abstract][Hide abstract] ABSTRACT: We present a timing analysis of X Persei (X Per) using observations made between 1998 and 2010 with the Proportional Counter
Array (PCA) onboard the Rossi X-ray Timing Explorer (RXTE) and with the INTEGRAL Soft Gamma-Ray Imager (ISGRI). All pulse arrival times obtained from the RXTE-PCA observations are phase-connected and a timing solution is obtained using these arrival times. We update the long-term
pulse frequency history of the source by measuring its pulse frequencies using RXTE-PCA and ISGRI data. From the RXTE-PCA data, the relation between the frequency derivative and X-ray flux suggests accretion via the companion's stellar wind.
However, the detection of a transient quasi-periodic oscillation feature, peaking at ∼0.2 Hz, suggests the existence of an
accretion disc. We find that double-break models fit the average power spectra well, which suggests that the source has at
least two different accretion flow components dominating the overall flow. From the power spectrum of frequency derivatives,
we measure a power-law index of ∼− 1, which implies that, on short time-scales, disc accretion dominates over noise, while
on time-scales longer than the viscous time-scales, the noise dominates. From pulse profiles, we find a correlation between
the pulse fraction and the count rate of the source.
Monthly Notices of the Royal Astronomical Society 10/2014; 444(1):457-465. DOI:10.1093/mnras/stu1351 · 5.23 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present timing analysis of RXTE-PCA and INTEGRAL-ISGRI observations of X
Per between 1998 and 2010. All pulse arrival times obtained from the RXTE-PCA
observations are phase connected and timing solution is obtained using these
arrival times. We update long term pulse frequency history of the source by
measuring pulse frequencies from RXTE-PCA and INTEGRAL-ISGRI data. From
RXTE-PCA data, the relation between frequency derivative and X-ray flux
suggests accretion via companion's stellar wind. On the other hand, detection
of the transient QPO feature peaking at $\sim 0.2$ Hz suggests the existence of
an accretion disc. From the average intensity power spectra, we find that
double break model fits the power spectra well which suggests that the source
has at least two different accretion flow components dominating the overall
flow. From the power spectra of frequency derivatives, we measure a power law
index of $\sim -1$ which implies that on short time scales disc accretion
dominates over noise, while on time scales longer than the viscous time scales
the noise dominates. From pulse profiles, we find a correlation between pulse
fraction and count rate of the source.
[Show abstract][Hide abstract] ABSTRACT: The type I X-ray burster 1E1724-3045 located in the globular cluster Terzan 2 was observed by the Rossi X-ray Timing Explorer (RXTE) while it was in a Island state, displaying both intense flickering and a hard Comptonized spectrum. No statistically significant High Frequency Quasi-Periodic Oscillations (HFQPOs) were detected. A 3σ upper limit on the fractional Root Mean Squared (RMS) of ∼ 2.5% has thus been derived in the 5–30 keV band. Attenuation of the QPO signal in a scattering corona of optical depth ∼ 3 as inferred from the spectral analysis could provide an explanation for the non-detection of HFQPOs from 1E1724-3045.
Advances in Space Research 07/2013; 25(s 3–4). DOI:10.1016/S0273-1177(99)00763-2 · 1.35 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this document, we describe the scientific potential of blazar
observations with a X-ray polarimetry mission like GEMS (Gravity and
Extreme Magnetism SMEX). We describe five blazar science investigations
that such a mission would enable: (i) the structure and the role of
magnetic fields in AGN jets, (ii) analysis of the polarization of the
synchrotron X-ray emission from AGN jets, (iii) discrimination between
synchrotron self-Compton and external Compton models for blazars with
inverse Compton emission in the X-ray band, (iv) a precision study of
the polarization properties of the X-ray emission from Cen-A, (v) tests
of Lorentz Invariance based on X-ray polarimetric observations of
blazars. We conclude with a discussion of a straw man observation
program and recommended accompanying multiwavelength observations.
[Show abstract][Hide abstract] ABSTRACT: We examine the expected X-ray polarization properties of neutron-star X-ray
sources of various types, e.g., accretion and rotation powered pulsars,
magnetars, and low-mass X-ray binaries. We summarize the model calculations
leading to these expected properties. We describe how a comparison of these
with their observed properties, as inferred from GEMS data, will probe the
essential dynamical, electromagnetic, plasma, and emission processes in
neutron-star binaries, discriminate between models of these processes, and
constrain model parameters. An exciting goal is the first observational
demonstration in this context of the existence of vacuum resonance, a
fundamental quantum electrodynamical phenomenon first described in the 1930s.
[Show abstract][Hide abstract] ABSTRACT: We present here a summary of the scientific goals behind the Gravity and
Extreme Magnetism SMEX (GEMS) X-ray polarimetry mission's black hole (BH)
observing program. The primary targets can be divided into two classes:
stellar-mass galactic BHs in accreting binaries, and super-massive BHs in the
centers of active galactic nuclei (AGN). The stellar-mass BHs can in turn be
divided into various X-ray spectral states: thermal-dominant (disk), hard
(radio jet), and steep power-law (hot corona). These different spectral states
are thought to be generated by different accretion geometries and emission
mechanisms. X-ray polarization is an ideal tool for probing the geometry around
these BHs and revealing the specific properties of the accreting gas.
