PoS(Integral08)101 itions across http://pos.sissa.it/ n our galaxy recently discovered theory in high mass X-ray binaries e expected to be characterized by at SFXTs which display very large tter and the neutron star magneto- In particular, we concentrate on the 2591), F-91191, Gif sur utron star surface is largely inhibited; rriers. We show that very large lumi- 044 Rome, Italy vity displayed by supergiant fast X-ray ht thus provide a unique opportunity to ia della Ricerca Scientifica 1, a e Commons Attribution-NonCommercial-ShareAlike Licence. L. Stella c or more on time scales as short as hours) can result from trans 4 10 ∼ M. Falanga b , a ∗ In this paper we summarize some aspects of the wind accretion different types of interaction between the inflowing wind ma nosity swings ( hosting a magnetic neutron star and a supergiant companion. sphere that are relevant when accretion ofthese matter onto include the inhibition ne by the centrifugal and magnetic ba with INTEGRAL. According to this interpretation weluminosity argue swings th and host a slowly spinning neutron star ar different regimes. This scenario is thentransients applied (SFXTs), to the a acti new class of high mass X-ray binaries i magnetar-like fields. Supergiant fast X-ray transientsdetect mig and study accreting magnetars in binary systems. Speaker. ∗ Copyright owned by the author(s) under the terms of the Creativ c Yvette, France Dipartimento di Fisica - Università di Roma “Tor Vergata”, v 00133 Rome, Italy CEA Saclay, DSM/DAPNIA/Service d’Astrophysique (CNRS FRE 7th INTEGRAL Workshop September 8-11 2008 Copenhagen, Denmark INAF - Osservatorio Astronomico di Roma, Via Frascati 33, 00

E. Bozzo Are There Magnetars in High MassThe X-ray Case Binaries? of SuperGiant Fast X-Ray Transients c b a PoS(Integral08)101 1 9 2 3 7 − 10 , op- ⊙ erg s E. Bozzo 32 (Sguera et 30M 1 10 ∼ − ∗ ∼ , would require 5 al identifications . In some cases, erg s rita Heras, 2007). , wind velocity of 1 1 -10 − creting matter from 37 − 4 rding to the standard ) have been seen on 5 yr in quiescence and out- 10 me SFXTs showed also Sguera et al., 2006) and X-ray binaries (HMXB), erg s ⊙ ∼ 165 d. This is interpreted as d for spin periods in the -10 4 ∼ retion onto a NS immersed and 10 M 33 anion stars (see e.g., Walter . A recent list of confirmed 1 5 10 36 − -10 ally a mass of M ∼ 10 31 -10 icity of ∼ 7 10 − , 2007). Thus Walter & Zurita Heras (2007) ∼ =10 w ˙ M 2 . These sources are observed to exhibit sporadic outbursts, last- 5-6, mass loss rate of ∼ ) ⊙ /L ∗ INTEGRAL 6) SFXTs is given by Walter & Zurita Heras (2007). Between outbursts, , and are persistent soft X-ray sources with luminosity around 1 ∼ − It is widely believed that SFXTs contain neutron stars (NS) sporadically ac Only IGR J11215-5952 showed recurrent flaring activity, with a period Supergiant Fast X-ray transients (SFXTs) are a new class of high mass 1000-2000 km s 1 5) and candidate ( ∼ 1000-2000 s range. Proposed models for SFXTs generally involve acc w ∼ outbursts from a systems with anexcluded unusually these long sources orbital from period their (Sidoli SFXT et list. al. (Cassinelli et al., 1981). 1. Introduction Contents 1. Introduction Magnetars and SFXTs 2. Stellar wind accretion 3. Transitions and paths across different regimes 4. Application to SFXT sources 5. Conclusions v timescales comparable to the outburstflare-like duration activity (e.g., at In’t intermediate luminosity Zand, levels 2005). (e.g.,of Walter So et SFXTs al., show 2006). that these Optic sources are& associated Zurita to OB Heras, supergiant 2007, comp and reference therein). These stars have typic tical luminosity of log (L in the clumpy wind ofHowever, its if supergiant accretion onto companion the (In’t collapsed Zand,burst, then object 2005; the of Walter corresponding SFXTs X-ray & luminosity takes Zu swing, place typically a both factor of SFXTs remain in quiescence with luminosities in the range ( a supergiant companion, and inIGR the prototypical J16479-4514 SFXTs (Walter XTE et J1739-302 al.,∼ ( 2006) some evidence has been reporte al., 2005). No firm orbital period measurement has been obtained yet very high peak-to-quiescence X-ray luminosity swings (factor of recently discovered with wind inhomogeneities with a very large density and/or velocity contrast (acco ing from minutes to hours, with peak X-ray luminosities between PoS(Integral08)101 ), w nd V 6 NS (2.2) (2.1) ere are R π eric radius E. Bozzo /(8 a co 2 M R R R µ tter from the regime ation radius (dashed Direct accretion ollowing radii are wind can be eased ursued (Negueruela nclude that if SFXTs (we assumed circular w V NS radius and mass at ⊙ Shock the theory of wind accre- is the binary orbital period s seconds), then they must . Davidson & Ostriker, 1979). of the stellar wind mass loss co 2 10d a 4 M R , w R R − 10d − w ˙ a regime ,P M v / 4 cm 3 w / − 8 2 ˙ Subsonic propeller 1 30 v M capt − 8 5 ˙ M v − M 3 / 10 2 10d w g matter from its supergiant companion. All the regimes 10 V 10 × =P Shock and it is assumed that the orbital velocity of × 2 1 7 10d − . 