Supersoft X-ray Sources in M31 in Be binaries? An astronomical game of “Guess Who”

1. Supersoft X-ray Sources in M31 in Be binaries?An astronomical game of “Guess Who” • Thomas Nelson and Marina Orio • INAF-Padova and University of Wisconsin

2. Supersoft X-ray Sources • First discovered by Long et al. (1981) in Magellanic clouds with Einstein, became “class” with ROSAT • Characterized by very soft emission - no flux at energies >1 keV • Fit with blackbody models of 105 < Teff <106 K, and 1036 < Lx < 1038 erg/s • A large fraction must be WD systems • Unerstanding whether they are the “single degenerate” progenitors of type Ia SN is crucial CAL 83 CAL 87

3. Type Ia supernovae SN1994D in NGC 4526 (HST) Sne Ia are often in young populations! Rate~SFR Favorite single degenerate models are “very old” (recurrent novae, symbiotics)

4. Optical counterparts of Galactic and Magellanic Cloud sources • Proximity of 2(4) Galactic and 14 MC sources allows for study of optical counterpart • SSS can be foreground objects, SNRs and PN, but most are WD binaries: classical novae in outburst (1), symbiotics, and other WD in binary systems burning accreted hydrogen in a shell • Trying to narrow down the phenomenological definition in X-ray range makes things even more confusing (e.g. RS Oph vs S And) • The nature of some sources (e.g. MR Vel, Cal 87) is still uncertain. • Sources can be persistent, recurrent or transient in soft X-rays. SMC mostly persistent. • ~75% of SSS contain a white dwarf - possible SN Ia progenitors? CAL 87 finding chart (Pakull et al., 1998) CAL 87 V band lightcurve (Callanan et al., 1989)

5. M31 - the largest SSS population ROSAT PSPC survey (Supper et al., 1997) XMM-Newton EPIC survey (Pietsch et al., 2005, Orio 2006) Chandra ACIS-S (Di Stefano et al., 2004)

6. M31 SSS population properties • Most of the sources are transient (especially ROSAT!) • A few are recurrent, and have been detected at different epochs with different instruments • Larger distance to M31, and increased crowding in the galaxy make optical counterparts hard to identify • >=33% of M31 SSS have been identified with novae

7. A UV and optical counterpart search • Search for counterparts to the M31 SSS population using GALEX, WIYN and the images of the Local Group Survey (Massey et al., 2006) • If (and only if) the SSS hosts a shell burning white dwarf, it should be detected as a UV source with GALEX, and a very blue star (U-B, B-V negative) in the Massey survey • Both the GALEX and LGS images suffer from source confusion in the bulge of M31: no counterparts within central circle of radius 5’ • This leaves 60 sources which could be detected with GALEX • The majority of the sources are ROSAT objects - only 4 Chandra and 12 XMM sources lie outside of the bulge

8. The Nearby Galaxy Survey with GALEX Thilker et al. (2005) • 50 cm telescope • Simultaneous imaging in 2 bands • FUV: 1350-1750 A • NUV: 1750-2800 A • Average exposure time ~ 2700 s

9. The Local Group Survey Massey et al. (2006) • UBVRI photometry of 370,000 stars in M31 • 1% photometry at U=B=V=R=I = 21 • <10% photometry at U=B=V=R=I = 23 • Coverage not as extensive as GALEX, but covers all but 5 of the 60 SSS • Follow up with WIYN images

10. FUV RX J0039.7+4030 RX J0043.3+4118 NUV

11. A surprising result! • We find that only 9 SSS have a UV counterpart in the X-ray error circle and they are all are inside, or within ~30” of an OB association in M31! • In addition, several of the sources not detected with GALEX also lie within or near an OB association, including one Chandra source. • So we find a number of SSS that appear to be associated with young stellar population. • We have proof in a few cases that only one UV object among them is a B star that has colors consistent with a binary WD system SSS. • This is in contradiction with most accepted models of accreting white dwarf SSS, which have a long delay time and so should be associated with OLD populations.

12. Is the proximity to OB associations a coincidence? • Assume that SSS really are an old population phenomenon • Therefore, should be distributed randomly over the ROSAT survey area total area of ROSAT survey = 23,400 arcmin2 total area of OB associations in M31 = 1293 arcmin2 probability of chance alignment ~ 6% We find 20% of all ROSAT sources are inside an OB association! This is not just a coincidence!

13. So what are these sources? • Detections could be spurious - ROSAT count rates are very low: an effect of spread of soft tails of “many stellar winds”? • Could be supernova remnants. Young supernova remnants can be quite soft, anomalous SNR in cavities and bubbles. • Some kind of new LMXB? Ot LMXB in new state? • Something else?

14. One possibility: WD-Be binaries • Be stars are rapidly rotating stars, which may be formed as a result of massive star binary evolution • Raguzova (2001) predicts that 70% of all Be stars formed as a result of binary evolution should have a WD companion • WD accretes wind from the Be star and begins to burn H in a shell • Could this be what we are seeing? • Kahabka et al. (2006) report the XMM detection of a new supersoft source, XMMU J052016.0-692505 in the LMC, coincident with Be star in the LMC

15. An exciting new XMM-Newton source!A clue to this puzzle? We found a new SSS in the XMM-Newton archive, better error box 0.15 - 10 keV 0.6 - 10 keV

16. FUV U One star inside error circle V = 21.827 B-V = -0.33 U-B = -0.705 V-R = 0.151 Colors are consistent with a B star in M31! NUV

17. Blackbody model with T = 5 - 6 x 105 K EPIC-pn spectrum. Lx in range 1037 - 1038 erg/s

18. Summary • We have carried out a search for optical and UV counterparts of SSS in M31 • Of 60 sources which could be detected with GALEX, we find only 9 have UV counterparts • These sources all lie inside or near OB associations in M31, and it is likely the UV sources are single, extremely hot stars in these young populations. • “Conventional” SSS should not be found in young populations - these sources are therefore likely to be different, and if they are WD binaries, they may account for prompt component of SNe Ia.