Explosion, turn-off and recovery of
accretion in novae revealed by X-rays
Margarita Hernanz
Institut de Ciències de l’Espai (CSIC-IEEC) - Barcelona (Spain)
The X-Ray Universe 2011 - Berlin, 27-30 June 2011
M. Hernanz
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OUTLINE
 Classical
scenarios
and
recurrent
novae
explosions:
 Origin of X-ray emission
 Summary of X-ray observations - Theoretical
implications
 Possibility to accelerate cosmic rays in novae
(symbiotic recurrent) : RS Oph and V407 Cyg
 Conclusions
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White dwarfs
• Endpoints of stellar evolution (M< 10M): no Enuc available;
compression until electrons become degenerate
• Chemical composition: He, CO, ONe; masses: typical 0.6
M, maximum: MChandrasekhar (~1.4M)
• When isolated, they cool down to very low L (~10-4.5L):
• When in interacting binary systems, they can explode
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White dwarfs in close binary systems
Cataclysmic variable:
Symbiotic system:
WD + Main Sequence
WD + Red Giant
Roche lobe overflow
accretion from a red giant wind
Classical nova
Hydrogen
burning in
degenerate
conditions
on top of the
white dwarf
Recurrent nova
Credit: David Hardy
Prec~104-105 yr; Porb~hr-day
a~few 105 km~ few 1010cm
rate ~35/yr in Galaxy
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Prec<100 yrs; Porb~100’s days
a ~ 1013-1014 cm
rate ~10 known in Galaxy
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In recurrent novae, initial mass of the WD should be
very large (close to Chandrasekhar mass), to drive
such frequent outbursts
Feasible scenario of type Ia supernova explosions,
provided that less mass is ejected than accreted in
each explosion
X-ray observations: way to study if WD mass grows
or diminishes after each nova explosion
Solve the controversy between single degenerate and
double degenerate scenarios of type Ia supernovae
Credit: David Hardy
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Scenario for classical novae
Mass transfer from the
companion star onto
the white dwarf
(cataclysmic variable)
Hydrogen burning in
degenerate conditions on
top of the white dwarf
Thermonuclear runaway
Explosive H-burning
Decay of short-lived radioactive
nuclei in the outer envelope
(transported by convection)
Envelope expansion, L increase
and mass ejection
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apparent luminosity (mv)
Novae observations: optical light curve
time
L increases very fast by factors greater than 104 -absolute Lmax~104-5L
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Novae observations: light curves
UV satellites:
Lbol(LV+LUV)=ct.
V+UV
FH Ser 1970 - Gallagher & Code 1974
V
V
TOT
UV
Lbol(LV+LUV+ LIR) =ct.
IR
Nova Cyg 1978 – Stickland et al. 1981
IR emission
dust formation
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Photosphere recedes as matter
expands and becomes transparent
Supersoft X-ray emission reveals the
hot white dwarf photosphere, close to
the burning shell
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Origin of X-ray emission (I)
•
Residual steady H-burning on top of the white dwarf:
photospheric emission from the hot WD:
Teff ~ (2-10)x105K (L~1038erg/s)
supersoft X-rays
 detected by ROSAT/PSPC in only 3 classical novae, out
of 39 observed up to 10 years after explosion:
GQ Mus (N Mus1983), N Cyg 1992, N LMC 1995
(Orio et al. 2001). A few more detections with BeppoSAX,
Chandra, XMM-Newton; many more with Swift/XRT
 duration related to H-burning turn-off time. “Old” theory:
τnuc~100yr; observations: <9 - 12 yr; typically: < 2yr
 new models: L-MH,rem-Teff compatible with short duration
of soft X-ray phase (Tuchmann & Truran 1998; Sala &
Hernanz, 2005)  very small remnant H-mass
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Origin of X-ray emission (II)
•
Internal (external) shocks in the ejecta: thermal plasma
emission
 detected early after explosion (N Her 1991, N Pup
1991, N Cyg 1992, N Vel 1999): internal shocks;
recurrent nova RS Oph: external, V2491 Cyg 2008 ?
•
Reestablished accretion: emission “as a CV” (idem)

Hard (but also soft) X-rays, depending on the thermal
plasma T
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Origin of X-ray emission (II, cont’d)
Restablished accretion:
 emission “CV-like”
How and when?