3 )= a 3 2 R M = co regime R a R 4 2 w through (Frank et al., 2002) c propeller regime convective motions at the base of the atmosph ( v a / / ine is used when the magnetospheric boundary at the magnetosph 2 a osition of the magnetospheric radius (solid line), the corot G). R NS CDE 15 ≃ w V GM w -10 , at which the pressure of the NS magnetic field ( 2 ˙ M M 14 = / Shock a , respectively. The fraction 10 , R R ⊙ capt ∼ co ˙ is the total mass of the binary in units of 30 M R M is the distance at which the inflowing matter is gravitationally focused a M R R a 30 , R inhibition regime =1.4 M cm is the orbital separation, a B NS that remains closed during quiescence, halting most of the accretion flow, a 10d w V a 12 Shock 10 barrier × M cm and M is the wind velocity in units of 1000 km s Schematic view of a magnetized NS interacting with the inflowin R co ) captured by the NS depends on R a R 6 8 R w ˙ M We investigate here the conditions under which a magnetized NS can accrete ma inhibition regime =10 Superkeplerian magneticSuperkeplerian magnetic Subkeplerian Supersonic propeller NS A Figure 1: Magnetars and SFXTs described in the text are shown, together with the relative p wind of a massive companion.defined In (e.g., the Illarionov theory & of Sunyaev, wind 1975;- accretion Stella The et in accretion al., HMXBs, radius 1986): the f toward the NS. It is usually expressed as rate ( opens up in outbursts, leading totion direct in accretion. HMXBs After (§ reviewing 2 briefly and 3),host we slowly apply rotating barrier NSs scenarios (spin to SFXTspossess periods in magnetar-like of § fields several 4 ( hundreds and co to thousand Models involving accretion of extremely dense clumpset are al., still being 2008). actively p Theif requirement there on is the a density and/or velocity contrasts in the 2. Stellar wind accretion in units of 10 days, and M Here a=4.2 represented with small eddies. wind accretion, the mass capture rate onto the NS scales like line), and the accretion radius (dottedis line). Kelvin-Helmholtz A unstable. wavy solid In l the supersonic and subsoni the star is negligible (FrankR et al., 2002). Throughout the paper we fix the orbits). - The magnetospheric radius where v PoS(Integral08)101 6 − (2.4) (2.5) (2.6) (2.3) /10 w ). In the ˙ E. Bozzo M . In sys- 2 w ) v = a 6 w 1977), with R , subject to a − at is shocked ρ d the regimes ˙ > , M tion is also su- 1 ergy. The power M − different regimes ble X-ray sources results in rotational , and (Harding et al., 1992; M 1 erg s 3 gy at a rate − M 2 .3), which can vary on a the wind parameters can − , the accretion radius and s3 bly short timescales, thus t R P egion around the stagnation G cm 2 erg s . In this case the magneto- . − 10d 6 co 33 a − 6 cm ˙ − M /10 , R 3 ˙ a 2 / M µ 1 33 − 10d R = . a µ > 2 8 3 33 4 M10 / v M µ cm R 1 10d a 3 29 6 / 2 M10 / : R 2 s3 1 R 10 P − 8 29 v × 10 6 7 / 10 . 10 1 4 6 3 − × − × ˙ s. 7 ≃ M 7 . . 3 2 4 10 1 ) = Ω 10 should be used. = M 3 w × M co v R 3 ( w R . w ρ 3 v 2 M w = R ), a NS dipolar field with ρ cannot proceed further inward, due to the rotational drag of the NS 2 w π M , at which the NS angular velocity equals the Keplerian angular ve- 2 M v R co R 2 M ≃ a π a , which is satisfied for a very wide range of parameters. π , R ≃ ≃ M /(4 shock , the magnetospheric radius is given by (Davies & Pringle, 1981) sd a w L L ˙ R the mass flow from the companion star interacts directly with the NS magneto- M > cm), and is mainly radiated in the X-ray band. We distinguish two different a ∼ ) M R 10 M > M 10 (R / w M is the NS spin period in units of 10 ρ . In the following sections we discuss the range of applicability of Eq. 2.3, an 1 the NS magnetic moment) balances the ram pressure of the inflowing matter ( 2 s3 − The superKeplerian magnetic inhibition regime energy dissipation and thus, NS spin down. This