On the Possible Wind Nebula of Magnetar Swift J1834.9−0846: a Magnetism-Powered Synchrotron Nebula

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RAA 2016 Vol. 16 No. 9, 143 (4pp) doi: 10.1088/1674–4527/16/9/143 Research in http://www.raa-journal.org http://iopscience.iop.org/raa Astronomy and Astrophysics

On the possible wind nebula of magnetar Swift J1834.9−0846: a magnetism-powered synchrotron nebula

Hao Tong Xinjiang Astronomical Observatory, Chinese Academy of Sciences, Urumqi 830011, China; [email protected]

Received 2016 May 5; accepted 2016 May 17

Abstract Recently, the magnetar Swift J1834.9−0846 has been reported to have a possible wind nebula. It is shown that both the magnetar and its wind nebula are understandable in the wind braking scenario. The magnetar’s rotational energy loss rate is not enough to power the particle luminosity. The required particle luminosity should be about 1036 ergs−1 to 1038 ergs−1. It is obtained in three different approaches: considering wind braking of Swift J1834.9−0846; the spectral and spatial observations of the wind nebula; and an empirical upper bound on wind nebula X-ray luminosity. The nebula magnetic ﬁeld is about 10−4 G. The possible wind nebula of Swift J1834.9−0846 should be a magnetar wind nebula. It is powered by the magnetic energy released from the magnetar.

Key words: pulsars: individual (Swift J1834.9−0846) — stars: magnetars — stars: neutron — winds and outﬂows

1 INTRODUCTION However, observations of magnetar wind nebulae have not been conclusive. For example, the extended emission around magnetar Swift J1834.9−0846was thought to be a Magnetars are a special kind of isolated neutron star. They dust scattering halo (Kargaltsev et al. 2012). Later, Younes may be neutron stars powered by their strong magnetic et al. (2012) proposed that some component may be due field (Duncan & Thompson 1992; Mereghetti et al. 2015). to a wind nebula. However, Esposito et al. (2013) used The magnetic energy that is released will dominate the ob- more observations to say that it should be a dust scattering servational behaviors of the magnetar’s persistent X-ray halo. Recently, Younes et al. (2016) confirmed the idea of luminosity, bursts and spin-down. During the outbursts, a wind nebula using two new deep XMM-Newton obser- the magnetar X-ray luminosity will decrease, accompanied vations. With the addition of recent observations, the ex- by a decaying spin-down rate and/or other timing events tended emission around Swift J1834.9−0846 is likely to (Scholz et al. 2014; Younes et al. 2015; Pintore et al. 2016; be a wind nebula. In the following, we will calculate the Tong & Xu 2013; Tong 2014). The variable spin-down rate properties of both the magnetar and its possible wind neb- of magnetars may be due to a strong particle wind flowing ula. This may be the foundation of more detailed studies in out from the magnetar magnetosphere (Tong et al. 2013). the future. This scenario is called “wind braking of magnetars.” In the case of normal pulsars, the particle outflow is pow- 2 WIND BRAKING OF MAGNETAR SWIFT ered by the rotational energy of the central neutron stars. J1834.9−0846 This particle outflow will finally be seen as a wind nebula around the pulsar (Gaensler & Slane 2006;Kargaltsev et al. The magnetar Swift J1834.9−0846 has a rotational pe- ˙ 2015). Analogous with normal pulsars, wind braking of riod of P = 2.48s and period derivative of P = 7.96 × −12 −1 magnetars predicted the existence of a magnetar wind neb- 10 ss (Kargaltsev et al. 2012). For a typical neutron 45 2 ula (Tong et al. 2013). The difference between a magnetar star moment of inertia of 10 g cm , the corresponding ˙ 34 −1 wind nebula and a traditional pulsar wind nebula is that the rotational energy loss rate is Erot = 2.1 × 10 ergs . magnetar wind nebula can be powered by the magnetic en- Assuming magnetic dipole braking in a vacuum, the char- P acteristic age of the magnetar is τc = =4.9kyr and the ergy that is released. The central neutron star’s magnetic 2P˙ energy may first be converted to a system of non-thermal characteristic magnetic field at the magnetic pole is Bc = particles. These particles will affect the radiation spectra of 6.4 × 1019 P P˙ = 2.8 × 1014 G. Swift J1834.9−0846 the magnetar and contribute a braking torque to the mag- may be associatedp with the supernova remnant W41 (Tian netar. When they flow out to the surroundings,they may be et al. 2007; Kargaltsev et al. 2012). The distance and age seen in the form of a wind nebula. provided by the supernova remnant is about 4kpc and 60- 143–2 H. Tong

