403 research outputs found
The Kepler Light Curves of AGN: A Detailed Analysis
We present a comprehensive analysis of 21 light curves of Type 1 AGN from the
Kepler spacecraft. First, we describe the necessity and development of a
customized pipeline for treating Kepler data of stochastically variable sources
like AGN. We then present the light curves, power spectral density functions
(PSDs), and flux histograms. The light curves display an astonishing variety of
behaviors, many of which would not be detected in ground-based studies,
including switching between distinct flux levels. Six objects exhibit PSD
flattening at characteristic timescales which roughly correlate with black hole
mass. These timescales are consistent with orbital timescales or freefall
accretion timescales. We check for correlations of variability and
high-frequency PSD slope with accretion rate, black hole mass, redshift and
luminosity. We find that bolometric luminosity is anticorrelated with both
variability and steepness of the PSD slope. We do not find evidence of the
linear rms-flux relationships or lognormal flux distributions found in X-ray
AGN light curves, indicating that reprocessing is not a significant contributor
to optical variability at the 0.1-10% level.Comment: 39 pages including 2 appendices. Accepted for Publication in the
Astrophysical Journal, with higher resolution figure
MEASURING THE MASS OF 4UO900-40 DYNAMICALLY
Accurate measurements of neutron star masses are needed to constrain the equation of state of neutron star matter - of importance to both particle physics and the astrophysics of neutron stars - and to identify the evolutionary track of the progenitor stars that form neutron stars. The best measured values of the mass of 4UO900-40 (= Vela XR-l), 1.86 +/- 0.16 Msun (Barziv et al. 2001) and 1.93 +/- 0.20 Msun (Abubekerov et al. 2004), make it a leading candidate for the most massive neutron star known. The direct relationship between the maximum mass of neutron stars and the equation of state of ultra-dense matter makes 4UO900-40 an important neutron star mass to determine accurately. The confidence interval on previous mass estimates, obtained from observations that include parameters determined by non-dynamical methods, are not small enough to significantly restrict possible equations of state. We describe here a purely dynamical method for determining the mass of 4UO900-40, an X-ray pulsar, using the reprocessed UV pulses emitted by its BO.5Ib companion. One can derive the instantaneous radial velocity of each component by simultaneous X-ray and UV observations at the two quadratures of the system. The Doppler shift caused by the primary's rotational velocity and the illumination pattern of the X-rays on the primary, two of the three principal contributors to the uncertainty on the derived mass of the neutron star, almost exactly cancel by symmetry in this method. A heuristic measurement of the mass of 4UO900-40 using observations obtained previously with the High Speed Photometer on HST is given in Appendix A
Canonical Timing and Spectral Behavior of LMC X-3 in the Low/Hard State
We present results from three observations with the Rossi X-ray Timing
Explorer (RXTE) of LMC X-3, obtained while the source was in an extended
'low/hard' state. The data reveal a hard X-ray spectrum which is well fit by a
pure power law with photon index Gamma=1.69+/-0.02, with a source luminosity at
50 kpc of 5-16x10^{36}erg/s (2--10 keV). Strong broad-band (0.01-100 Hz) time
variability is also observed, with fractional rms amplitude 40+/-4%, plus a
quasi-periodic oscillation (QPO) peak at 0.46+/-0.02 Hz with rms amplitude
\~14%. This is the first reported observation in which the full canonical
low/hard state behavior (pure hard power law spectrum combined with strong
broad-band noise and QPO) for LMC X-3 is seen. We reanalyze several archival
RXTE observations of LMC X-3 and derive consistent spectral and timing
parameters, and determine the overall luminosity variation between high/soft
and low/hard states. The timing and spectral properties of LMC X-3 during the
recurrent low/hard states are quantitatively similar to that typically seen in
the Galactic black hole candidates.Comment: 5 pages, 3 figures, accepted for ApJ Letter
KSwAGS: A Swift X-ray and UV Survey of the Kepler Field. I
We introduce the first phase of the Kepler-Swift Active Galaxies and Stars
survey (KSwAGS), a simultaneous X-ray and UV survey of ~6 square degrees of the
Kepler field using the Swift XRT and UVOT. We detect 93 unique X-ray sources
with S/N>3 with the XRT, of which 60 have observed UV counterparts. We use the
Kepler Input Catalog (KIC) to obtain the optical counterparts of these sources,
and construct the X-ray to optical flux ratio as a first approximation of the
classification of the source. The survey produces a mixture of stellar sources,
extragalactic sources, and sources which we are not able to classify with
certainty. We have obtained optical spectra for thirty of these targets, and
are conducting an ongoing observing campaign to fully identify the sample. For
sources classified as stellar or AGN with certainty, we construct SEDs using
the 2MASS, UBV and GALEX data supplied for their optical counterparts by the
KIC, and show that the SEDs differ qualitatively between the source types, and
so can offer a method of classification in absence of a spectrum. Future papers
in this series will analyze the timing properties of the stars and AGN in our
sample separately. Our survey provides the first X-ray and UV data for a number
of known variable stellar sources, as well as a large number of new X-ray
detections in this well-studied portion of the sky. The KSwAGS survey is
currently ongoing in the K2 ecliptic plane fields.Comment: Accepted for publication in the Astronomical Journal. 19 pages, 8
figures, 3 table
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
X-Ray and UV Orbital Phase Dependence in LMC X-3
The black-hole binary LMC X-3 is known to be variable on time scales of days
to years. We investigate X-ray and ultraviolet variability in the system as a
function of the 1.7 day binary phase using a 6.4 day observation with the Rossi
X-ray Timing Explorer (RXTE) from December 1998. An abrupt 14% flux decrease,
lasting nearly an entire orbit, is followed by a return to previous flux
levels. This behavior occurs twice, at nearly the same binary phase, but it is
not present in consecutive orbits. When the X-ray flux is at lower intensity, a
periodic amplitude modulation of 7% is evident in data folded modulo the
orbital period. The higher intensity data show weaker correlation with phase.
This is the first report of X-ray variability at the orbital period of LMC X-3.
Archival RXTE observations of LMC X--3 during a high flux state in December
1996 show similar phase dependence. An ultraviolet light curve obtained with
the High Speed Photometer aboard the Hubble Space Telescope shows orbital
modulation consistent with that in the optical, caused by the ellipsoidal
variation of the spatially deformed companion.
The X-ray spectrum of LMC X-3 can be acceptably represented by a
phenomenological disk-black-body plus a power law. Changes in the spectrum of
LMC X-3 during our observations are compatible with earlier observations during
which variations in the 2-10 keV flux are tracked closely by the disk geometry
spectral model parameter.Comment: 11 pages, 7 figures, ApJ in pres
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