Swift Observations of Novae

1. Swift Observations of Novae Julian Osborne & the Swift nova-CV group http://www.swift.ac.uk/nova-cv/

2. Swift Observations of Novae Julian Osborne & the Swift nova-CV group http://www.swift.ac.uk/nova-cv/ Nova: Thermo-nuclear runaway on an accreting white dwarf

3. Previous best nova in X-rays • Nova Cyg 1992 • (V1974 Cyg) • Rosat PSPC • 18 observations • Super-soft Source lasts ~400 days • Duration of SSS phase generally unknown – important constraint on WD mass (Tturnoff ~ M-6.3) Krautter et al 1996

4. X-rays from novae • Potential sources of X-ray emission from classical/recurrent novae: • Thermal emission from hot white dwarf • shock breakout • residual nuclear burning after ejecta dispersal • High velocity shocks • internal shocks within the ejecta • shock of ejecta with shell from previous nova or planetary nebula • shock of later fast wind with earlier slower wind • Re-established accretion • Compton degraded gamma-ray emission (before optical peak) • not yet seen Swift Swift Swift

5. Nova X-ray light-curves

6. Swift novae stats • Swift has observed ~40 novae within 4000 days of outburst • 25 detected in X-rays • 7 novae have >100 ksec each: U Sco, KT Eri, N LMC 2009, CSS 081007, V2491 Cyg, V458 Vul & RS Oph • Observations start within 1 day (pre-nova for V2491 Cyg & U Sco)

7. RS Oph: XRT light curve • 0.3-10 keV light curve shows: • Cooling hot gas emerging from red giant wind • Bright interval which lasts ~64 d • Shock breakout from wind since last outburst 21 yrs ago during bright phase Osborne et al 11

8. RS Oph: XRT light curve • 0.3-10 keV light curve shows: • Cooling hot gas emerging from red giant wind • Bright interval which lasts ~64 d • Shock breakout from wind since last outburst 21 yrs ago during bright phase 32 milliCrab Osborne et al 11

9. RS Oph BAT detection BAT 14-25 keV • RS Oph was detected by the BAT in the softest band • Emission was likely thermal rather than compton degraded radioactivity Bode et al 2006

10. RS Oph shock development Asymmetric shock propagates into RG wind Walder et al 2008

11. RS Oph: XRT light curve • 0.3-10 keV light curve shows: • Cooling hot gas emerging from red giant wind • Noisy onset of super-soft phase, which lasts ~64 day in total • Turnoff time → MWD~1.35 M☼ Osborne et al 11

12. RS Oph X-ray spectra • 1st appearance of hot WD on day 26 • kTBB=31 eV on day 29, = 56 eV on day 50.5 • Variable neutral oxygen absorption (0.54 keV) • Late flux decline consistent with temperature drop to kT = 41 eV at day 76.9 Osborne et al 11

13. RS Oph X-ray spectra • NLTE WD model atmosphere (Rauch+ 10) spectral fits (declining wind absorption with O=OSun/3, Ness et al 07) • Initial low temperature and large radius change to changes to high temperature and smaller radius • Peak Luminosity ~ LEdd • Late count rate decline due to cooling & shrinkage wavelength Osborne et al 11 days

14. A quasi-periodic modulation Period near 35s in soft X-rays between days 33-59 WD spin? Nuclear burning instability? Day 33-45 Day 45-58 Beardmore et al 2009

15. A quasi-periodic modulation Period near 33 sec also seen by Chandra LETG on day 40 Period near 35 sec confirmed by XMM Day 33-45 Orio et al 2007 Ness et al 2007

16. Periods in SS novae Drake et al 2003 Ness et al 2003 • Nova Sgr 2002c • (V4743 Sgr) • Chandra grating • 22 minute period • Softening decline in ~2 hrs • Nova 1999b Aql • (V1494 Aql) • Chandra ACIS • ~42 minute period

17. Previously seen modulation has been multiply periodic – therefore not due to WD rotation These 20-30 min periods have been ascribed to non-radial g-mode oscillations in the white dwarf (cf hot planetary nebula nuclei) induced by compression-opacity instability [Drake et al 2003] RS Oph periods at ~35 sec are very much shorter Possible origin in the ε mechanism (nuclear burning instability) for which models predict periods 4 min - 1 sec [Kawaler 1988, Sastri & Simon 1973] Consistent with modulation (mostly) visible during SS phase Quasi-periods in SS novae

18. Extreme early variability 100 sec hardness and count rate values compared to expected hardness for kT=52 eV blackbody and ISM plus variable neutral absorption HR = (0.6-2.0keV)/(0.3-0.6keV) GK Per

19. Rapid variability in SS novae Flux variation unexplained Brief flux increase Does not look like variable absorption Drake et al 2003 • Nova 1999b Aql • (V1494 Aql) • Chandra ACIS • X-ray burst lasts ~14 mins

20. Variable absorption? General trend of hardening at times of low flux is consistent with neutral absorption model

21. Variable absorption? General trend of hardening at times of low flux is consistent with neutral absorption model Counter-examples exist, due to ionization of oxygen? - Reduction of variability due to blob expansion/ionization?