process releases ener which adds to the shock luminosity (Eq. 2.5). spheric radius is larger thanand both halted the close accretion to and R corotationmagnetosphere radii. which is Matter locally th superKeplerian.personic, the Since interaction between magnetospheric the rota NS magnetic field and matter at R =R P µ - Outside the accretion radius: the magnetic inhibition of accretion (R Changes in the relative position of these radii result into transitions across We approximated a-R yr 2 • M10 ⊙ Toropina et al., 2006). At least inpoint, the the front whole part kinetic of energy the of shock, thereleased i.e. inflowing in matter in this is the region converted r into is thermal of en order tems with R sphere without significant gravitational focusing, forming a bow shock a for the NS (Illarionov & Sunyaev,magnetospheric 1975; radius Stella depend et on al., the 1986). windwide In parameters range particular (see of Eqs. timescales 2.1 (from and hourscause to 2 the months). NS Therefore, to variations undergo in transitionsopening across the different possibility regimes to on explain compara through the them. properties of Below we some summarise classesvarying the of stellar different wind. highly regimes varia of a magnetic rotating NS locity, i.e. Here regimes of magnetic inhibition of accretion: (R - The corotation radius case in which R M in which a different prescription for R Magnetars and SFXTs with Here we assumed adensity non-magnetized spherically symmetric wind (Elsner & Lamb, PoS(Integral08)101 or can and t the M (2.9) (2.8) (2.7) e a ρ / i E. Bozzo ρ gime can elevant to , e to R 1 is paper we embling an we consider − , etween R 1 nov & Pustil’nik, through Kelvin- − stable layer, i.e. at it is orders of erg s 1 rbulent motions are − erg s ) inside and outside the e 1 e ρ − is deposited there. Three ρ / ) i ction between matter and e ρ ρ / and + i . In this case the magneto- i ρ 1 co ( ρ , 2 + R ) / 1 1 < M ( ) e 2 M R / ( ρ 1 R depends on the conditions at the inner ff / ) i v e n < i ρ a ρ ρ ( can be assumed for the pressure and den- (this produces an almost negligible contri- t / 6 i h n ; the atmosphere is stationary on dynamical a − ρ / . In this case the rotational velocity of the NS 3 : R is inside the accretion radius, matter flowing ˙ M 1 ( a M 6 R + 1 R M 1 − − 8 ˙ < ≃ ρ 5 v M , and the densities M ∝ 2 2 sh R − 10d resulting from the KHI depends on the efficiency fac- − 10d v conv a a < v M M e co ρ . Once R 3 M10 2 M10 R a are negligible 2 M R , respectively. The luminosity released by accretion of this R 1 R a R M − s3 KH < P η M equal to the largest of the above two values). The ratio 34 KH η 10 KH is supersonic; the interaction with matter in the atmosphere leads to 34 × M 10 5 . (Davies et al., 1979; Davies & Pringle, 1981). A hydrostatic equilibrium 3 × M 8 = . (Bozzo et al., 2008). The contribution of Bohm diffusion to the total mass 8 M KH = R L = 0.1, the shear velocity at R t is the height of the unstable layer (Burnard et al., 1983). Throughout th KH t ∼ L KH η matter onto the NS is given by magnetospheric boundary at R where h The subKeplerian magnetic inhibition regime tor assume h inflow rate through the magnetosphere in the subKeplerian magnetic inhibition re be calculated according to Ikhsanovmagnitude & smaller Pustil’nik than (1996), that due andthis to we work. the found Similarly, KHI th the over contribution themagnetospheric to whole boundary the range can total of be luminosity neglected parameters resulting in r from this regime. the shock a The supersonic propeller regime: if the shear velocity is taken to be the gas velocity in the post-shock region clos or be calculated from the equation of the mass conservation across the KHI un spheric drag is subKeplerian and matter canHelmholtz penetrate instabilities the (KHI, NS Harding magnetosphere et al., 1992) and1996). Bohm The diffusion (Ikhsa mass inflow rate across R the rotational velocity ofthroughout the this NS paper magnetosphere, L respectively (for simplicity, magnetosphere at R dissipation of some of the star’s rotational energy and thus spin-down. Tu We checked this is verified for all the case of interest for this paper. this matter redistributes itself into an approximately spherical configuration (res 3 • • M sity of the atmosphere. The value of the polytropic index - Inside the accretion radius: R boundary of the atmosphere, and indifferent particular regimes on can the be rate distinguished: at which energy Magnetars and SFXTs from the companion star is shocked adiabatically at R bution to the total luminosity) and halted at the NS magnetosphere. In the region b ensues when radiative losses inside R “atmosphere”), whose shape and propertiesNS are magnetic determined field by at the R intera R timescales, and a polytropic law of the form p PoS(Integral08)101 , co (see R (2.10) (2.12) (2.14) (2.13) (2.11) (2.15) < opeller capt avies & M E. Bozzo ˙ M , lace, direct gime 1 . − . 