Interaction between
ejecta and new accretion
flow?
Magnetic or non
magnetic white dwarf?
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Origin of X-ray emission (III)
Compton degradation
of γ-rays emitted by
classical novae CAN
NOT be responsible of
their early hard X-ray
emission:
• Cut-off at 20 keV
(photoelectric abs.)
• Fast disappearence:
2days (w.r.t Tmax, i.e.,
before visual outburst)
Gómez-Gomar,Hernanz,José,Isern,
1998, MNRAS
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Observations – Supersoft X-ray emission
 EXOSAT and ROSAT discoveries:
GQ Mus (1983): 1st detection of X-rays in a nova, EXOSAT
(Ögelman et al. 1984). One of the longest supersoft X-ray
phases: 9 yr Ögelman et al.1993; Shanley et al. 1995; Orio et
al. 2001; Balman & Krautter 2001
V1974 Cyg (1992): complete light curve with ROSAT- rise,
plateau and decline – 1.5 yr Krautter et al. 1996, Balman et al.
1998
N LMC 1995: ROSAT & XMM-Newton – 8 yrs
ROSAT discovery Orio &Greiner 1999 [XMM-Newton obs.
Orio et al. 2003]
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V1974 Cyg (1992): ROSAT’s soft X-ray light curve
rise: until day 147
plateau: 18 months
BB fits not good – too large L
Krautter et al. 1996, ApJ
The X-Ray Universe 2011 - Berlin, 27-30 June 2011
ONe WD atmospheres
MacDonald & Vennes
Balman et al. 1998, ApJ
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V1974 Cyg (1992): ROSAT’s soft X-ray spectra
F=6x10-10erg/cm2/s
kTBB=21eV, kTbr=0.32keV
F=3.2x10-9erg/cm2/s
kTBB=30eV ( kTbr=0.002keV)
F=3x10-11erg/cm2/s
kTBB=20 eV,kTbr=0.29keV
F=3.1x10-9erg/cm2/s
kTBB=30eV (kTbr=0.002keV)
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Models that best explain the supersoft X-ray
emission of V1974 Cyg 1992 and its evolution
WD envelope models with steady H-burning (no accretion)
• Mwd=0.9 M, 50% mixing with CO core (but V 1974Cyg 1992
was a neon nova!)
or
• Mwd=1.0 M, 25% mixing with ONe core
[in goog agreement with models of the optical and UV light curve
(Kato & Hachisu, 2006)]
• Menv~2x10-6 M
WD properties from X-ray observation of turn-off
Sala & Hernanz, A&A 2005
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Observations – Supersoft X-ray emission
 BeppoSAX
V382 Vel (1999): supersoft X-ray flux not constant; model
atmosphere not a good fit; emission lines from highly ionized
nebula were required (Orio et al 2002)
Chandra grating observations detected emission lines (Burwitz
et al., 1992, Ness et al. 2005).
Turn-off 7-9 months
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Observations – Supersoft X-ray emission
 Chandra
LETGS:
V382 Vel
(1999)
Burwitz et al.
2002
Ness et al.
2005
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Observations – Supersoft X-ray emission
Chandra and XMM-Newton (novae in outburst)
puzzling temporal behaviours
grating observations
V1494 Aql (1999) - burst and pulsations Drake et al. 2003
V4743 Sgr (2002) - strong variability and complex spectra
Ness et al 2003, Rauch, Orio, González Riestra et al., 2010:
fits with NLTE WD atmospheric models
 see Rauch’s talk – C1
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Observations
– Supersoft Xray emission
V4743 Sgr (2003)
Temporal variability:
P ~ 22 min.
Ness et al. 2003,
ApJ
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Observations
– Supersoft Xray emission
V4743 Sgr (2003)
Non LTE model
atmospheres
Rauch, Orio,
González-Riestra et
al., 2010, ApJ
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Observations – Supersoft X-ray emission
XMM-Newton
Monitoring campaigns of post-outburst novae
Nova LMC 1995 - Orio et al. 2003: H-burning still on in 2000
 see Orio’s talk C2, about Nova LMC 2009
Galactic novae V5115 Sgr and V5116 Sgr 2005: Hernanz,
Sala et al.
