Could 2S 0114+650 be a magnetar? X.-D. Li1,2 and E. P. J. van den Heuvel2 ABSTRACT We investigate the spin evolution of the binary X-ray pulsar 2S 0114+650, which possesses the slowestknownspin period of ∼2.7 hours. We arguethat, to interpretsuchlong spin period, 9 the magnetic field strength of this pulsar must be initially ∼> 1014 G, that is, it was born as a 9 magnetar. Since the pulsar currently has a normal magnetic field ∼1012 G, our results present 9 support for magnetic field decay predicted by the magnetar model. 1 n a Subject headings: binaries: close - pulsars: individual: 2S 0114+650 - stars: neutron - X-ray: J stars 0 1 2 1. Introduction v 4 Neutron stars are thought to be born as rapidly rotating (∼ 10 ms) radio pulsars created during a 8 0 type II/Ib supernova explosion involving a massive star. The dipolar magnetic fields of radio pulsars, as 1 inferred from their observed spin-down rates, range from 108 G to 3×1013 G (Taylor, Manchester, & Lyne 0 1993). However, it has been proposed that there may exist “magnetars” - neutron stars with magnetic 9 9 field strengths in excess of ∼ 1014 G (Thompson & Duncan 1992). These objects have been invoked to / model soft gamma-ray repeaters (SGRs) and the 6−12 s anomalous X-ray pulsars (AXPs) (Thompson & h p Duncan 1996; see also Kouveliotou et al. 1993, 1994; van Paradijs, Taam, & van den Heuvel 1995; Corbet - et al. 1995; Vasisht, Frail, & Kulkarni 1995; Vasisht & Gotthelf 1997), though unambiguous evidence for o r the existence of magnetars comes from recent observations of the AXP-like object 1E 1841−045(Vasisht & t s Gotthelf 1997), SGRs 1806−20 (Kouveliotou et al. 1998a) and 1900+14(Kouveliotou et al. 1998b). a : According to Thompson & Duncan (1992, 1996), magnetars are neutron stars born with millisecond v i periods that generate magnetic fields above 1014 G by dynamo action due to convective turbulence, X magnetic field decay powers the X-ray and particle emission of magnetars. It is conceivable that when the r a magnetic fields in magnetars have decayed to, say, 1012 G, and accretion occurs, they are not distinguished from X-ray pulsars born with normal (1012−1013 G) magnetic fields, except that they may have relatively longer spin periods. One may then expect to find magnetar descendants among long-period binary X-ray pulsars, because they are much more luminous in X-rays than isolated objects accreting from interstellar medium. Here we presentarguments indicating that 2S 0114+650, the X-ray pulsarwith the slowestknown spin, may have a magnetar evolutionary history. 2. The slowest X-ray pulsar 2S 0114+650 The X-ray source 2S 0114+650 was discovered in 1977 by the SAS 3 galactic survey (Dower & Kelly 1977). Its optical counterpart, LS I+65 010, was recently identified as a supergiant of spectral type B1 Ia 1 DepartmentofAstronomy,NanjingUniversity,Nanjing210093, China 2 AstronomicalInstitute, UniversityofAmsterdam,Kruislaan403,1098SJAmsterdam,TheNetherlands – 2 – (Reig et al. 1996). Thus 2S 0114+650 belongs to the class of high-mass X-ray binaries (HMXBs), systems in which a compact star - generally a neutron star - accompanies a high-mass donor star (cf. Bhattacharya & van den Heuvel 1991 for a review). With a distance of 7.2 kpc derived from this spectral classification, the X-ray luminosity is a few 1035−1036ergs−1. An orbital period of 11.59 days was reported from optical radial velocity measurements by Crampton, Hutchings, & Cowley (1985). There has been some weak evidence ofa pulsationperiodof850−895s(Yamauchietal. 1990;Koenigsbergeretal. 1983). Incontrast, Finley, Belloni, & Cassinelli (1992) have discovered periodic X-ray outbursts with a 2.78 hour period. The same period was confirmed by ROSAT observations (Finley, Taylor, & Belloni 1994). Recent Rossi X-ray Timing Explorer (RXTE) observations show presence of modulation on 11.59 day orbital period as well as 2.7 hour pulse period (Corbet, Finley, & Peele 1998). If this pulse period indeed represents the rotation period of the neutron star, 2S 0114+650would be by far the slowest known X-ray pulsar. 