1 n of the mag- 1 − − where the inter- . 2 ergy dissipation) 2 erg s / damps convective me rate luminosity is M / 3 (Davies & Pringle, tmosphere changes erg s 1 erg s − ) 15 2 ˙ 2 ate because R M / )) M / capt 1 and matter outside the 1 ) ˙ 35 /R e M )) M10 co ρ 10 NS . R R / . i 5 ) × M10 < ( e ρ ller regime. cm ( =3/2), and the magnetospheric 2 R / ρ cm M 2 9 n 3 / / / 7 i ( ≃ 3 4 a10 / 33 ρ / 4 33 1 R is expected to accrete onto the NS, µ )=(2GM )) . The break down of the supersonic µ − 9 + a M a10 / 7 . IfR 16 1 4 8 / R lim M10 ( 8 8 v (R ˙ + 8 lim v 9 R M erg s 16 / v 7 ˙ ff 1 5 2 / M 6 ( 4 < ( 2 6 + − − / 2 10d − 8 > w 2 − − ˙ )=v 1 / v ˙ a w M ˙ ( 5 M10 M 2 ˙ M M 9 a10 s3 , M R / 2 7 − 10d P R 4 10d / / 8 , (R a co 1 M10 4 10d v a s 6 6 a 16 co R − 10 1 , R 2 10d ≃ , i.e., when the magnetosphere rotation is no longer 10 ˙ + a − 8 M a , R ) 10 co v 1 3 R a 10 and c ( 35 . We found that, in all the cases of interest to this paper, M 2 R − × s3 R < a 4 6 − 10d R × P < 3 10 − ( R M a < . 2 2 3 s ˙ 6 M 2 M M 10 × c − 10 ) ≃ : R − 2 R =1/2, and the balance between the magnetic and gas pressure ˙ , reaches the magnetospheric boundary at R ≃ M M a n × M . This is the standard accretion regime; the system achieves the 3 M10 = R M 1 R 31 8 R R ( =10 . − R ∼ 2 ρ 10 NS onto the NS surface is permitted. KH a10 2 M R gs = η × / R 6 4 35 15 capt π . − ˙ 5 2 capt M 10 ˙ lim /10 which convect this energy up through the atmosphere, until it is lost at its M ˙ = × M M NS 8 capt . sd ˙ 1 M L = GM = 15 = ˙ M KH L acc L Eq. 2.2) at which it flows towards the magnetosphere. The corresponding Matter that is shocked at In the subsonic propeller regime, the centrifugal barrier does not oper is reached, at which the gas radiativemotions cooling inside (bremsstralhung) the completely atmosphere (Daviesaccretion & at Pringle, a 1981). rate If this cooling takes p The direct accretion regime: outer boundary. In this case generated at R The subsonic propeller regime applies until the critical accretion rate action with the NS magneticPringle field (1981), draws this gives energy the from largest NS contribution rotation. to the According total luminosity to in D this re magnetosphere cools efficiently, accretion onto the NS takes place at the sa gives In the above equation R The subsonic propeller regime propeller regime occurs when R 1981). Nevertheless a fraction of the matter inflow at R but the energy input at theis base still of too the high atmosphere for (due matter to to NS penetrate rotational the en magnetosphere at a rate and the transition to the subsonic propellernetosphere regime is takes subsonic, place. the Since atmosphere the is rotatio roughly adiabatic ( supersonic with respect to the surrounding material. The structure of the a radius is approximated by (Davies & Pringle, 1981): regime. By using similar arguments to those above, we estimated in this case the KHI provides the largest contribution to the total luminosity in the subsonic pr due to the KHI and Bohm diffusion highest mass to luminosity conversion efficiency. where To our knowledge this is the first application of the KHI to the subsonic prope 4 • • Magnetars and SFXTs PoS(Integral08)101 co 100, (3.4) (dashed < 6 co . depends E. Bozzo (solid line), − , and R ˙ M M a M gs for a sys- < ;; (3.2) (3.3) a a , R R R : Relative position of M > < ) barriers. Corre- ime, to the super- a =1, and investigate . regimes take place. M M g, since R w er system parameters G), such abrupt lumi- ˙ 10d M egime, giving rise to a 12 direct accretion Lower panel → plerian magnetic inhibition unction of the mass loss rate from the ; (3.1) =1.2. 8 . The equations that define the 6 − ˙ 5 variation of M (dotted line), and the corotation radius R ) and magnetic (R =1, and v a ∼ s3 from the companion star. In this case the parameters co . We note that a large part of this swing 1 (dashed line), as a function of the mass loss rate from the − co , and =0.8, P 8 33 subsonic propeller magnetic barrier centrifugal barrier with R centrifugal barrier with R µ erg s : Relative position of the magnetospheric radius, R , v at 2 3 37 s3 7 21 / / / 2 10d 1 2 33 33 r. 10 a µ µ , P 16 33 2 3 ∼ 11 8 µ / / 33 v 1 10d 2 8 µ 21 to v a 2 Lower panel 33 / 3 4 / µ / 8 60 31 1 1 6 v − − 8 − 45 =1.2. 