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XMM-Newton - AO1 Cycle -Summary
Target
Discovery
date
N Sco 1997
V1141 Sco
June 5
N Sgr 1998
V4633 Sgr
March 22
N Oph 1998
V2487 Oph
Date of observation – Time
after outburst
Detection
Oct. 11, 2000 – 1224d, 3.4yr
Mar. 24, 2001 – 1388d, 3.8yr
Sep. 7, 2001 – 1555d, 4.3yr
NO
Oct. 11, 2000 – 934d, 2.6yr
Mar. 9, 2001 – 1083d, 3.0yr
Sep. 7, 2001 – 1265d, 3.5yr
YES
but no
SSS
June 15
Feb. 25, 2001 – 986d, 2.7 yr
Sep. 5, 2001 – 1178d, 3.2 yr
Feb. 2002 – 1352d, 3.7yr
Sept. 24, 2002 – 1559d, 4.3yr
YES
but no
SSS
N Sco 1998
V1142 Sco
October 21
Oct. 11, 2000 – 721 d, 2.0 yr
Mar. 24, 2001 – 885 d, 2.4 yr
Sep. 7, 2001 – 1052 d, 2.9 yr
2.60.3
2.20.4
1.20.2
(10-2 cts/s)
N Mus 1998
LZ Mus
December
29
Dec. 28, 2000 – 730 d, 2.0 yr
Jun. 26, 2001 – 910 d, 2.5 yr
Dec. 26, 2001 – 1093 d 3.0 yr
NO?
• No supersoft X-ray emission related to residual H-burning detected
 all novae had already turned-off
• 3 out of 5 were emitting [thermal plasma (+ BB)] spectrum  ejecta/accretion
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Date of observation – Time
after outburst
Target
Discovery
date
Detection
N Oph 1998
V2487 Oph
June 15
Mar. 24, 2007 – 8.8yr
AO6 long exposure
N Cyg 2005
V2361 Cyg
February
10
May 13, 2006 - 15mo – bkg
Oct. 20, 2006 - 20months
AO5
-YES
marginal: (4.00.8)x10-3
cts/s
N Sgr 2005a
V5115 Sgr
March 28
Sep. 27, 2006 – 18months
Apr. 4, 2009 – 49 months
YES supersoft source
YES but no SSS
N Sgr 2005b
V5116 Sgr
July 4
Mar. 20, 2007 – 20 months
Mar. 13, 2009 – 44 months
YES supersoft source
YES but no SSS
N Cyg 2006
V2362 Cyg
April 2
May 5, 2007 – 13 months
affected by bkg AO6
Dec. 22, 2008 – 32 months
YES but no SSS
YES
but no SSS
YES but no SSS
N Oph 2006a
V2575 Oph
February
9
Sep. 4, 2007 – 19 months
AO6
NO
N Oph 2006b
V2576 Oph
April 6
Oct. 3, 2007 – 18months
AO6
NO
Supersoft X-ray emission related to residual H-burning found in 2
novae from 2005 (V5115 Sgr & V5116 Sgr) novae had not
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turned-off
yet
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June 2011
Nova Sgr 2005 b – V5116 Sgr – 610 days post-outburst
partial eclipse by an asymmetric disk? Sala, Hernanz, Ferri & Greiner, ApJL 2008
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Nova Sgr 2005 b – V5116 Sgr – 610 days post-outburst
RGS spectra
Sala,Hernanz, Ferri,
Greiner, AN (2010)
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Nova Sgr 2005 b – V5116 Sgr: new obs. March 2009
1348 days post-outburst
L=(3-7)x1032 erg/s (10 kpc)
U filter
Swift/XRT light light curve
SSS turn-off: 2 - 3 years postoutburst
compatible with Hachisu & Kato
(2007) prediction
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SUMMARY of XMM-Newton campaign on
Galactic novae
11 novae have been observed between 3 months and 5
years after outburst (9 years)
• Only 2, V5115 Sgr 2005a and V51116 Sgr 2005b, were
still bright in supersoft X-rays, revealing remaining Hnuclear burning – one of them with a puzzling temporal
behavior
• SSS phase absent means that either we missed it or
Mejected > Maccreted: Mwd decreases after each nova outburst
 WD can’t reach MCHANDRA and explode as SNIa
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Observations – Supersoft X-ray emission
Swift/XRT
Ness et al. 2007, Osborne (today’s talk, C1)
The largest sample. Example: two extreme cases
• V723 Cas (1995): L and Teff not well determined (BB) Ness et
al. 2008 – Still SSS 12 yrs after outburst. New XMM-Newton
observations in 2010, still active
• V2491 Cyg (2008): duration SS phase 10 days
Also observed with XMM-Newton and Suzaku: Ness et al.
