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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