10 . v ˙ 8 21 M / 0 4 ∼ / 3 10d 30 1 / 6 . 2 10d a − − =1.2. The lower panel of this figure shows that, for 0.1 a 6 ˙ 8 21 8 M − / =1, and v . ˙ 6 (dotted line), and corotation radius R 1 s3 . M 15 6 a 2 − − . ˙ ⇒ . M s3 a (solid line), with respect to the accretion radius R : Variation of the luminosity through different regimes, as a f 5 P . s3 R =1, and v M =0.001, P 4 P s3 33 > ⇒ µ & ⇒ M co s3 R co R P R Upper panel =0.8, P > : Variation of the luminosity as a function of the mass loss rate > 33 (A) M µ M R R We explore here the conditions under which transitions across different Fig. 2B shows that, in the presence of a standard NS magnetic field (10 Upper panel As an example, in Fig. 2A we use the above equations to compute the luminosity swin of the model are fixed at line), as a function of the(B) mass loss rate from the companion sta companion star. 3. Transitions and paths across different regimes only weakly on the orbital period and the total mass of the system, we fix a variations in the other four parameters: change in response to variations in the stellar wind parameters. In the followin to the direct accretion regimes, which take a mere factor of (about five decades) is attained across the transitions from the superKe the magnetospheric radius, R nosity jumps are not expected for a very slowly rotating (1000 s) NS (the oth with respect to the accretion radius R the magnetospheric radius crosses both the centrifugal (R spondingly, the system moves fromsonic the and superKeplerian subsonic magnetic propeller inhibition regime, reg six-decade luminosity and, swing finally, from to the direct accretion r As emphasised in § 2, these transitions occur when the relative positions of R tem with conditions for transitions between different regimes are Figure 2: Magnetars and SFXTs companion star. In this case the parameters of the model are fixed PoS(Integral08)101 , 5 co ∼ and R E. Bozzo persistent a luminosity 5 and short spin 10 egimes through 0.1) for similar ). In the second ∼ 1 ≪ − itions between dif- an both R 33 are required in order esides the centrifugal µ first case the centrifu- . Such transitions take erg s 0.1) in order for a large =0.1. The wind velocity B shows that an increase 33 s3 & 33 (dashed line), as a function of the mass -10 µ co unction of the mass loss rate. In this case 32 10 ≃ : Relative position of the magnetospheric radius, =2.2. 8 =0.01 and P 33 µ Lower panel =0.1, and v s3 =1.2. 8 8 =0.01, P and accretion does not take place; such systems might 33 µ ). M 1 (dotted line), and corotation radius R − =0.1, and v a s3 1) must posses lower magnetic fields ( erg s ≪ 1) require magnetar-like B-fields ( s3 37 =0.01, P & . Therefore, the direct accretion regime applies, with the luminosity 33 s3 µ -10 w ˙ =2.2 in Fig. 3B. These two figures show that, for sub-magnetar fields, 35 M 8 10 ) to arise from modest variations in the wind parameters (e.g. a factor ≃ 5 : Variation of the luminosity through different regimes, as a f 10 . ∼ w ˙ M for a wide range of wind parameters, accretion can take place, and a high 100 in the mass loss rate is required, in this case, to achieve a factor Upper panel co ∼ R (A) < ). These luminosity swings might result from transitions across different r M 6 − In Fig. 3 we show the transitions for a system with =1.2 in Fig. 3A, and v ˙ 8 (solid line), with respect to the accretion radius R M Few or no transitions are expected for systems with either high magnetic fields Shorter spin period systems (P Long spin period systems (P M for any reasonable value of proportional to swing; this is comparable with theTaking magnetar into case account of of Fig. the 2A. examples- discussed above, we conclude that: in luminosity swing ( are the same as those of Fig. 2A), since the magnetospheric radius is smaller th loss rate from the companion star. (B) Same as (A) but for R a 100 s spinning NS can undergo a transition across the magnetic barrier (b barrier), for suitable parameters (aplace high over a wind more velocity extended in intervalby of the a mass case factor loss at rates. hand) For instance Fig. 3 is v transitions to take place. Somewhat largerto variations achieve in similar the luminosity