Swift Observations of Novae

Swift Novae – Julian Osborne – MAXI 1st year meeting – Shibuya, Japan (30 Nov – 2 Dec)

Julian Osborne & the Swift nova-CV group

http://www.swift.ac.uk/nova-cv/

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

Previous best nova in X-rays

Krautter et al 1996

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)

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

Nova X-ray light-curves

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)

RS Oph: XRT light curve

Osborne et al 11

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

32 milliCrab

RS Oph BAT detection

• RS Oph was detected by the BAT in the softest band

• Emission was likely thermal rather than compton degraded radioactivity

BAT 14-25 keV

Bode et al 2006

RS Oph shock development

Asymmetric shock propagates into RG wind Walder et al 2008

• 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

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 76.9Osborne et al 11

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 & shrinkageOsborne et al 11days

wavelength

A quasi-periodic modulation

Day 33-45 Day 45-58

Beardmore et al 2009

Period near 35s in soft X-rays between days 33-59

WD spin?

Nuclear burning instability?

A quasi-periodic modulation

Day 33-45

Period near 35 sec confirmed by XMM

Ness et al 2007

Orio et al 2007

Period near 33 sec also seen by Chandra LETG on day 40

Nova 1999b Aql(V1494 Aql)• Chandra ACIS• ~42 minute period

Drake et al 2003Nova Sgr 2002c(V4743 Sgr)• Chandra grating• 22 minute period• Softening decline in ~2 hrs

Ness et al 2003

Periods in SS novae

Quasi-periods in SS novae

• 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

100 sec hardness and count rate values compared to expected hardness for kT=52 eV blackbody and ISM plus variable neutral absorption

GK Per

Extreme early variabilityH

Nova 1999b Aql(V1494 Aql)

• Chandra ACIS• X-ray burst lasts ~14 mins

Drake et al 2003

Flux variation unexplained

Brief flux increase

Does not look like variable absorption

Rapid variability in SS novae

General trend of hardening at times of low flux is consistent with neutral absorption model

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?

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

RS Oph Summary

• 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?

• 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 ~ 3x10-7 M_sun• To reach ignition: M_acc ~ 4x10-6 M_sun• From shock: M_ej ~ 10-7-10-6 M_sun• Nova cycle mass budget: accreted mass >4x

ejected mass → future SN • but uncertainties large

RS Oph Summary

V2491 Cyg

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.

Page et al 2010

V2491 Cyg

Short-term X-ray variability is seen, though there is no strict periodicity.

Similar (unexplained) variability has been seen in other SS novae.

Numbers in the top right corner indicate the day after outburst.

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.

V2491 Cyg

Page et al 2010

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.

V2491 Cyg

Page et al 2010

McDonald CO LTE model fits

Show same trends of rising temperature and shrinking radius as BB fits

V2491 Cyg

Page et al, subm

McDonald LTE models are worse spectral fits than BB

V2491 Cyg

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

Page et al 2010

0.3-10 keV

V2491 Cyg

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.

Page et al 2010

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.

KT Eri

Nova Eri 2009

mV,peak = 5.4

14 Nov 2009

Early SS X-ray behaviour shows large-scale variability like RS Oph

KT Eri

Clear spectral variations of a very super-soft spectrum

KT Eri

Spectrum varies on both low and high energy sides

KT Eri

Ratio of bright over faint spectra shows variation is mostly Oxygen line emission

Ratio:

day 83.5 over day

Energy (eV)

KT Eri

No correlation of X-ray with optical flux

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%).

XRT Chandra XRT

06 Feb 2010 = day 83.6

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

Behaviour in 1945

Previous outburst with template from 7 outbursts(Schaefer 2010)

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

Osborne et al ATEL 2442

Days: 12.1-13.8, 14.3-16.2, 16.3-18.5, 19.2-20.6

Quiescent eclipse (Schaefer arXiv:0912.4426)

1 mag.

Optical eclipses only seen after day 14

Early optical eclipses are broad and shallow

• BB spectral fits show shrinking photosphere revealing higher temperatures

• Luminosity peak ~ Eddington

• Radius is cold WD value by day ~27

Summary

• U Sco is the fastest declining nova known, suggesting a very high mass

• It is the only eclipsing RN, offering a unique opportunity to dissect the progress of the outburst

• The SS phase onset between day 8-12 coincides with the optical plateau, although this is not generally true of novae

• Broad and shallow eclipses are seen in the UV & optical from day 14 only.

• Lack of strong X-ray eclipses appears to imply scattering

• Very strong optical emission line wings (‘horns’) will constrain the ejecta geometry

• The UV spectrum is very rich, giving great hope of a radial deconstruction of the properties of the evolving nova ejecta

V407 Cyg

Summary

Abdo et al 2010 Science

Fermi detection of Nova V407 Cyg at <100 MeV

V407 Cyg

Fermi detection during initial hard phase

Short-lived bright X-ray phase ends at time of UV faster decline

Shore et al 2011

X-ray flux

X-ray HR

UV flux

V407 Cyg

Complex X-ray spectrum:

Enhanced abundances & 2 temperatures required for initially highly absorbed cooling optically thin emission (N>250, O~35, Ne<3, Na>20, Mg<2.3)

BB-like soft component seen before day 60

Shore et al 2011

V407 Cyg

• First nova of V407 Cyg

• Secondary is Mira-type with strong wind

• RS Oph-like nova evolution of X-rays and optical line profiles

• Enhanced abundances seen in X-rays indicative of nuclear explosion

• High ionization lines eventually became highly asymmetric

• Indicates explosion off-centre in Mira wind

• Mira has longest P~750 d and does not fill Roche lobe

• Measured shock velocity of ~2500 km/s consistent with Fermi detection at ~3 days

• Orbital period likely >30 years

Shore et al 2011

Summary

• The scheduling flexibility and multi-wavelength capability of Swift make it the best rapid-response nova observing facility

• Extensive datasets are revealing novel behaviours

• The short SS phase durations measured suggest high MWD

→SN1a?

Nova SSS years

V723 Cas >12

V574Pup 3.5

V5116 Sgr 2.9

V1281 Sco 1.2

V597 Pup 1.4

RS Oph 0.25

V2491 Cyg 0.16

V458 Vul >1.4

CSS 081007 0.41

N LMC 2009 0.63

• Blackbody fits show similar trends to static NLTE model atmosphere fits, but lack of outflow in model limits inference from SSS spectral fits

• Periodic & aperiodic X-ray variability still to be understood. Not shown: Orbital modulation likely in some cases. X-ray & UV/opt are correlated/anti-correlated/unrelated

Swift Observations of Novae

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