Transcript powerpoint - Astronomy at Swarthmore College

A Large ROSAT Survey of X-Ray Time Variability in O Stars
1
S. Adelman ,
2
Cohen
Allison
David H.
(1) Bryn Mawr College, (2) Swarthmore College
Background
Analysis Procedure
OBSERVATIONS
O Star X-Rays
Bright stars in the spectral range earlier than about
B3 are soft X-ray sources, with LX ~ 10-7 LBol
THEORY
•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 Xrays 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, & Rybicki 1988).
• Shocks occur stochastically, so X-ray output should 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.
The 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.
wavelength - velocity
Linear Fits to Variable Stars
• 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 thoroughly quantified.
•A few individual cases of X-ray variability have been
detected:
-
q1
Ori 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.
• We ran c2 tests of the null hypothesis (of a constant count rate).
• 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 ranges separately.
-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.
HD36861 - A Representative
Non-Variable Star
Project
• 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.
mean = 0.31078
HD149797
mean = 0.41460
bins = 20
Reduced c2= 0.93
P = 45%
st. dev. = 0.05171
K-S Probability = 0.829
bins = 12
Reduced c2 = 0.80
P= 36%
st. dev. = 0.05509
K-S Probability = 0.981
HD36861J
Time (days)
•We extracted source counts for each O star, and performed
several types of time variability analyses on each object
Data
UV observations of O star
winds show small- and
large-scale variability;
often periodic but also
stochastic.
Two Sample Images of ROSAT fields
ObsID rp200112n00
Flow timescales and cooling timescales are of
order 1000s of seconds.
11 of the 17
observations
showing
variability have
non-zero slope of
their count rates
Hard and Soft Energy Channel K-S Probability Results
Examples of K-S Test Results
HD 149757 (rp200199a00)
0.981 Probability of Variability
HD 36861J (rp200200a01)
0.829 Probability of Variability
Cumulative 
Deviation of data
& model scaled
 to 0.3
 99%
 95%
 90%
 Cumulative
 Deviation of
data & model
scaled to 0.3
 99%
 95%
 90%
ObsID rp200066a02
O Star
K-S
Probability
K-S Prob. for Soft K-S Prob. for Hard
Energy Channels
Energy Channels
HD66811
.941
.373
.945
HD24912
.928
.523
.864
HD193322
.985
.999
.926
HD168112
.974
.554
.968
HD57060
.997
.948
.992
HD46223
.904
.869
.451
HD46150
.953
.789
.447
HD37742J (rp200198a00)
.990
.935
.984
HD37742J (rp900386n00)
.999
.788
.999
HD37468
.998
.169
.999
HD15570
.988
.999
.104
Note: K-S test is generally more sensitive than c2 (can see in lightcurve at
the bottom of the previous column too)
Numerical radiation
hydrodynamic
simulations (snapshot
at left) show highly
time-dependent
structure, with shock
waves advecting
through the wind.
HD149757 - A Representative
Variable O Star
- 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.
The ROSAT PSPC is a gas proportional counter with a twodegree 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)
There are numerous sites of Xray emission on the sun. Each
one evolves in time, as does
the overall distribution, leading
to significant X-ray variability.
Sample Light Curves of O Stars
Conclusions
HD152247
HD57060
HD152003
HD57061
HD152424
•Nearly 30% of the O stars in the
sample are variable
•No dramatic variability (e.g. no flares)
• A pointed observation of O star
HD57060
•A pointed observation of a young open
cluster of O stars (NGC 6231)
• O star HD57061 is also present, but
overlapping another point source,
smeared out near the edge of the
image
•3 O stars are present in the central cluster;
however many were too indistinguishable to
extract data
•The support structure of the telescope
is clearly visible
•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
Note: the spatial resolution (FWHM~5”) degrades rapidly off-axis.
•Much of it is ‘long’ timescale
variability (t > few ksec)
•Significant amount of hard variability