wind parameters swings toferent regimes those occur of in the most long cases period- through case, the and centrifugal barrier. trans luminosity is released ( periods, or systems with lower magneticgal fields barrier and halts long the spin inflowing periods. matter In at R the Figure 3: Magnetars and SFXTs case R thus be observable only at very low (X-ray) luminosity levels ( both the centrifugal and magnetic barriers. - the parameters of the model are fixed at PoS(Integral08)101 t 1 n w − ˙ . M 4 =10, 6 10 − erg s NS as a =0.001, ˙ E. Bozzo × M 33 In’t Zand, 36 , such that µ 6.5 co 10 & R , Cassinelli et gal barrier (as 2), while both × s rates, and the ibition regime, 1 peller regime is > 2 < − , (c) the outburst 6 M ≃ ity in this regime, 1 -2619 light curve, The spin period of − =200 and X − ˙ 6 J17544-2619 can be ctor of M ed by relatively mild − erg s nce. According to this ˙ , a SFXT observed by < 3.6 kpc, Rahoui et al., and R M ly spinning NS. We use ponds to the supersonic 32 Ts. erg s observation: (a) a quies- (this is a consequence of ∼ a 2619, a weaker magnetic 500. In this interpretation 1 ed by adopting R 10 34 − = > at four different stages, with 6 ∼ 10 − M ˙ × M erg s 3, where a slight decrease in Chandra 1.5 ∼ 32 6 ≃ − 10 X ˙ M ∼ 1000 s, then it must host a magnetar. > 1 hour-long outburst, yielding the first de- ∼ 9 =1. For this somewhat faster spin (and lower magnetic 8 20 the above values give R luminosity in the subsonic propeller regime compares well < 1 6 translates into an upper limit on the luminosity in this regime ) is dominated by accretion of matter onto the NS due to the − 1000 s periodicities in XTE J1739-302 and IGR J16479-4514, − h , (b) a rise stage with L 1 ˙ 1 > M − − erg s , and (d) a post-outburst stage (or“tail”) with L 1 =0.4, and v erg s 34 − erg s s3 34 10 32 0.6), the rise stage to the subsonic propeller (0.6 ∼ 10 erg s < 10 for which the supersonic propeller regime is attained in this interpretation). ∼ 10 higher than that given above (Bozzo et al., 2008). During the outburs 6 × w 37 ∼ − 2 ˙ =0.08, P ˙ M 10 =1.4, and show in Fig. 4(b) the different regimes experienced by such a ≃ M 33 8 X × =2. The µ 4 8 , is likely outshined by the X-ray luminosity of the supergiant star (the companio ≃ 1 X − =1.3, v during a complex transition to and from a s3 erg s Panel (c) of Fig. 4 shows an alternative interpretation of the IGR J17544 Motivated by the evidence for In this section we propose that transitions across different regimes caus As a case study we consider IGR J17544-2619 (Sunyaev et al., 2003) 31 =1, P =0.01, and v 10 33 s3 µ would cause the magnetic barrier tointerpretation, close if and IGR the J17544-2619 source has to a return spin to period quiesce of direct accretion must also be at work in the outburst tail at which is a factor of peak the direct accretion regime must apply at a mass loss rate of We note that, the whole luminosity swing takes place for a wider range of mass los 2008). The maximum luminosity swing observed across these stages was a fa the higher value of (see panel (a) of Fig. 4; these luminosities are for a source distance of where we fixed now significantly higher than the quiescence luminosity of with the luminosity in the rise stage. However, the luminosity of the supersonic pro KHI. The uncertainty in the value of P cent state with L IGR J17544-2619 is presently unknown.very In’t different Zand luminosity (2005) levels, could showed be th singled out during the peak with L tailed characterization of a SFXT light curve over a wide range of luminosity. variations of the wind parameters are responsible for the outbursts of SFX Chandra al., 1981). We concludeexplained that in the this lowest way, with emission the2005). state companion (quiescence) star The of dominating the IGR rise highwhere stage energy the is luminosity luminosity ( in ( good agreement with the subKeplerian magnetic inh the peak of the outburst and tail take place in the direct accretion regime at star’s luminosity is not shown in Fig. 4, but it is typically of order field), the luminosity variationopposed is to mainly the driven magnetic by barrier). apropeller transition regime In ( across this the case, centrifu the quiescent state corres function of the mass loss rate. For Magnetars and SFXTs 4. Application to SFXT sources respectively. Assuming an evenfield faster would NS be required. spin In period panel (d) for of IGR Fig. J17544- 4, we show the results obtain superKeplerian magnetic inhibition of accretion∼ applies. The expected luminos we discuss first the possibility that IGR J17544-2619 contains a very slow PoS(Integral08)101 . 