[vsnet-chat 7562] Re: Outbursting object in Centaurus

[substellar at Safe-mail.net]

KT said "I suspect that the object is a classical nova in its supersoft X-ray
state, similar to V598 Pup = XMMSL1 J070542.7-381442.

Past X-ray detection:
115111.2 -623734 (2000.0) 2RXP_J115111.3-623735 0.1068P 0.82 -0.55 930202"

And as Sebastian already pointed out, a candidate slow nova, very slow nova.

What the route of 'detective work' has led to is the best likelihood of it being a very slow nova now in supersoft xray mode of the ilk of V723 Cas (and another very slow nova that I can never remember the name of), and that now the probable likely situation is that it's about a magnitude or a bit less above the quiescent 15th-16th magnitude situation and may be varying by up to an amplitude of one magnitude or less as has occurred for V723 Cas in the past, with some of this variation being likely due to orbital signature, and both having some evolution.  So, observations would be nice.

The route to there and evidence towards this comes like this, in general :-

Joe Patterson noted a new xray source brought to his attention by Jules Halpern in a CBA news note

http://cbastro.org/communications/news/messages/0763.html

Checking the object out showed _two_ previous xray detections, the 2RXP one and a more recent, but _earlier_ than the current one, XMM one (in the 2XMMi catalogue), which seemed pretty soft, or at least weak.

A plethora of sources showed the thing to be photometrically white enough, with DENIS, 2MASS and SPITZER data ranging from only mag 14.5 to 15.0 for a suite of NIR passbands from J to 4.5 microns.

DENIS had two gunn i measures at 15.5 ish (late 1990s stuff usually) and a Caldwell 1991 paper on a deepish CCD search for Cepheids in the immediate region had Ic around 15.5 for it.

The ASAS3 data then showed the lightcurve history, or _potentially_ did so.

Then I got myself muddled a bit whilst examining the Galactography, ie where it lay (very near the Galactic Plane on the outskirts of Crux OB1).  Possibly some SDOR or WR alternative wasn't entirely excluded.

Thus I notified some Southern observers and made some notes to JH.

The general situation was i) the ASAS3 data, it's a bit noisy, and there's pre-outburst stuff on the lightcurve ii) the decline looks like a very slow nova, and not particularly like an SDOR/WR, but it lies where it lies and there are such stars in that region iii) another potential ASAS3 problem was the possibility of a happenstance contamination by a field Mira (there is a nearby IR source listed in the NSV).

The muddled thinking I managed was because if it _was_ SDOR or WR (remember, it's relatively bright at quiescence, a nova at the distance of that Spiral Arm likely wouldn't be) then interstellar extinction would apply and it turned out E(B-V) around here is about ~~ 3!  I'd already shown the thing wasn't red when at quiescence, never mind very red.

About the same time Sebastian Otero, who has much exprience with the use of and the publishing with ASAS3, data kindly confirmed that the outburst was real, that the line could be cleaned up for a better curve as some data was problematic, and that the pre-outburst blips could be traced to an adjacent field star.  Further, I'd decided that a Mira was a no go as the projected period would've been many hundred days, which although not unprecedented, it'd've had to have been high amplitude too, and that that combination was unlikely.  Also he confirmed, as I'd checked, that the relatively nearby NSV object did not show any signal in ASAS3, and whatever the vagaries of wide field many arcsec per pixel sky surveys, a star doesn't show up away from its position yet not show up at its position.  So the outburst was real (and from the looks of things the very actual maximum might've been missed).

Sebastian also noted, without prompting or pre-bias, that it looked like a slow nova decline to him, and further confirmed that it didn't look like any outbursting sdor or wr that he knew of.

And Jules Halpern kindly confirmed that the xray source was unabsorbed, such that it was likely the the object was a foreground object, given its colours and the amount of interstellar extinction in that direction if it had been associated with the Spiral Arm.

That left slow nova, very slow nova, probably taking about two years to re-approach quiescence.  Jules Halpern again kindly noted that the xray source nature was amenable to something of the ilk of V723 Cas.

And that, more or less, is it, probably slightly muddled in order, but mostly like that, but probably not quite, as I also spent a lot of time looking at various images... for instance it's in supercosmos Halpha and also short red images at around mag 15.5 too, and the Halpha image showed a nearby ISM bowshock roughly centred on the object's position which biased some of the sdor/wr thinking for a time.

Predictions of such stuff on the whole only go so far.  Follow up observations are needed.  It might be something outbursting again, but it now more likely might be having a similar situation now to what late state V723 Cas and one or two other very slow novae have had optically, as it appears to be doing so xray-ly.  If nobody looks, we won't know.  And of course spectra solve lots of questions (sometimes).

I guesstimate the UVOT uvw2 ultraviolet band image at mag 13.5 to 14, depending on how much of that is a UV excess, and depending on what quiescence actually was in V, the thing might currently be around V 15 or brighter, unfiltered a touch brighter still, and varying with with some orbital signature which might be up to half a day or so, or maybe more but less than a few days.

Apparently it's late in the season for this object, but anyone wanting to follow it might find it interesting.

And just in case of any misunderstandings, you don't have mention me if you note it, you don't have to credit me with anything, just go out and enjoy the damn thing.  It'd be nice to know if it is what appears to be, is all.  Having said that, I'll probably personally and privately refer to it as Nova Cen 2008 No. 2 until something comes along to show that's wrong.

Potential missed novae candidates are rife in archival data, very few turn out to be real, a lot of sidetracking, process of elimination stuff, and some cul-de-sac investigations, have to be gone through before bothering to trouble folk with it.  Weigh the above information as you will before deciding to invest precious photon collecting time upon it.

Cheers

John