Uncovering the Origin of Hard X-rays from γ Cas star

1. Turning the tide: the origin of hard X-rays from γ Cas* Myron Smith1, Raimundo Lopes de Oliveira2 & Christian Motch3 1National Optical Astronomical Observatory, (Tucson, AZ, USA), 2Universidade Federal de Sergipe (São Cristóvão, Brazil), 3Université de Strasbourg (France) *See poster P16 on census, properties of the “γ Cas class.”

2. Optical Characteristics of γ Cas • Optical spectrum: a B0.5 IVe star with very broad lines; CS disk emission in IR, hydrogen Balmer, yellow-red Fe II lines, • γ Cas known as prototype of a class of hard-XR emitting stars. They are old: at least some of them are Blue Stragglers (see poster P16 by same authors), • LBOI (interferometery) of star-disk gives inclination of 42-44o, • Prot = 1.21581 day robust signature in APT B, V light curves, • spect. binary in circular orbit (RV study: Porb = 203.55 ±0.20 d), [Secondary’s evol. status is unknown; mass ≈ 0.8±0.4 M] • optical & UV lines: red-to-blue moving “migrating subfeatures.”

3. Migrating subfeatures (msf) in UV: Hubble/GHRS spectra (1996) (note gray, irregularly-spaced, diagonal striations) “UV continuum“ Si IV 1394Å Si IV 1403Å Wavelength (After Smith & Robinson 1998, ApJ, 517, 866.) Lines from many ions are visible. The msf acceleration is high (95 km s-2). If ascribed to rotational advection of surface structures this movement would imply a supercritical rotation rate! The msf were first reported in γ Cas by Yang et al. 1988 (PASP, 100, 233). Smith & Balona (2006) observed them in another γ Cas star, HD110432.

4. X-ray characteristics: • XR spectrum: emitting volume is optically thin, multiple components, with kThot = 14 keV (160 million degrees K), • * XR light curve is variable on a variety of distinct timescales, • o ubiquitous rapid “shots”(duration: a few secs to 1 minute), • o XR “undulations” - vary over one to a few hours, • o varies over long cycles, quasi-periodic, ~70 days, • o episodic - related to optical Be “outbursts.” We will focus on these variabilities now.

5. 1996 RXTE Campaign light curves. The rapid shots (“flares”) and undulations: 4-second decay time! 1998, ApJ, 503, 877; “SRC” Apply plasma cooling arguments to the flares’ rapid decay: they must originate in hot plasma of density Ne >1014 cm-3 .

6. Simultaneous UV/XR campaign in March, 1996: Correlation of X-ray and (inverted) UV light curves *INVERTED* UV light curve (After “SRC”) UV dips features are associated with X-ray undulation maxima – not shared by optical light curve. UV dips are explained best by translucent clouds forced into corotation above the star’s surface (as are the migrating sufeatures).

7. A slide slipped in after the talk: Other correlations during this simultaneous RXTE/HST campaign. Correlations of UV continuum: with Si III, Si IV abs. lines, (less excited). Anticorrelation: with Fe V lines (more excited ion than photospheric Fe IV).

8. OPTICAL LONG CYCLES: From 15 years of monitoring with an APT system, γ Cas’s B & V-filter light curves exhibit fragile long cycles of P = 55-91 days. (After Henry & Smith 2012, ApJ, 760, 10) (The V amplitude of these variations is larger than B. This suggests these long period variations arise in the cooler Be disk, not the star.) Q: Are X-ray variations correlated with these optical cycles?...

9. OUR EARLY ANSWER: From simultaneous observations in 1999-2000, apparently so! APT: “binned, continuous” V band mag RXTE: avgs of 6 extended obsns X-ray (RXTE) flux Days from MJD=51440 (Robinson et al. 2002, ApJ, 575, 435; data from G. Henry’s APT)

10. Long periods: NEW: this slide is embargoed pending acceptance of a submitted paper.

11. Episodic Optical - X-ray Activity During 2010 outburst of γ Cas, correlation we found in p.e. absorption of soft X-rays and B-V color (a disk-building index). The greater the Be star’s release of matter (brightening in V, B-V), the greater the column density that absorbs soft X-rays. (But no change in hard X-rays). nH2(1022 cm-2) (B – V) Color X-rays (even the hard ones) must be produced from behind the absorbing column, i.e. close to the Be star..

12. FINALLY, here’s the photometric rotational signature for γ Cas: Periodogram for seasons 1998-2004: (Prot = 1.215811 ±0.000030 days) The waveform Variance Factor (observing window spectrum) After 2003 the amplitude has faded to near undetectabilty. Amplitude (detection limit) # of obsns Year 2005 2000 2010 Maybe this is why C. Neiner couldn’t detect a field? (2011, IAUS272, 172)

13. Conclusions: • the hard XRs are emitted in hot, high density plasma (shots), • XRs emitted close to Be star (behind outflowing column), • XRs emitted close to Be star (correlation of UV lines with ””XR undulations”) • long cycle variations mediated by conditions in Be disk, • neither the few-hour UVC dip/XR maxima (1996 campaign) nor the phasing of optical/XR long cycles exhibit XR delays. This argues that the XR production does not arise following a particle migration from the Be primary to an accreting star. (Consistent with these other conclusions.)

14. The following pages are outtakes!

15. (Example of a magnetic star-disk interaction)

16. XMM spectrum of γ Cas with “unfolded” plasma components XMM (2004, red) vs XMM-#4 (2010, black) Spectra

17. The following pages are outtakes!

19. Migrating subfeatures in optical/UV lines (diagonal striations) He I 6678A line in HD110432 - probably cut this

20. (sinefitplotph4vb.ps under papers/old/hs12)