Astrophysical Explosions in Binary Stellar Systems. Dr. Maurizio Falanga

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1 Astrophysical Explosions in Binary Stellar Systems Dr. Maurizio Falanga

2 The X-ray binary systems

3 haracteristics Classification after the mass of the companion Kuulkers, in t Zand & Lasota 2009 Falanga et al. 2008

4 INTEGRAL image IGR J ( kev)

5 Burst oscillations SAX J Mag. (D. Chakrabarty et al. Nature, 2003! What are they telling us about B?

6 Low-mass X-ray Binary (LMXB)

7 Binary evolution UCXB The companion star should fill ist Roche lobe to allow sufficient accretion on the compact star "!Brown dwarf models at different ages (Chabrier et al. 2000) "!Cold low-mass white dwarfs with pure-helium composition "! IGR J "! SAX J H-rich donor, brown dwarf "! XTE J "! XTE J "! XTE J H-poor, highly evolved dwarf "! XTE J !! What burns on the NS in 4U 0614? How to get He bursts from a C O accretor (Kuulkers et al. 2010)!!Bursts from UCXB that appear to be accreting carbon/oxygen/neon!! What happens at Mdot ~ 0.1 Mdot_Edd?

8 Brief history: the first X-ray burst

9 ! Detected in 1969 with Vela 5b Published by Belian et al. (1972) First cited in 1976 Still the brightest X-ray burst ever: 1.4! 10-6 erg s -1 cm -2 (Bright enough to disturb earth s ionosphere) Re-investigated by Kuulkers et al. (2009). Happened few days prior to accretion outburst

10 Detected in 1975 with first pointed X-ray satellite ANS (Grindlay & Heise 1975) Prompted a spur of subsequent burst discoveries, particularly with SAS-C (Lewin, Hoffman et al.) Explained as thermonuclear shell flash on NS by Maraschi & Cavaliere (1977), Woosley & Taam (1976), based on theoretical work by Hansen & van Horn (1975) Grindlay & Heise, IAUC, dec 1975 Grindlay et al., ApJ, 1976

11 Thermonuclear X-ray bursts

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13 X-ray Bursters

14 !! 11~20 UCXB: hydrogen-deficient accretors!! 40 Persistently accreting < 2% Eddington 2-100% Eddington!! 52 Transients Sometimes cover large M and all burst regimes!! 20% of bursts reach Edddington limit!! 0.02% are intermediate long bursts!! 0.02% are superbursts Recurrence times between 20 min (Cir X-1) and years (superbursts)!! (in 20 t Zand burst et oscillators al. 2007; Galloway et al. 2008; Keek et al. 2009)!! ~50% of LMXBs don t burst (~25% are BHs)

15 Accumulation of accreted matter for hours on the neutron star surface! Unstable nuclear burning for seconds " Thermonuclear X-ray burst. Accretion onto neutron star 24 s! Rise time " seconds! Decay time " seconds! Recurrence time " minutes to day! Energy release in 10 seconds " erg Sun takes more than a week to release this energy. Typical X-ray burst lightcurve 4.8 hr (4U )! ~10 43 erg! rare (every few years?) Superburst

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17 Burst energy thermonuclear E b = " T 0 F bol (t) dt Persistent flux gravitational energy Power of accretion: GM NS m p R NS " 200 Mev / nucleon Energy release of nuclear burning to heavy elements is: # nuc " 1.6 Mev / nucleon for pure He and # nuc " 5 Mev / nucleon for solar composition material " # $ $ F p dt F b dt % GM /R Q nuc % (40 &100) Very little matter if any is ejected by an X-ray burst

18 Observational quantities

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20 Spectra pure black body Strohmayer & Bildsten 2006 Strohmayer & Brown 2002

21 Time resolved spectral analysis 1. Modelling of the net burst emission by blackbody (BB) 2. Modelling of the total burst emission by BB+PL (PL is fixed by pre-burst persistent emission) Type I X-ray bursts are characterized by a 2-3 kev (T K) BB emission and exponential decay with cooling. Spectral intensity Planck Function: Wien displacement law: hν Max = 2.82 kt Maximum burst emission (5-9 kev)

22 ), to obtain:

23 Flux conservation: 2 4π R BB σ SB T 4 eff = 4π d 2 ξ p F BB = L BB R BB = d 2 T eff F BB σ SB (Stefan s law) 1. Burst emission is assumed isotropic (ζ p =1) 2. Gravitational redshift effects L = L bol,* ( 1+ z) 2 T = T eff,* ( 1+ z) 1 R = R eff,* ( 1+ z) " z(r) +1 = 1+ 2GM NS $ # R NS c 2 % 1 2 ' = 1.31 & 3. What is actually observed is a colour temperature

24 Gravitationally redshifted absorption lines in the X-ray burst spectra of a neutron star Observed z = 0.35 " z(r) +1 = 1+ 2GM NS $ # R NS c 2 % 1 2 ' = 1.31 & J. Cottam, Paerels & Mendez, Nature, 2002 Ø If a spectral line can be indentified => redshift Ø Rapid rotation broadens lines.

25 Deviations from blackbody emission of hot neutron stars (kt > kt Edd 2.4 kev) Nakamura et al., 1989: High energy tail due to comptonization of photons in a hot plasma around NS Lewin et al., Space Sci. Rev. 62 (1993): Modification of BB emission by electron scattering in the atmosphere of NS T c = f c T eff, f c 1.5 R underestimated by factor 2 Strohmayer & Brown, ApJ 566 (2002): Reflection from accretion disk of 4U [suggested by Day & Dove, MNRAS 253 (1991)]

26 Spectral energy distribution of X-ray bursts 4U Temperature varies during the burst Nearly a black body Molkov et al. 2000

27 Observational Burst parameters Spectral quantities from observations: R BB, T BB, F BB,peak, E b = T 0 F bol (t) dt Effective burst duration: α = F pers E b Δt τ = E F b peak ~ e-folding decay time 10<α<10 3 : a measure of burst energetics γ = F F pers peak : burst strength relative to persistent emission ατ Note: γ = Δt Observational persistent emission prior to the Burst L = 4πd 2 F = GM R dm dt = M ηc 2 Mass accretion rate per unit are: η = GM Rc 2 : m = M A acc = M 4πR 2 NS

28 Intermediate Long Thermonuclear X-ray Bursts

29 Preliminary interpretation Basic inferences from burst flux profile Ø Fluence è amount of fuel Ø Decay time è thickness of fuel layer Ø Peak luminosity è amount of fuel X production rate of nuclear energy (or, type of nuclear process) Ø Flux + distance è radius (r = d F/σT 4 = Stefan Boltzmann) Ø PRE + peak flux è distance (d = Ledd/4πF peak ) Ø The previous records seem to indicate a transition between two nuclear burning regimes.

30 Burst parameter relationships: α vs. γ The correlation of α with γ is consistent with previous conclusion and seems to indicate that steady nuclear burning limiting the burst energy release does increase with accretion rate (Van Paradijs et al, 1988).

31 Burst parameter relationships: τ vs. γ The decrease of burst duration with persistent luminosity indicates that hydrogen becomes less important in the energetics of the burst as the mass accretion rate increases (Van Paradijs et al, 1988).

32 Preliminary interpretation The previous records seem to indicate a transition between two nuclear burning regimes. Evidences of increasing time between bursts as persistent emission increases may indicate an increase of the accretion area implying that the local accretion rate per unit area, m, actually decreases with the accretion rate M. The influence of the accretion rate per unit area is an indication that only a fraction of the NS is covered by freshly accreted fuel.

33 Photospheric Radius Expansion

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35 Eddington-limited bursts Faster burning and thicker piles result in higher nuclear energy rate # larger L# may reach Eddington limit and drive photosphere to heights Eddington# L plateaus & R Increases# kt decreases # PRE/Eddington-limited bursts 20% of all bursts are Eddington-limited (Galloway et al. 2008) (Weinberg et al. 2006) (e.g., Kuulkers et al. 2002)

36 Blackbody cooling track and Stefan s law: F R 2 T Touchdown Expansion/contraction phase Cooling track at touchdown L Edd T = Edd 2 4π R NS σ GX 17+2 (Kuulkers et al., 2002)

37 Constraining mass and radius with burst spectra!! if we know the distance and f C => R infinity!! Using Eddington limited bursts and distance => M,R separately. Assumes peak T = touchdown Güver et al. (2010) Suleimanov et al. (2010) Steiner et al. (2010) What are the systematic errors in determining M, R from burst spectra?