22. Variable absorption? Ratio of bright phase spectrum to faint phase shows complexity of the spectral change with intensity This is not simple absorption Its probably not absorption at all

23. Recurrent nova in 455 day binary Initially very hard X-ray source due to wind shock Super-soft phase lasted ~64 days, short duration suggests high mass WD SS phase started with v large variability. X–ray spectral behaviour not simple, UV unaffected 35 sec qpo present at some times during SS phase – WD instability? RS Oph Summary

24. Early decline of SSS phase strongly suggests a very high mass WD (early fuel exhaustion due to ignition at low accreted mass) From SSS: M_burnt ~ 3x10-7M_sun To reach ignition: M_acc ~ 4x10-6M_sun From shock: M_ej ~ 10-7-10-6M_sun Nova cycle mass budget: accreted mass >4x ejected mass → future SN but uncertainties large RS Oph Summary

25. V2491 Cyg Page et al 2010 V2491 Cyg was observed – and detected - pre-outburst as part of the BAT survey follow-up. This X-ray source may have been the counterpart of the BAT source.

26. V2491 Cyg Numbers in the top right corner indicate the day after outburst. Short-term X-ray variability is seen, though there is no strict periodicity. Similar (unexplained) variability has been seen in other SS novae.

27. V2491 Cyg The UV (1928 Å) evolution is very different from that seen in X-rays, showing a fading trend beyond at least 20 days after the outburst. Earlier observations were saturated. The UV curve has inflections at ~40 and ~57 days, at the times of the X-ray peak and the end of the rapid decline.

28. V2491 Cyg Blackbody fit parameters while the super-soft source was visible Absolute values probably not reliable A rising temperature and shrinking radius are seen between days 30 - 57 Where is the expected constant luminosity nuclear burning phase? Grey points show luminosity upper limits from linking X-ray to UV. Page et al 2010

29. V2491 Cyg McDonald CO LTE model fits Show same trends of rising temperature and shrinking radius as BB fits Page et al 2010

30. V2491 Cyg McDonald LTE models are worse spectral fits than BB Page et al, subm

31. V2491 Cyg 0.3-10 keV 0.3-10 keV Page et al 2010 Flickering is visible after day ~57. If this is the recovery of accretion due to viscous expansion of a disk, then disk was removed to 2x1010 cm. With Porb=0.96 days, Rdisk ~ 3x1010 cm. Inner disk only destroyed by nova

32. V2491 Cyg Page et al 2010 Estimates of the time to reach ignition pressure for V2491 Cyg derived from the observed pre-nova luminosities of Ibarra et al (2009). Curves are from the Nauenberg (1972) WD M-R relationship (points from Althaus et al 2005), ranges reflect distance and accretion efficiency uncertainties.

33. V2491 Cyg It is highly unusual to see a nova in X-rays pre-outburst. The only others which have been detected pre/inter-outburst have actually been recurrent novae (e.g. RS Oph and V2487 Oph). A high inter-outburst accretion rate is expected if a nova is to be observably recurrent. It seems possible that V2491 Cyg will be a recurrent nova.

34. KT Eri Nova Eri 2009 mV,peak = 5.4 14 Nov 2009 Early SS X-ray behaviour shows large-scale variability like RS Oph

35. KT Eri Clear spectral variations of a very super-soft spectrum

36. KT Eri Spectrum varies on both low and high energy sides

37. KT Eri Ratio: day 83.5 over day 82.0 Energy (eV) Ratio of bright over faint spectra shows variation is mostly Oxygen line emission

38. KT Eri No correlation of X-ray with optical flux

39. KT Eri Is there a ~35 sec modulation in KT Eri, just like in RS Oph? Yes, there is! (ATEL 2423, Beardmore et al) NB: No such modulation is seen in the AGN Mkn 421, which also had ~150 XRT c/s (ie <0.4%).

40. KT Eri XRT Chandra XRT 06 Feb 2010 = day 83.6

41. KT Eri

42. KT Eri

43. KT Eri • Summary • SS phase onset around day 60 shows large scale variability like RS Oph • Spectrum is very soft (even for a SSS), kT_BB = 16 eV • Large spectral variability does not look like simple temperature or absorption variations • X-ray and optical flux unrelated • Variable strength ~35 sec qpo during SS phase, period & amplitude not a strong function of SSS intensity • This is the second SS nova to show ~35 sec qpo, favouring a WD instability origin over a rotation-based mechanism

44. U Sco – the fastest nova U Sco is the only eclipsing recurrent nova. Porb = 1.2 day Outbursts regularly every 10.3 yrs, next was predicted at 2009.3±1.0 Behaviour in 1945 Previous outburst with template from 7 outbursts (Schaefer 2010)

45. U Sco

46. U Sco

48. U Sco Osborne et al ATEL 2442

49. U Sco Days: 12.1-13.8, 14.3-16.2, 16.3-18.5, 19.2-20.6

50. U Sco 1 mag. Quiescent eclipse (Schaefer arXiv:0912.4426) Optical eclipses only seen after day 14 Early optical eclipses are broad and shallow