2011, Takei et al. 2011
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Observations – Supersoft X-ray emission
V723 Cas (1985)
Swift observations in 2007: Ness et al. 2008, MNRAS
• not turned-off yet
XMM-Newton obs. in 2010: still on
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Observations – Supersoft X-ray emission
V2491 Cyg
(2010)
Page et al.,
2010, MNRAS
(Other
interest of this
nova: later)
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Observations – Supersoft X-ray emission
V2491 Cyg (2010) Ness et al. 2011
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SUMMARY of Swift/XRT campaign
Nova
From Julian Osborne:
see talk in C1
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
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2007-2008
Novae in M31
XMM-Newton &
Chandra monitoring:
d and line of sight
absorption known
Henze et al. 2011,
A&A
2008-2009
 see talks by
Henze C1
Pietsch C2
this afternoon
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Observations of novae where H has
turned off :
Recovery of accretion and/or ejecta
emission
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Nova Oph 1998 = V2487 Oph - 4.3 yrs post explosion
neutral Fe Kα fluorescence line
6.4 keV
6.7 keV Fe XXV
6.97 keV FeXXVI
• Identification of three
Fe Kα emission lines:
~neutral Fe: 6.4 keV
He-like Fe: 6.68 keV
H-like Fe: 6.97 keV
• If Thigh ~ (10-20) keV,
He-like and H-like lines
well reproduced & only
6.4 keV fluorescent line
added
 If complex absorption
-partial covering
absorber- low (ISM)+
high NH  Thigh~(10-20)
keV
Fluorescent Fe Kα line at 6.4 keV reveals reflection on
cold matter (disk and/or WD): accretion
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Nova Oph 1998 = V2487 Oph 4.3 yrs post explosion
Lunabs[0.2-10 keV]= 8.4
+0.2
x1034
-0.4
erg/s
NH=2x1021 cm-2 (frozen)
+0.3
Tbb=120
d=10 kpc
+20
eV
-30
Tlow=0.3 -0.1 keV
+0.6
EMlow=0.5 -0.4 x1057 cm-3
+8
Lbb=4±1 x1034 erg/s Thigh=13 -3 keV
EMhigh=6±1 x1057 cm-3
Covf=0.6±0.1
22
-3
NHPCA=24 +10
-8 x10 cm
• LBB ~ 50% LTOT[0.2-10] keV - f(emitting surface/wd surface)~10-4 (hot spots)
 Luminosity, spectral shape .. Intermediate polar? need Pspinvs. Porb
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N Oph 1998 = V2487 Oph
Mar. 24, 2007 8.8yr post outburst
 Spectral model:
similar to previous
observations
 No clear periodicities
in X-rays, but hint of
orbital period ≈ 6.5 hrs
 Optical observations
seem to confirm the
orbital P
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V2487 Oph (1998): 1st nova seen in X-rays before
its explosion (ROSAT)
Positional correlation with a source previously discovered
by ROSAT (RASS) in 1990 suggests that the “host” of the
nova explosion had been seen in X-rays before the outburst
(Hernanz & Sala 2002, Science)
 new case: V2491 Cyg (2008b): previous ROSAT, XMM
and SWIFT detections (Ibarra et al. 2009, A&A)
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Nova Oph 1998 = V2487 Oph Hard X-rays
• Detection with INTEGRAL/IBIS survey in the 20-100
keV band (Barlow et al. 2006, MNRAS): kT=25 keV ; flux
compatible with our XMM-Newton results, but the IBIS
spectrum has low S/N.