38 X-ray bursts as estimators of distance!! The maximum luminosity during the burst is the Eddington luminosity L Edd ~ erg/s (for a 1.4 M $ neutron star).!! It can be used as a standard candle to estimate the distance: L edd," = 4#d 2 F bol,peak Observationally (globular clusters ): L Edd # 3.8x10 38 erg/s (Kuulkers et al., 2003)!! IF L peak = L Edd " d : Distance!! IF L peak= 4#d 2 F bol,peak + L Edd " d Upper limit to distance For canonical NS parameters (R NS = 10 km, M NS = 1.4 M $ )!! Eddington luminosity!! Eddington temperature!! Eddington accretion rate

39 Burst recurrence time

40 In order to ignite hydrogen and helium a certain pressure and temperature are needed. They are achieved when sufficient amount of fuel has accreted on to the surface. Trec "M = # M (t)dt $ 0 M T rec $ cont % T rec & M '1 & F '1 Strohmayer & Bildsten, 2003

41 - Bursts are fuelled by accreted material at rate dm dt = M M tot =,t M -! Mass burned during a burst is given by M b = E b " 1+ z ( ) ; " : burning efficiency erg# g-1 -! Recurrence time between bursts is then: M M E L pers 2 $ c # b b " t = =!! 1 z ( + ) [ ] 't ((L pers ) -1 should reflect the time needed to accumulate the nuclear burning fuel But things are not that simple-

42 <F bol,pers > -1.1

43 Burning regimes

44 Nuclear burning regimes! m < 900 g/cm 2 /s : Mixed H/He burning triggered by thermally unstable H ignition. Long burst duration (> 100s s) due to rp- process. $#150.! < m < 2000 g/cm /s : H stable burning (hot CNO cycle) to He " Pure He flash (3-$). Frequent PRE. $#200.! 2000 g/cm 2 /s < m < medd : Mixed H/He burning triggered by thermally unstable He ignition. Burst duration > 10s due to rp- process. $~ He unstability H unstability 100% m-dot Edd. Stable H/He burning Pure He accretion (e.g. from white dwarf) " powerful pure He bursts. Deep Carbon burning in superbursts.

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46 Burst ignition regimes 3 cases, in order of increasing accretion rate (e.g., Fujimoto et al. 1981) 1) H-burning is unstable, ignition is from H in mixed H/He fuel; 2) H-burning is stable, H is exhausted prior to unstable Heignition, pure He Burst; 3) H is not exhausted prior to Heignition, mixed H/He burst accretion rate He ignition curve Case 3 case 2 Case 1 H ignition curve Taam, Woosley, Joss, Fujimoto (late 1970s, early 80s), Bildsten (1998)

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49 Why study Type I XRBs?!! Nature of dense matter in NS: crust and core To constrain M NS, R NS and T cor!!gr in strong field regime z GR from discrete spectral features (absorption lines and edges)!! Nuclear physics: Nucleosynthesis from top to bottom In particular properties of nuclei near the drip lines (rp-process, neutron rich nuclei in the crust)!! Combustion physics: Stable/unstable nuclear burning, how does the burning front spread over the surface? Can it be confined?

50 Why study Type I XRBs?!! Binary evolution donor composition, using XRBs to light up the surrounding gas!! Stellar physics mixing, settling, diffusion, convection, angular momentum transport - all on rapid timescales!! Accretion physics!! MHD does the accretion geometry affect the burst properties? how does B field affect propagation, what is B strength and geometry in the accreted layer between and during bursts?

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53 Fuel accumulation and ignition!! Local accretion rate in low-b NSs is 10 to 10 5 g s -1 cm -2!! For M-dot>10% Edd, H burns through hot CNO cycle, producing pure He layer!! After hours to days, accumulate columns of y= g cm -2 or g!! Pressure (y*g) builds up to ignition condition for explosive triple-alpha, CNO cycle and rp-capture processes, 1 m deep!!heating (:) T 17, cooling (:) T 4 # thermonuclear shell flash!! Layer heats up to 10 9 K within milliseconds and then cools radiatively over tens of seconds # X-ray burst