[Show abstract][Hide abstract] ABSTRACT: The Gravity and Extreme Magnetism Small Explorer (GEMS) will realize its scientific objectives through high sensitivity linear X-ray polarization measurements in the 2-10 keV band. The GEMS X-ray polarimeters, based on the photoelectric effect, provide a strong polarization response with high quantum efficiency over a broad band-pass by a novel implementation of the time projection chamber (TPC). This paper will discuss the basic principles of the TPC polarimeter and describe the details of the mechanical and electrical design of the GEMS flight polarimeter. We will present performance measurements from two GEMS engineering test units in response to polarized and unpolarized X-rays and before and after thermal and vibration tests performed to demonstrate that the design is at a technology readiness level 6 (TRL-6).
[Show abstract][Hide abstract] ABSTRACT: 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
The Astrophysical Journal 08/2012; 757(2). DOI:10.1088/0004-637X/757/2/159 · 6.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The Cosmic X-ray background (CXB) contains significant information about
the energy content of the universe. However, the total X-ray background
flux itself is still a matter of some controversy. A recent compilation
of 10 CXB 2-10 keV flux measurements by Moretti et al. (2003) found
statistical errors of 5%, with some values differing by up to 25%. Here
we present preliminary results of a new technique to measure the X-ray
background, using the dark side of the moon as an occulting shutter
within the RXTE PCA field of view. This technique has the benefit of
measuring the total X-ray background emission, rather than concentrating
on the point-like sources. Observations were carefully designed to allow
the moon to pass over the center of the PCA field of view, obscuring
about 20% of the total field of view. Multiple observations throughout
the year 2010, at different celestial locations, allow improved
statistics and a measure of cosmic variance. In this work, we show the
first results from this technique and compare to previous results, with
the goal of achieving better than 5% statistical errors in the 2-10 keV
[Show abstract][Hide abstract] ABSTRACT: We use data from twice-weekly scans of the Galactic bulge by the Rossi
X-ray Timing Explorer (RXTE) to study fast x-ray transients, including
Supergiant Fast X-ray Transients (SFXTs). Using up to 11 years of data
at this cadence for some sources, we explore the duty cycle of activity
in these sources and the timescales of correlation; lack of correlation
on timescales of a week or more is characteristic of the SFXTs but not
other classes of x-ray binary. We confirm the orbital periods reported
by other groups for SFXTs SAX J1818.6-1703 and IGR J17544-2619, but not
for XTE J1739-302. The bulge source XTE J1743-363 remains a mystery,
showing a combination of fast variability and a long-term decline in
activity. Using a spectrum of the companion taken with the Andalucia
Faint Object Spectrograph and Camera (ALFOSC) on the Nordic Optical
Telescope in La Palma, Spain, we suggest that this system may be a
symbiotic neutron star binary rather than an SFXT.
[Show abstract][Hide abstract] ABSTRACT: We present the results of a long-term observation campaign of the
extragalactic wind-accreting black-hole X-ray binary LMC X-1, using the
Proportional Counter Array on the Rossi X-Ray Timing Explorer (RXTE). The
observations show that LMC X-1's accretion disk exhibits an anomalous
temperature-luminosity relation. We use deep archival RXTE observations to show
that large movements across the temperature-luminosity space occupied by the
system can take place on time scales as short as half an hour. These changes
cannot be adequately explained by perturbations that propagate from the outer
disk on a viscous timescale. We propose instead that the apparent disk
variations reflect rapid fluctuations within the Compton up-scattering coronal
material, which occults the inner parts of the disk. The expected relationship
between the observed disk luminosity and apparent disk temperature derived from
the variable occultation model is quantitatively shown to be in good agreement
with the observations. Two other observations support this picture: an inverse
correlation between the flux in the power-law spectral component and the fitted
inner disk temperature, and a near-constant total photon flux, suggesting that
the inner disk is not ejected when a lower temperature is observed.
The Astrophysical Journal 07/2011; 742(2). DOI:10.1088/0004-637X/742/2/75 · 6.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report the most recent results of our analysis of X-ray monitoring of new
Galactic Black Hole (BH) candidates XTE J1752-225 and MAXI J1659-152 performed
by Rossi X-ray Timing Explorer (RXTE). We investigate various aspects of the
RXTE data including energy and power spectra, variability energy distribution
and phase lags between soft and hard energy bands. The sources generally
exhibit the spectral states and evolution expected from an accreting stellar
mass BH. The energy distribution of different variability components show that
the aperiodic noise has a spectrum consistently softer with respect to the
total rms spectrum, while the spectrum of the quasi-periodic (QPO) features is
harder. Particularly interesting behavior is observed in phase lags. Namely,
XTE J1753-223 shows that QPO in the hard band lags the QPO in the soft band.
This is opposite to what was previously reported in other bright BH candidates
and also found in our analysis from MAXI J1659-152. We report the results of BH
mass estimations using the spectral-timing correlation scaling technique.