• Hints for large MWD from the optical light curve (Hachisu
& Kato, 2002, ApJ)
 also large MWD from large Thigh deduced from X-ray
spectra – but Thigh not well constrained
 The recent nova – V2491 Cyg (2008b) – has also been
detected in hard X-rays with Suzaku (Takei et al. 2009)
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Observations wih
Suzaku and
XMM-Newton:
V2491 Cyg (2008)
prompt and short
duration hard Xrays
Takei et al. 2009
and 2011
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Nova Oph 1998=V2487 Oph - Recurrent Nova
• Previous outburst in 1900 June 20, discovered in the Harvard College
Observatory archival photograph collection Pagnotta and Schaefer,
IAUC 8951, 200; 2009 AJ)
 recurrent nova - P=98 yrs
MWD very close to MCHANDRA  relevance for the SNIa scenario
challenge for theory to get recurrent nova explosions with such
short time scales
X-ray emission CV-like ≠ RN scenario
• The recent nova – V2491 Cyg (2008b) – has also been claimed to be
recurrent. It was also a very fast nova, expected to be massive, very
luminous in X-rays (Ibarra et al. 2009, A&A), and detected in very hard
X-rays (Takei et al. 2009)
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Models of recurrent novae – TNR on
accreting WDs
Search combinations of initial conditions leading to short
recurrence periods:
 Prec = ΔMacc / (dM/dt) = 98 yrs (21 years for RS Oph)
 ΔMacc: required accreted mass on top of the WD to
power the outburst through a TNR
 Mwdini? Accretion rate? Lwdini?
 Accretion rate: related to mass loss from the red giant wind
 effective dM/dt onto the WD: 2x10-7 - 10-8 M/yr
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Accreted masses to reach H-ignition conditions
.
M=10-8 M/yr
Lini
critical accreted
mass does not
depend only on
Mwd
Mwd very close
to MCHANDRA
Hernanz & José 2008
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Recurrence Periods
.
M=10-8 M/yr
V2487 Oph 1998: Prec=98 yr
Lini
*
RS .Oph: Prec=21 yr
*
-7 M /yr
M=2
10

*
& L=10-2 L
Hernanz & José 2008
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CONCLUSIONS (recovery of accretion)
• X-rays are crucial to study the recovery of accretion in post-outburst
novae: type of CV, mass of the WD
• Magnetic WD: challenge for accretion –traditionally assumed to
occur through a normal accretion disk in a non magnetic WD. But
some cases of novae in magnetic CVs are known: V1500 Cyg
(1975), V4633 Sgr (1998) – asynchronous polar as a consequence of
the nova outburst (Lipkin & Leibowitz, 2008), V2487 Oph (1998),
V2491 Cyg (2008)
 see Pietsch’s talk C2: M31N 2007-12b, an IP?
• Massive WD: if Thigh(plasma) is large and/or the nova is recurrent.
Novae as scenarios for type Ia supernovae
 but very “ad-hoc” conditions are required to obtain a recurrent
nova (Precurrence < 100 years)
 but XMM spectra (V2487 Oph) looks CV-like
RN scenario
M. ≠
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An interesting case: the recurrent nova RS Oph,
which erupted in 2006
Previous eruption in 1985 – Prec ~21 yrs
Short recurrence period  large MWD close to
MChandra (deduced from models)  possible SNIa
scenario (but should be CO WD!)
