294 research outputs found
Analyzing X-ray variability by State Space Models
In recent years, autoregressive models have had a profound impact on the
description of astronomical time series as the observation of a stochastic
process. These methods have advantages compared with common Fourier techniques
concerning their inherent stationarity and physical background. If
autoregressive models are used, however, it has to be taken into account that
real data always contain observational noise often obscuring the intrinsic time
series of the object. We apply the technique of a Linear State Space Model
which explicitly models the noise of astronomical data and allows to estimate
the hidden autoregressive process. As an example, we have analysed a sample of
Active Galactic Nuclei (AGN) observed with EXOSAT and found evidence for a
relationship between the relaxation timescale and the spectral hardness.Comment: 4 pages, Latex, uses Kluwer Style file crckapb.cls To appear in Proc.
of Astronomical Time Series, Tel Aviv, 199
Quantifying Rapid Variability in Accreting Compact Objects
I discuss some practical aspects of the analysis of millisecond time
variability X-ray data obtained from accreting neutron stars and black holes.
First I give an account of the statistical methods that are at present commonly
applied in this field. These are mostly based on Fourier techniques. To a large
extent these methods work well: they give astronomers the answers they need.
Then I discuss a number of statistical questions that astronomers don't really
know how to solve properly and that statisticians may have ideas about. These
questions have to do with the highest and the lowest frequency ranges
accessible in the Fourier analysis: how do you determine the shortest time
scale present in the variability, how do you measure steep low-frequency noise.
The point is stressed that in order for any method that resolves these issues
to become popular, it is necessary to retain the capabilities the current
methods already have in quantifying the complex, concurrent variability
processes characteristic of accreting neutron stars and black holes.Comment: To be published in the Proceedings of "Statistical Challenges in
Modern Astronomy II", University Park PA, USA, June 199
Updating the orbital ephemeris of Her X-1; rate of decay and eccentricity of the orbit
We present an update of the orbital ephemeris of the binary X-ray pulsar Her
X-1 and determine an improved value for the rate of orbital decay. In addition,
we report the first measurement of the orbital eccentricity. We have analyzed
pulse timing data of Her X-1 from X-ray observations by RXTE (Rossi X-Ray
Timing Explorer) and INTEGRAL over the period 1996-2007. Accurate pulse arrival
times were determined from solar system bary-centered photon arrival times by
generating pulse profiles averaged over appropriately short integration times.
Applying pulse phase connection techniques, it was possible to determine
sufficiently accurate local ephemeris data for seven observation periods
distributed over 12 years. Combining the new local T90 values with historical
values from the literature we update the orbital ephemeris of Her X-1 to T90 =
MJD 46359.871940(6) and Porb = 1.700167590(2) d and measure a continuous change
of the orbital period of dPorb/dt = -(4.85 +/- 0.13) x 10-11 s/s. For the first
time, a value for the eccentricity of the orbit of Her X-1 is measured to be e
= (4.2 +/- 0.8) x 10-4.Comment: 7 pages, 4 figures, accepted by A&A on 30.03.200
Discovery of orbital decay in SMC X-1
The results are reported of three observations of the binary X ray pulsar SMC X-1 with the Ginga satellite. Timing analyses of the 0.71 s X ray pulsations yield Doppler delay curves which, in turn, provide the most accurate determination of the SMC X-1 orbital parameters available to date. The orbital phase of the 3.9 day orbit is determined in May 1987, Aug. 1988, and Aug. 1988 with accuracies of 11, 1, and 3.5 s, respectively. These phases are combined with two previous determinations of the orbital phase to yield the rate of change in the orbital period: P sub orb/P sub orb = (-3.34 + or - 0.023) x 10(exp -6)/yr. An interpretation of this measurement and the known decay rate for the orbit of Cen X-3 is made in the context of tidal evolution. Finally, a discussion is presented of the relation among the stellar evolution, orbital decay, and neutron star spinup time scales for the SMC X-1 system
