(rp200200a01) 0.829 Probability of
A Large ROSAT Survey of X-Ray Time
Variability in O Stars Allison S. Adelman1, David H. Cohen2 (1) Bryn Mawr College, (2)
O star X-ray emission comes from shock-heated gas present in their stellar winds; for B
stars, the situation is more uncertain, and their X-rays may be related to
magnetic fields, at least in some cases.The same may be true for certain O stars too.
Unstable mass flow
driven by the radiation field of the star produces strong shocks as faster
wind material impacts on slower material (Owocki, Castor, &
stochastically, so X-ray outputshould
be time variable.
X-ray variability should be due to evolution of individual
shock structures and due to time variability in the number of
individual shock zones in the wind.
ήThe X-ray variability
properties (timescales and spectral properties, as well as amplitude of variability) should tell us about the
properties and underlying physics of wind shocks.
At the simplest level, the amplitude of X-ray variability is proportional to the
square root of the number of individual shocks.
Two Sample Images of ROSAT fields
A pointed observation
of O star HD57060
O star HD57061 is also
present, but overlapping another point source, smeared out nearthe edge of the image
The support structure of the telescope is clearly visible
A pointed observation of a young open cluster of O stars (NGC
3 O stars are present in the central cluster; however many were too
indistinguishable to extract data
2 other O stars with good data are present in the field, as well as 1
more that is smeared out at the edge of the image
Sample Light Curves of O
- A Representative Non-Variable Star
- A Representative Variable O Star
O Star X-Rays
We searched the ROSAT PSPC archives of pointed
observations for O stars, observed both intentionally and serendipitously
Our sample includes 60
O stars in 86 separate observations, including many O stars not
previously reported on in the X-ray literature.
We extracted source counts for each O star, and performed several types of time
variability analyses on each object
dev. = 0.05509
Probability = 0.981
Reduced c2= 0.93
P = 45%
dev. = 0.05171
Probability = 0.829
Examples of K-S Test
ίDeviation of data & model scaledto 0.3
(rp200199a00) 0.981 Probability of Variability
We ran c2tests of the null hypothesis (of a constant
And we also calculated
the one-sided K-S statistic.
For longer pointings,
which were broken up into several observations, we calculated the K-S
statistic for each observation.
If the K-S test gave a
positive result (>90% probability of variability) we tested
the hard (E>0.5keV) and soft (E<0.5keV) spectral
Very few large,
systematic studies of X-ray variability among O stars.
It is generally accepted that O star X-ray emission is not variable, but this has not been
A few individual cases of X-ray variability have been detected:
- q1Ori C: periodic
modulation: a young magnetic rotator.
-z Pup: Very long observations have turned up very low level (~2%) periodic (P=18h) variability
correlated with Ha variability.
-z Ori: one-time brightening of ~15%: possibly also magnetic in origin.
-d Ori and Cyg OB2-8 detected with Einstein; also some interacting binaries.
- No systematic survey
of O star X-ray variability has been carried out using the largest,
high-sensitivity database of X-ray observations: The ROSAT archive.
There are numerous sites
of X-ray emission on the sun.Each one evolves in time, as does the overall distribution,
leading to significant X-ray variability.
UV observations of O star
winds show small- and large-scale variability; often periodic but also stochastic.
Linear Fits to Variable Stars
K-S Prob. for Hard Energy Channels
K-S Prob. for Soft Energy Channels
Hard and Soft Energy Channel K-S Probability Results
Bright stars in the spectral range earlier
than about B3 are soft X-ray sources, with LX~ 10-7LBol
Note: K-S test is
generally more sensitive than c2(can see in lightcurve at the bottom of the previous column too)
11 of the 17 observations showing variability have non-zero slope of their count rates
Nearly 30% of the O stars in the sample are variable
No dramatic variability
(e.g. no flares)
Much of it is long timescale variability (t > few ksec)
Significant amount of
Numerical radiation hydrodynamic simulations (snapshot at left) show highly time-dependent structure, with shock waves advecting through the wind.
Flow timescales and cooling timescales are of order 1000s of seconds.
The ROSAT PSPC is a gas proportional counter with a two-degree field of view and
some very modest energy resolution (each photon is tagged with an approximate
energy, as well as a position and arrival time in the detector)
spatial resolution (FWHM~5) degrades rapidly off-axis.