Porb=456d; RG companion – symbiotic recurrent nova
Detected as a very variable SSS by Swift/XRT (Bode
et al. 2006), XMM-Newton (Nelson, Orio et al. 2008,
Ness et al. 2009)
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Supersoft X-ray light curve of the recurrent nova RS
Oph (Swift observations, Bode et al. 2006)
Mwd=1.35 M
Menv= 4x10-6 M
Kato & Hachisu, 2007
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RS Oph in quiescence observed with XMMNewton
Nelson, Mukai, Orio, et al., 2011: observations in
quiescence, 537 and 744 after outburst  accretion
rate ≈ theoretical
In previous eruptions: very faint X-ray source in
quiescence, hard to reconcile with large accretion
rates needed to explain frequent (every 20 yrs)
outbursts
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RS Oph grating observations with XMM-Newton
and Chandra: Ness et al, Drake et al. 2009
See as well observations of U Sco (another recurrent
nova, eclipsing, not of the symbiotic nova type: Ness
talk C2
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RS Oph
 Prec ~21 yr: last eruptions in 1985, 2006
 MWD > 1.35 M (deduced from models; not measured)
 Mejec ~ (3-4)x10-6 M
 not all accreted mass is ejected (deduced from models)
MWD increases & M close to MCHANDRA  SNIa scenario
 Interaction between nova ejecta and red giant wind: expanding shock
wave sweeps through the red giant wind (“mini” SN remnant)
 Detected from radio to X-ray wavelengths
 X-rays: reveal interaction between ejecta and wind (hard) and hot
white dwarf surface with remaining nuclear burning (soft)
 Acceleration of particles (Tatischeff & Hernanz 2007, ApJL)
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RS Oph
A supernova remnant-like, but faster and dimmer
free expansion phase: days
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RS Oph (2006 eruption): blast wave evolution
• IR (Das+’06,
Evans+’07)
• X-rays: RXTE &
Swift (Sokoloski+’06,
Bode+’06)
*
2 caveats
•Why shock cooling started at 6 days, when Ts was 108K and radiative
cooling was not important? Particle acceleration - CRs
• Why vshock (X-rays) < v (IR): * (for test particle strong shock)
underestimates vshock when particle accel. is efficient, because Ts is
lower (particle ecape and softer EOS)
RS Oph (2006 eruption)
Non-linear diffusive shock acceleration: model of Berezhko &
Ellison (1999)
 accelerated proton spectrum and post shock temperatures as a
funtion of ηinj - the fraction of shocked protons injected into the
acceleration process
Tatischeff & Hernanz, ApJL 2007
RS Oph (2006): cosmic-ray modified shock
 Good agreement with X-ray
measurements of Tshock for
moderate CR accel. efficiency
ηinj~10-4 and Alfvén wave heating
of the precursor
 Energy loss rate due to particle
escape
~100 times larger than Lbol of
postshock plasma  energy loss
via accelerated particle escape
much more efficient than
radiative losses to cool the shock
RS Oph (2006): predicted gamma-ray emission
• π0 production: from εCR and (dM/dt)RG
• IC contribution: from non thermal synchrotron L (Kantharia et
al.’07, radio 1.4 GHz), Lsyn~5x1033 td-1.3 erg/s, and ejecta L,
Lej~LEdd = 2x1038 erg/s: LIC = Lsynx Urad/(B2/8π) ~ Lsyn
π0 production dominates
RS Oph would have been detected by Fermi!
V407 Cyg
 Detected by Fermi/LAT 2 days after outburst: Cheung et
al. 2008, Science
Main differences wrt RS Oph
• Not a standard recurrent novae: no regular eruptions before 2010
(Munari 2010)
• Porb ~ 43 yr (456d in RS Oph)  a ~ 15 AU (10 times larger than for
RS Oph)  shock wave needs ~7 days to reach d~a, so it propagates
through the RG wind perturbed by the orbital motion in V407 Cyg (RS
Oph, free exp. unperturbed wind at 1d)
• Lsyn needed to compute IC not available from early radio observations
Preliminary estimation gamma-ray flux from π0 production
The X-Ray Universe 2011 - Berlin, 27-30 June 2011
M. Hernanz
58
SUMMARY (of GeV emission from symbiotic RNe)
• Recurrent novae in symbiotic binaries are expected
to accelerate particles and emit VHE gamma-rays
detectable with Fermi, because of the shock wave
propagation in the dense red giant wind
• RS Oph would have been detected by Fermi
• V407 Cyg, detected by Fermi, did not behave as
RS Oph, regarding X-ray and radio emission. So
computing IC contribution is difficult.
• Other similar systems exist in the Galaxy:
eventually 1-2 novae with RG companion per yr are
expected (but not necessarily detected in the optical)
Summary
• Variety of behaviours of post-outburst novae: still need more
observations
• Grating spectra are very rich, but still lack of emission models
(e.g. WD expanding atmospheres) to interpret them. Blackbodies
give wrong L and Teff & emission is often a mixture of photospheric
and ejecta components
• WD Mass and envelope chemical composition (mainly H
content) determine duration of SS X-ray phase
• Duration of SS X-ray phase observed indicates in general Menv
< Macc-Meject from hydro models  mass loss (wind and/or
others?)
• Recurrent novae: very short duration of SS phase compatible
with small Menv. Challenging for theory: narrow parameter range:
Mwd extremely large & accretion rate large.
The X-Ray Universe 2011 - Berlin, 27-30 June 2011
M. Hernanz
60