[vsnet-chat 7569] V725 Tau rebrightening again...

[substellar at Safe-mail.net]

...at high energies :-

http://www.astronomerstelegram.org/?read=2754

http://maxi.riken.jp/top/index.php?cid=1&jname=J0538+263

http://maxi.riken.jp/top/maxi_data/star_data/J0538+263/J0538+263_00055058g_lc.png

but what it's doing optically is not really known, as the data are about a year out of date :-

http://www.astrouw.edu.pl/cgi-asas/asas_variable/053854+2618.9,asas3,0,0,1500,300

though it's still a bit of a morning object at present.

The big spike in xrays may well have been the last big event for a while anyway.  The model has the connection with the neutron star causing the flaring at periastron.  Now the main star is a Be supergiant I think, and those occasionally spew off outer layers, usually with an equatorial bias, making a torus, from time to time, them being so big and hot and rapidly rotating and all.  These tori come and go in Be stars, you'll see such things monitored in Halpha line profile from many excellent examples from Christain Buil's work http://astrosurf.com/buil/us/becat.htm .

Now as these gamma Cas - esque object cast of some outer atmosphere there's an orientation dependent affect upon the optical lightcurve, and sometimes it gets in the way of the light coming to us, and the star appears to fade for a while, until the torus thins again.

If you look at the ASAS3 optical lightcurve and look at the SWIFT xray lightcurve history http://swift.gsfc.nasa.gov/docs/swift/results/transients/1A0535p262/ you'll see a faint state during the last run of xray outbursts.  And a little while ago, but no longer, ASAS3 lightcurve was more uptodate for a short while and again showed a faint state during the last xray peak, which coincided with a fainter bit of that faint state, with it's echoing ever smaller peaks every 111 days, and all possibly, according to that scenario, happening when the neutron star ploughed through a nice fresh batch of plasma cloud torus as said torus reached the distance of its orbit.

What the object does optically in the short term, in the duration of one of these xray outbursts, in a time series sense, is probably unknown, after all no superhumps are harmed in the course of these events, so no interest is shown.

Whether the current mild rebrightening in xrays is likely to give any optical signature is not clear either.

But the optical lightcurve shows that on a times scale of a coupla thousand days enough atmosphere gets thrown off to give some dense material for the neutron star to pass through in its orbit, or their mutual orbit, or whatever you prefer.  So, optically, last October's big burst could've been predicted, possibly not to the month, but a likelihood of such events could've been predicted.  Look at how the xrays got slowly brighter in peaks, then there was a big peak, and now slowly fading off.  You can almost imaging an expanding ring and the neutron star clips the outer edges to start with, then the main ring, and now is on to the inner edges, and will soon stop xray outbursts due to ploughing through this stuff altogether.  And maybe that's all wrong and just a flight of fancy.

But come the next optical fade (in time optically it should return to the bright state, unless some further expulsion of matter occurs before then), which might well be the hint of another torus of fresh material expanding outwards, that concept leads to another xray outburst period, and it'd be nice to know if during the biggest one whether there's any other spike at other wavelengths.

Just a thunk, based on the coincidence of xray outbursts and optical fade states and the nature of the system.

The lack of connection may simply be because the xray guys have never heard of V725 Tau, and the optical guys don't realise 1A 0535+262 is an optically variable object.  Both are a Be/xray binary combo.

Cheers

John