1 1 − − 100 s erg s ≪ erg s E. Bozzo netic NS 36 , such that 35 10 capt 10 × ˙ mass loss rate the luminosity eriods 100 s. Whether × M etion might take > . In combination 1000 s. directly from the ed that their large n direct accretion e observed values only for very high rce states occur, in ith a spin period of ∼ ccretion away from wer ) can be attained in ocesses between the rst at 5 h extended high sen- s a very clumpy wind 5 , then a considerably riod the system undergoes en 2 in the wind velocity el of 5 ll the way to the lowest 10 se the NS magnetic field ∼ s require lower magnetic and discovered the 228 s ∼ 1 be driven mainly by either: − assing the subsonic propeller: erg s INTEGRAL 34 Based on the above discussion, we 0.004 would be obtained by imposing 10 ≃ × 33 µ must be lower than the limit fixed by Eq. 2.14. 10 co =R M 3000, an extremely high value even for an OB supergiant. ≃ 6 − ˙ revealed the source at 5 M (assuming a distance of 12.5 kpc, Lutovinov et al., 2005; Smith, 0.07 would be required. signalled that the source entered the subsonic propeller regime, while ≃ 33 µ 2-3). On the other hand, an increase by a factor of XMM-Newton INTEGRAL > 8 XMM-Newton The above discussion emphasizes the importance of determining, throug Interpreting the properties of IGR J17544-2619 in terms of a NS with a spin p As another example we discuss the case of IGR J16465-4507, a SFXTs w In this paper we reviewed the theory of wind accretion in HMXBs hosting a mag About a week later, pulsations (Lutovinov et al., 2005). Ifluminosity the level observed direct so accretion far, regime then applied an a upper limit of direct accretion occurred only during the outburst detected by that the NS didmeasured not by enter the subsonic propeller regime.higher On magnetic the field of other hand, if Magnetars and SFXTs outburst peak luminosity requires response to relatively modest variationstransitions across of different the regimes. wind We found parameters, that such provided transitions can 2004), which did not detect the source before the outburst down to a lev 228 s. The luminosity behaviour of this source is still poorly known. An outbu with a supergiant companion, and consideredinflowing in plasma some and detail the the interactiononto magnetosphere, pr the that NS surface are is expected inhibited.luminosity to We then swings take applied between this place quiescence theory whe to and SFXTs outburst and show (up to a factor of was observed with the magnetic barrier or thethe centrifugal outbursts, barrier will sets depend on in, whether causing the inhibition spin of period a is longer or shorter th the subsonic and the direct(unless accretion unrealistically regime high luminosities mass fallconclude loss that shortwards IGR rate J17544-2619 of likely are hosts th a considered). slowly rotating NS, with spin pe sitivity observations, the luminosity atparticular which the transitions lowest between luminosity different level sou with the for NS which spin, direct this can accretionplace be is unimpeded used still at to all infer at luminosity the work levels NSas of magnetic SFXTs, envisaged field. a in Alternatively possibility other accr which scenarios require (Waltercan & be Zurita considerably Heras, lower 2007). than discussed In here. this ca 5. Conclusions (with respect to the longer spin period solutions) would give a substantially lo is more difficult.corresponding For to instance, the for transition the across subsonic R propeller regime to set in, the supersonic propeller to the direct accretion regimetherefore, (or vice the versa), rise byp stage would remainfields unexplained. for Since fast direct rotating accretion NS towind take velocities place (v while in outburst, Eq. 2.14 is satisfied If instead the transition takes place at higher mass loss rate, the system goes PoS(Integral08)101 erent 05), the =0.4, and s3 E. Bozzo P t compan- , in the quiescence =0.08, d showed that 1 to outburst transition − 33 =2. µ 8 asise that in both v es the velocity and erg s long as 1000-2000 s. systems is still miss- hanism requires long 37 44-2619 (whose spin ely high luminosities NS rotation becomes se. We discussed also ind interaction regime 10 e a new prospective for htly shorter spin period chanism, when the mag- × Liu & Yan, 2006), clear =0.01, and G, respectively) are in the s3 that were not affected by pile-up (see P 13 , and 4 1 10 − G). On the other hand, magnetar- × =0.001, 8 erg s 14 33 & µ 36 10 10 & × ion from quiescence to outburst. Panel (a): A schematic =1. Panel (c): Interpretation of the quiescence to outburst ,2 1 G and 33 − µ 15 el are fixed at tted horizontal lines mark the luminosity that divide the diff erg s observation of this source are well matched by 10 34 osity stages are clearly visible. According to In’t Zand (20 11 & 10 =1.4, and 8 × v ier model. The parameters of the model are fixed at 5 . respectively. Panel (b): Interpretation of the quiescence ,1 1 Chandra =1.3, − s3 P erg s 32 10 × light curve of IGR J17544-2619 obtained by using the segments Chandra ) in NSs with spin periods of hundreds seconds. 1000 s) coupled with magnetar-like fields ( 1 & − Application of the gated model to the IGR J17544-2619 transit erg s 36 While the possibility that magnetars are hosted in binary system with supergian Evidence has been found that the spin periods of a few SFXTs might be as 10 =1. Panel (d): Same as panel (c) but here the parameters of the mod & 8 ions has been investigated byobservational several evidence authors for (e.g., such Zhang extremelying. high et According magnetic al., to field 2004; the NS presentdetecting in study, and binary long studying spin magnetars in period binary SFXTs systems. might provid magnetar range. the different regimes of wind-magnetosphere interaction expected in this ca an interpretation of this source(400 based s), on which the can centrifugal reproducesolutions barrier the the and luminosity required a stages magnetic slig comparably field well. strength ( We emph Motivated by this, we presentedperiod an interpretation is of unknown) the in activitythe of terms IGR luminosity of J175 stages the singled magnetic out barrier by in a a 1300 s spinning NS an like fields would also be( required if the centrifugal barrier sets in at relativ transition of IGR J17544-2619 based on the centrifugal barr of IGRJ17544-2619 in terms ofregimes. the The magnetic parameters barrier of model. the model The are do fixed at state, the rise state, the tail, and the peak of the outburst, (a) a centrifugal barrier mechanism,superKeplerian at which the halts magnetospheric radius, direct or accretionnetosphere (b) when extends a magnetic beyond the barrier the me accretionapplies radius. will depend Which sensitively mechanism on the andmass loss NS w rate spin in period the and supergiant’s wind. magneticspin In field, periods particular, ( besid the magnetic barrier mec v Figure 4: Magnetars and SFXTs representation of the both panels of figure 2 in In’t Zand, 2005). The different lumin count rates on the y-axis correspond to 2 PoS(Integral08)101 E. Bozzo F for grant support e University Press) omments and suggestions. This 12 EB thanks DTU Space, Technical University of Denmark and IASF-INA for this conference. We thank an anonymouswork referee was for supported his through useful ASI c and MIUR grant. References Bozzo, E., Falanga, M., Stella, L. 2008, ApJ, 683,Burnard, 1031 D. J., Arons, J., & Lea, S. M.Cassinelli, 1983, J. ApJ, P., 266, et 175 al. 1981, ApJ, 250, 677 Davidson, K. & Ostriker, J. P. 1979, ApJ, 179, 585 Davies, R. E., Fabian, A. C., & Pringle, J. E.Davies, R. 1979, E. MNRAS, & 186, Pringle, 779 J. E. 1981, MNRAS,Elsner, 196, R. 209 F. & Lamb, F. K. 1977, ApJ, 215,Frank 897 J., et al. 2002, Accretion Power in Astrophysics (3thHarding, ed., A. Cambridg K. & Leventhal, M. 1992, Nature, 357, 388 Ikhsanov, N. R. & Pustil’nik, L. A. 1996, A&A, 312,Illarionov, A. 338 F. & Sunyaev, R. A. 1975, A&A, 39,In 185 ’t Zand 2005, A&A, 441, L1 Liu, Q. Z. & Yan, J. Z. 2006, AdSpR, 38,Lutovinov, A., 2906 et al. 2005, A&A, 444, 821 Negueruela, I., et al. 2008, in press [astro-ph/0801.3863] Rahoui, F., Chaty, S., Lagage, P.-O., Pantin, E. 2008, A&A, 484,Sguera, 801 V., et al. 2005, A&A, 444, 221 Sguera, V., et al. 2006, ApJ, 646, 452 Sidoli, L., et al. 2007, A&A, 476, 1307 Smith, D. M. 2004, Astr. Tel., 338 Stella, L., White, N. E., & Rosner, R. 1986, ApJ,Sunyaev, R. 308, A., 669 et al. 2003, Astr. Tel., 190 Magnetars and SFXTs Acknowledgments PoS(Integral08)101 E. Bozzo 71, 569 13 Magnetars and SFXTs Toropina, O. D., Romanova, M. M., & Lovelace, R. V.Walter, E. R., 2006, et MNRAS, al. 3 2006, A&A, 453, 133 Walter, R. & Zurita Heras, J. A. 2007, A&A, 476,Zhang, 335 F., Li, X.-D. & Wang, Z.-R. 2004, ChjAA, 4, 320