Correlation between X-ray flux and rotational acceleration in Vela X-1
The results of a search for correlations between X-ray flux and angular acceleration for the accreting binary pulsar Vela X-1 are presented. Results are based on data obtained with the Hakucho satellite during the interval 1982 to 1984. In undertaking this correlation analysis, it was necessary to modify the usual statistical method to deal with conditions imposed by generally unavoidable satellite observing constraints, most notably a mismatch in sampling between the two variables. The results are suggestive of a correlation between flux and the absolute value of the angular acceleration, at a significance level of 96 percent. The implications of the methods and results for future observations and analysis are discussed
Study of the cyclotron feature in MXB 0656-072
We have monitored a type II outburst of the Be/X-ray binary MXB 0656−072 in a series of pointed RXTE observations during October through December 2003. The source spectrum shows a cyclotron resonance scattering feature at 32.8 +0.5
−0.4 keV, corresponding to a magnetic field strength of 3.67
+0.06 −0.04 × 10 12 G and is stable through the outburst and over the pulsar spin phase. The pulsar, with an average pulse period of 160.4 ± 0.4s,shows a spin-up of 0.45 s over the duration of the outburst. From optical data, the source distance is estimated to be 3.9 ± 0.1 kpc and this is used to estimate the X-ray luminosity and a theoretical prediction of the pulsar spin-up during the outburst
Two ~35 day clocks in Her X-1: evidence for neutron star free precession
We present evidence for the existence of two ~35 day clocks in the Her X-1/HZ
Her binary system. ~35 day modulations are observed 1) in the Turn-On cycles
with two on- and two off-states, and 2) in the changing shape of the pulse
profiles which re-appears regularly. The two ways of counting the 35 day cycles
are generally in synchronization. This synchronization did apparently break
down temporarily during the long Anomalous Low (AL3) which Her X-1 experienced
in 1999/2000, in the sense that there must have been one extra Turn-On cycle.
Our working hypothesis is that there are two clocks in the system, both with a
period of about ~35 days: precession of the accretion disk (the less stable
"Turn-On clock") and free precession of the neutron star (the more stable
"Pulse profile clock"). We suggest that free precession of the neutron star is
the master clock, and that the precession of the accretion disk is basically
synchronized to that of the neutron star through a feed-back mechanism in the
binary system. However, the Turn-On clock can slip against its master when the
accretion disk has a very low inclination, as is observed to be the case during
AL3. We take the apparent correlation between the histories of the Turn-Ons, of
the Anomalous Lows and of the pulse period evolution, with a 5 yr
quasi-periodicity, as evidence for strong physical interaction and feed-back
between the major components in the system. We speculate that the 5 yr (10 yr)
period is either due to a corresponding activity cycle of HZ Her or a natural
ringing period of the physical system of coupled components. The question
whether free precession really exists in neutron stars is of great importance
for the understanding of matter with supra-nuclear density.Comment: 6 pages, 5 figures, accepted for publication by A&
Warped accretion discs and the long periods in X-ray binaries
Precessing accretion discs have long been suggested as explanations for the
long periods observed in a variety of X-ray binaries, most notably Her X-1/HZ
Her. We show that an instability of the disc's response to the radiation
reaction force from the illumination by the central source can cause the disc
to tilt out of the orbital plane and precess in something like the required
manner. The rate of precession and disc tilt obtained for realistic values of
system parameters compare favourably with the known body of data on X-ray
binaries with long periods. We explore other possible types of behaviour than
steadily precessing discs that might be observable in systems with somewhat
different parameters. At high luminosities, the inner disc tilts through more
than 90 degrees, i.e. it rotates counter to the usual direction, which may
explain the torque reversals in systems such as 4U 1626-67.Comment: submitted to MNRAS, 17-Dec-97, revised submit 2-Nov-98. 15 pages
LaTeX, 11 postscript figures in-tex
- …