Slide 1

Cumulative à

Deviation of data

& model scaled

ß to 0.3

ß 99%

ß 90%

ß 95%

HD 36861J (rp200200a01)
0.829 Probability of Variability

A Large ROSAT Survey of X-Ray Time Variability in O Stars
Allison S. Adelman1, David H. Cohen2
(1) Bryn Mawr College, (2) Swarthmore College

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 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, & 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.

ÞThe 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

Analysis Procedure

Data

• A pointed observation of O star HD57060

• O star HD57061 is also present, but overlapping another point source, smeared out near the edge of the image

•

•The support structure of the telescope is clearly visible

ObsID rp200066a02

HD152247

HD152003

HD152424

•A pointed observation of a young open cluster of O stars (NGC 6231)

•

•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 Stars

HD36861 - A Representative Non-Variable Star

HD149757 - A Representative Variable O Star

Background

O Star X-Rays

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.

•We extracted source counts for each O star, and performed several types of time variability analyses on each object

HD149797

mean = 0.41460

bins = 12

Reduced c2 = 0.80

P= 36%

st. dev. = 0.05509

K-S Probability = 0.981

HD36861J

mean = 0.31078

bins = 20

Reduced c2= 0.93

P = 45%

st. dev. = 0.05171

K-S Probability = 0.829

Examples of K-S Test Results

ß Cumulative

ß Deviation of data & model scaled to 0.3

ß 99%

ß 90%

ß 95%

HD 149757 (rp200199a00)
0.981 Probability of 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.

OBSERVATIONS

• 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.

-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.

ObsID rp200112n00

HD57061

HD57060

Linear Fits to Variable Stars

K-S Prob. for Hard Energy Channels

K-S Prob. for Soft Energy Channels

K-S Probability

O Star

.104

.999

.988

HD15570

.999

.169

.998

HD37468

.999

.788

.999

HD37742J (rp900386n00)

.984

.935

.990

HD37742J (rp200198a00)

.447

.789

.953

HD46150

.451

.869

.904

HD46223

.992

.948

.997

HD57060

.968

.554

.974

HD168112

.926

.999

.985

HD193322

.864

.523

.928

HD24912

.945

.373

.941

HD66811

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 ~ 10-7 LBol

Note: K-S test is generally more sensitive than c2 (can see in lightcurve at the bottom of the previous column too)

refers to linear fits

11 of the 17 observations showing variability have non-zero slope of their count rates

Conclusions

•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 hard variability

wavelength - velocity

Time (days)

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)

Note: the spatial resolution (FWHM~5”) degrades rapidly off-axis.