3. The spin evolution in 2S 0114+650 How has 2S 0114+650 been spun down to the long period (P ≃ 104 s) if it was formed with much shorter period (∼0.01−1 s, say)? The neutron star’s spin evolution is divided in three phases (see Davies & Pringle 1981; Bhattacharya & van den Heuvel 1991). In the first radio pulsar phase, the star is an active radio pulsar and spins down by magnetic dipole radiation. This phase ends when the ram pressure of the ambient material overcomes the pulsar’s wind pressure at the gravitational radius (R = 2GM/V2, G where G is the gravitation constant, M is the neutron star mass, and V is the relative velocity between the neutron star and the ambient material). In the second propeller phase material enters the corotating magnetosphere and is stopped at R , the Alfv´en radius, where the energy density in the accretion flow A balances the local magnetic pressure. Further penetration cannot occur owing to the centrifugal barrier; that is, R >R , where R =[GM(P/2π)2]1/3 is the corotation radius. The star expels the material once A c c it spins up the material to the local escape velocity at ∼R (Illarionov & Sunyaev 1975). Propeller action A continuesuntilR ≃R ,whenthecentrifugalbarrierisremovedandthe spinperiodreachesits equilibrium A c value (Bhattacharya & van den Heuvel 1991) P ≃(20s)B6/7M˙ −3/7R18/7M−5/7, (1) eq 12 15 6 1.4 where B = 1012B G is the neutron star’s dipolar magnetic field strength, M˙ = 1015M˙ gs−1 the 12 15 mass accretion rate, R = 106R6 cm the radius, and M1.4 = M/1.4M⊙ (Throughout this Letter, we take M =R =1). In the following accretion phase the star becomes an X-ray pulsar, and its spin evolution 1.4 6 is determined by the net torque exerted on the star by the accretion flow. TheX-rayemissionobservedin2S 0114+650ismostlikelytobepoweredbyaccretionontotheneutron starviaastellarwindfromthecompanion. Withamass-lossrateofafew10−6M⊙yr−1 andawindvelocity of ∼ 103kms−1, the derived X-ray luminosity from a simple wind-fed model is in accordance with the mean detected one (Reig et al. 1996). A stable accretion disk is unlikely to form around the neutron star, which would require an extremely low (∼ 200kms−1) wind velocity (Wang 1981). Early two-dimensional numericalstudiesofBondi-Hoyleaccretionflow(e.g.,Matsuda,Inoue,&Swada1987;Fryxell&Taam1988, 1989) demonstrated that temporary accretion disks with alternating sense of rotation possibly form in a windaccretingsystem. Morerecenthighresolutionthree-dimensionalnumericalinvestigations(e.g., Ruffert 1992, 1997) found the so-called wind “flip-flop” instability with the timescale of the order of hours. This is consistent with the torque fluctuations in wind-accreting X-ray pulsars (Nagase 1989), suggesting that the long-term averaged angular momentum transferred to the neutron star by the accreted wind material is – 3 – very small. We therefore conclude that the spin period of 2S 0114+650has not been considerably changed during the present accretion phase. This means that the long spin period of 2S 0114+650 was actually attainedinanearlierevolutionaryphasebeforethecompanionstarbecameasuper-giant-itwasspundown by the propeller mechanism (Illarionov & Sunyaev 1975) when the companion star was on main-sequence and had a weakerwind. The magnitude of the equilibrium period, as seen in equation (1), is determined by the magnetic field strength and mass accretion rate of the neutron star during this phase. The magnetic field strength in 2S 0114+650 has not been measured, since no cyclotron features have been seen in its X-ray spectrum. But there is indirect evidence indicating that its magnetic field strength is similar to those in other X-ray pulsars: Its spectrum shows the typical shape of the usual X-ray pulsars having a power-law with an exponential high-energy cutoff at ∼ 7 keV (Yamauchi et al. 1990) or ≥ 15 keV (Koenigsberger et al. 1983). The iron emission line at about 6.4 keV, which is common among X-ray pulsars, was also discovered in 2S 0114+650 (Yamauchi et al. 1990; Apparao, Bisht, & Singh 1991). Observations of cyclotron lines in X-ray pulsars imply surface fields of about (0.5−5)×1012 G, and it has been suggested that the cutoff seen in the power-law spectra of X-ray pulsars is related to the magnetic field strength of the neutron star (Makishima & Mihara 1992) 3. This would imply a field of ∼1012 G for 2S 0114+650, consistent with those for typical X-ray pulsars which show cutoff energies of 10−20 keV. The mass of LS I+65 010 was estimated to be 16(±5)M⊙ from evolutionary models (Reig et al. 