Namely, we obtain the BH masses of 9.5 +/-1.5 and 20+/-3 solar masses for XTE
J1752-223 and MAXI J1659-152 correspondingly.
[Show abstract][Hide abstract] ABSTRACT: RXTE observed of the X-ray source Swift J164449.3+573451 / GRB 110328A (GCN Circ. 11823, 11824, 11842; ATEL #3242, #3244, #3250) on 2011-03-30 at 04:55 UTC for an exposure of 1.8 ksec. The PCA light curve is characterized by strong variations on a time scale of hundreds of seconds. Variations are a factor of ~2.5x peak to peak (50-125 mCrab 2-10 keV), with r.m.s. variability of 30%. A power spectrum shows red noise stochastic variability (power law with index f-2), although the mHz end of the spectrum is poorly characterized.
[Show abstract][Hide abstract] ABSTRACT: Following the detection of increased gamma ray emission from a location consistent with the Crab nebula by AGILE (Atel #2855) and FERMI/LAT (Atel #2861) RXTE performed a dedicated pointing observation on September 24, 2010 14:45 UT. Analysis of the PCU2 data has not shown any significant changes in the the overall spectral properties. A fit with the absorbed (N_H = 0.34 * 1022 cm-2 fixed) power law yielded the photon index of 2.128+/-0.002 and normalization at 1 keV of 10.96+/-0.06 which translates to unabsorbed fluxes of 3.7e-8 erg/cm2/s between 2 and 30 keV.
[Show abstract][Hide abstract] ABSTRACT: We report on the discovery and monitoring observations of a new galactic black hole candidate XTE J1752-223 by Rossi X-ray Timing Explorer (RXTE). The new source appeared on the X-ray sky on October 21 2009 and was active for almost 8 months. Phenomenologically, the source exhibited the low-hard/high-soft spectral state bi-modality and the variability evolution during the state transition that matches standard behavior expected from a stellar mass black hole binary. We model the energy spectrum throughout the outburst using a generic Comptonization model assuming that part of the input soft radiation in the form of a black body spectrum gets reprocessed in the Comptonizing medium. We follow the evolution of fractional root-mean-square (RMS) variability in the RXTE/PCA energy band with the source spectral state and conclude that broad band variability is strongly correlated with the source hardness (or Comptonized fraction). We follow changes in the energy distribution of rms variability during the low-hard state and the state transition and find further evidence that variable emission is strongly concentrated in the power-law spectral component. We discuss the implication of our results to the Comptonization regimes during different spectral states. Correlations of spectral and variability properties provide measurements of the BH mass and distance to the source. The spectral-timing correlation scaling technique applied to the RXTE observation during the hard-to-soft state transition indicates a mass of the BH in XTE J1752-223 between 8 and 11 solar masses and a distance to the source about 3.5 kiloparsec. Comment: Accepted for publication in The Astrophysical Journal
The Astrophysical Journal 08/2010; 723(2). DOI:10.1088/0004-637X/723/2/1817 · 6.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The Gravity and Extreme Magnetism Small Explorer (GEMS) is an astrophysical observatory dedicated to X-ray polarimetry (2-10 keV) and is being developed for launch in 2014. To maximize the polarization sensitivity of the observatory, GEMS uses polarimeters based on the photoelectric effect with a gas micropattern time projection chamber (TPC). We describe the TPC polarimeter concept and the details of the GEMS implementation, including factors that affect the ultimate polarization sensitivity, including quantum efficiency, modulation factor, systematic errors, and background.
Proceedings of SPIE - The International Society for Optical Engineering 07/2010; DOI:10.1117/12.857736 · 0.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Regarding the newly detected transient source SWIFT J1729.9-3437 (ATEL #2747), we report an X-ray position by Swift and the detection of possible pulsations at 530 sec by RXTE. Swift observed the source with the X-ray Telescope (XRT) starting at 2010-07-20 at 15:00 UT for a total observation of 2.3 ks. The image shows a bright point source at coordinates R.A.=262.53746, Decl.=-34.61239 (J2000) with an uncertainty of 1.7" (90% confidence).
[Show abstract][Hide abstract] ABSTRACT: We report the discovery of a new X-ray transient, designated SWIFT
J1729.9-3437, based on Swift BAT all-sky monitoring and RXTE PCA
monitoring scans of the galactic center region. Swift BAT detected
a new source starting on 2010-07-13 at a count rate (15-50 keV) of
0.0033 ± 0.0009 ct/cm2/s (~15 mCrab) and rising steadily to
0.0055 ± 0.0013 ct/cm2/s (~25 mCrab) on 2010-07-18. The
cumulative significance for the past eight days is 8.1 sigma.
[Show abstract][Hide abstract] ABSTRACT: Following the report of a new XMM slew transient, XMMSL1 J171900.4-353217 (Read & Saxton, ATEL #2607), we report the likely association to a transient designated XTE J1719-356, a faint transient source discovered in March 2010 by regular RXTE PCA monitoring observations of the galactic center. The monitoring observations detected enhanced emission from that region in March 2010. Follow-up dedicated PCA scanning and pointed observations 2010-03-10 detected a source.