1996). For a typical mass-loss rate of ∼ 10−8 −10−7M⊙yr−1 from a 16 M⊙ main-sequence star and a wind velocity of ∼ 103kms−1, the accretion rate of the neutron star is M˙ ∼ 1013−1014gs−1. Inserting the values of B and M˙ into equation(1), we find P ∼50−140 s, nearly two ordersof magnitude shorter than eq the observed period. (Even if B is enhanced to 1013 G, P never exceeds ∼1000 s.) This implies that the eq extremely long period of 2S 0114+650 cannot be reached via the propeller mechanism if the pulsar has possessed a constant magnetic field of ∼ 1012−1013 G. One may argue that the accretion rate during the propeller phase could be much lower than that adopted here, because of a lower rate of mass loss from the companion star or a higher wind velocity. For example, if M˙ ranges from ∼ 109gs−1 (for B = 1012 G) to ∼ 1011gs−1 (for B = 1013 G), the value of P can be indeed raised to ∼104 s. However, this would lead eq to a spin-down time during the radio pulsar phase (Davies & Pringle 1981) τ ≃ (2.5×1010yr)B−1M˙ −1/2V−1 s 12 9 8 ≃ (2.5×108yr)B−1M˙ −1/2V−1 (2) 13 11 8 (where V = 108V cms−1), which is much longer than the lifetime the companion star spends on 8 main-sequence (generally ∼< 107 yr). There exists another possibility that the neutron star was born rotating slowly (P ∼ 1 s), so that it went directly to the propeller phase. Propeller spin-down to P takes (Wang & Robertson 1985) eq τ ≃ (1.5×1010yr)B−1/2M˙ −3/4P−3/4 s 12 9 1 ≃ (1.5×108yr)B−1/2M˙ −3/4P−3/4 (3) 13 11 1 (where P =P/1 s), which is still much longer than 107 yr. 1 We are eventually led to the conclusion that 2S 0114+650 must initially have had a magnetic field much stronger than its present value, that is, it was born as a magnetar. Distinguished from neutron 3This relation is not quite accurate (see Reynolds, Parmar, & White 1993), but it may provide an order of magnitude estimateofthemagneticfields. – 4 – stars with normal magnetic field strengths, magnetars are radio-quiet, and the decaying magnetic fields power the X-ray and particle emission. For a magnetar in a binary system, during the early evolutionary phase, the pressure exerted by particle emission exceeds the stellar wind ram pressure at ∼R , preventing G accretion onto the neutron star (Thompson & Duncan 1996). After t ≃ 104−105 years, the star’s spin period increases to P ≃(10s)t1/2B R2M−1/2 (4) 4 15 6 1.4 by magnetic dipole radiation (where t = t/104 yr), the particle luminosity decays, and the wind material 4 from the companion star begins to interact with the magnetosphere. Again adopt a mass accretion rate of ∼ 1013−1014gs−1, the propeller effect, for a magnetic field of ∼ (1−4)×1014 G, can comfortably spin down the neutron star from ∼ 10 s to ∼ 104 s on a timescale ∼< 105 years (cf. equations [1] and [3]), i.e., before the magnetic field decays significantly (see below). After this long equilibrium period is reached, the period of the neutron star would remain close to it, due to inefficient angular momentum transfer during the subsequent wind-accretion process. Field decay in 2S 0114+650 should be slow enough to garantee the final, long spin period, and fast enough to allow a considerable reduction in the field within the lifetime of the system. There exist several mechanisms for field decay in non-accreting neutron stars: Ohmic decay, ambipolar diffusion and Hall drift (e.g., Goldreich & Reisenegger 1992). Ohmic decay dominates in weakly magnetized (B ∼< 1011 G) neutron stars, fields in intermediate strength (B ∼1012−1013 G) decay via Hall drift, and intense fields (B ∼> 1014 G) are mainly affected by ambipolar diffusion. For typical values of the characteristic length scale (105 cm) of the flux loops through the outer core and the core temperature (108 K), the field-decay timescale for ambipolar diffusion through the solenoidal mode in a magnetar is around 3×105 yr if B ∼ 1014 G (Thompson& Duncan1996). Comparedto the age(afew 106 yr)ofthe opticalcompanionof2S 0114+650, this implies that the magnetar has had enough time for its field to decay to its current value (a few 1012 G). Similar conclusion can also been found from the detailed calculations by Heyl & Kulkarni (1998). 4. Discussion SGRs, which are transient gamma-ray sources that undergo repeated outbursts, have been suggested to be the prototypes of magnetars. There are four known SGRs, with two (SGRs 1806−20 and 0525−66) associated with young supernova remnants (Kulkarni & Frail 1993; Vasisht et al. 1994). Recently, pulsations at a period of 7.47 s and 5.16 s with a spin-down rate of 2.6×10−3syr−1 and 3.5×10−3syr−1 were discovered in the persistent X-ray flux of SGRs 1806−20 (Kouveliotou et al. 1998a) and 1900+14 (Kouveliotou et al. 1998b), respectively; the magnetic field strengths of (2−8)×1014 G can be derived if the spin-down is due to magnetic dipole radiation. AXPs are another type of high-energy sources that have recently joined this group of highly magnetized neutron stars. They are a group of about eight pulsating X-raysourceswithperiods around6−12s (cf. Stella, Israel,& Mereghetti1998for areview),characterized by steady spin-down, relatively low and constant X-ray luminosities of ∼1035−1036 ergs−1, and very soft X-ray spectra. So far no optical counterpart has been detected. Nearly half of them are located at the centers of supernova remnants, suggesting that they are relatively young (∼< 105 years). Dipole magnetic fields of 1014−1015 G can also be derived from the measured spin-down rates, if they are spinning down due todipole radiationtorques. TheobservedX-rayluminosities,spinperiodsandspin-downratesinSGRs and AXPs follow naturally from the magnetar model (Thompson & Duncan 1996). A crucial test of the magnetar model should include the observational evidence of the hypothesized magnetic field decay in magnetars. This evidence is most likely to be found in slowly-rotating,binary (such – 5 – as 2S 0114+650) and isolated (such as RX J0720.4−3125; Haberl et al. 1997; Heyl & Hernquist 1998; Kulkarni& van Kerkwijk 1998)X-raypulsars. Figure 1 comparesthe magnetarcandidates (SGRs 1806−20 and 1900+14 in filled rectangles, and AXPs 1E 2259+586,4U 0142+61, 1E 1048.1−5937and 1E 1841−05 infilledstars)withtheir possibleX-raypulsardescendants(2S 0114+650inopentriangleandotherpulsars in open stars) in the magnetic field versus spin period diagram. A schematic view of the magnetar field and spin evolution of magnetars is clearly seen. The magnetic fields of SGRs and AXPs are derived from their observed spin-down rates (Kouveliotou et al. 1998a, 1998b; Stella, Israel, & Mereghetti 1998 and references therein) under the assumption that the spin-down is caused by magnetic dipole radiation. For 2S 0114+650we take a canonicalmagnetic field of 3×1012 G with the possible range of 1012−1013 G. The dashedlinesrepresentthe relationbetweenthe magnetic fieldstrengthandthe equilibriumspinperiodwith various mass accretion rates (1018, 1015 and 1012gs−1), from which an initial magnetic field of a pulsar can be constrained by the present spin period, given a reasonable estimate of the mass accretion rate during the propeller phase. The detailed evolutionary track of a magnetar depends on the particle luminosity, the properties of the stellar wind from the companion, the magnetic field and its decay timescale. Fig. 1.— The magnetic field versus spin period diagram for the candidates of magnetars (SGRs in filled rectangles and AXPs in filled stars) and the slow X-ray pulsars (in open stars except 2S 0114+650 in open triangle) with known magnetic fields. The dashed lines denote the relation between the magnetic field and the equilibrium spin period with various mass accretion rates. In Fig. 1 we have also plotted the slow (spin periods ∼> 100 s) X-ray pulsars (A 0535+26, Vela X−1, GX 1+4, 4U 1907+09, 4U 1538−52 and GX 301−2) with known magnetic field strengths as possible descendants of magnetars. Their magnetic field strengths are generally estimated from the observed cyclotron line features in X-ray spectra (Mihara & Makishima 1998 and references therein). For GX 1+4, it is determined from the observational evidence of the propeller effect (Cui 1997). These sources may have had a similar evolutionary history as 2S 0114+650, but for them the evidence is not as unequivocal as for 2S 0114+650. Their slow spins could be accounted for in terms of the propeller effects with normal magnetic field strengths (e.g., Illarionov& Sunyaev 1975; Waters & van Kerkwijk 1989,see also the dashed lines). However,a magnetar model presents an alternative explanation that can not be ruled out. 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