200 kyr, respectively (Tian et al. 2007). Assuming a dis- an enhanced spin-down rate. The corresponding character- tance of 4kpc, the peak X-ray luminosity can be as high as istic age will be much smaller than the true age. This will 1035 ergs−1 during the outburst, which quickly decreases also contribute to resolving the age discrepancy. with time (Kargaltsev et al. 2012). The quiescent X-ray luminosity of Swift J1834.9−0846 can be more than 103 3 DIAGNOSING THE POSSIBLE WIND NEBULA times fainter (Kargaltsev et al. 2012; Younes et al. 2016). Considering that only a small portion of the particle energy The possible wind nebula around Swift J1834.9−0846 is observed in the soft X-ray range, between 2-10 keV is converted to non-thermal X-rays, the particle luminosity ′′ of Swift J1834.9−0846 can be as high as 1036 ergs−1 or (Younes et al. 2016). Its angular radius is about 100 . For higher, which depends on the X-ray efficiency. This par- a distance of 4kpc, the radius of the wind nebula is about ticle luminosity is much higher than the rotational energy 2pc. Considering the asymmetrical shape of the wind neb- loss rate. Then, the spin-down of Swift J1834.9−0846 will ula, a spherical radius of 1pc is adopted in the follow- be dominated by the particle wind (Tong et al. 2013 and ing calculations. Assuming a synchrotron origin for the references therein). Applying the wind braking model to X-ray emission, the typical synchrotron photon energy is Swift J1834.9−0846, its dipole magnetic field and age can (Ghisellini 2013) be obtained. 2 eB In the case of wind braking of magnetars, the corre- hγ ≈ 5 keV, (3) 2πmec sponding magnetic field at the magnetic pole is (Tong et al. 2013, eq. (31)) where h is Planck’s constant, e is the absolute electron charge, me is the electron rest mass, c is the speed of light, ˙ γ is the Lorentz factor of electrons responsible for the soft 25 P −1/2 B0 =4.0 × 10 L G, (1) X-ray emissions, and B is the nebula magnetic field in P p,35 the emission region. A typical photon energy of 5 keV is where Lp,35 is the particle luminosity in units of adopted. There are possible signatures of spectra softening 1035 erg s−1. For a particle luminosity of 1036 erg s−1, with radius in the wind nebula (Younes et al. 2016). This the polar magnetic field of Swift J1834.9−0846 is B0 = may be due to the synchrotron burnoff or absorption ef- 4.1 × 1013 G. The rotational energy loss rate due to mag- fect. If due to synchrotron burnoff, it will require that the netic dipole braking is proportional to Ω4, where Ω is the synchrotron cooling time scale is smaller than the crossing angular velocity of the neutron star. However, the rota- time of the nebula region (Gaensler & Slane 2006; Chen tional energy loss rate due to wind braking is proportional et al. 2006). For a flow velocity of c/3 (Kennel & Coroniti to Ω2. Therefore, the magnetar will mainly be spun down 1984), the crossing time is about 10 years for a nebula with by magnetic dipole radiation during its early age. Later, radius of 1pc. Then, the synchrotron cooling time scale is when it has slowed down significantly, it will enter into the (Ghisellini 2013) wind braking domain. For Swift J1834.9−0846, the tran- 24.57 sition period between magnetic dipole braking and wind yr ≤ 10yr. (4) B2γ braking is P1 = 0.36s (Tong et al. 2013, eq. (36)). The age of Swift J1834.9−0846 in the wind braking model is Solving Equations (3) and (4) simultaneously, the nebula − (Tong et al. 2013, eq. (41)) magnetic field is B ≥ 2.4 × 10 4 G and the electron Lorentz factor is γ ≤ 4.2 × 107. P The nebula photon index is about Γ ≈ 2 (Younes t = 1+2log τc. (2) P1 et al. 2016). The underlying electron distribution may also have a power law form of N(γ) = Kγ−p. N(γ)dγ is The age of Swift J1834.9−0846 is about 20 kyr. For a the number of electrons per cubic centimeter in the en- higher particle luminosity of 1038 ergs−1, by repeating the ergy range γ-γ + dγ, K is the normalization in units of above calculations, the magnetar age is about 50 kyr. The cm−3, and p is the power law index. The photon index corresponding supernova remnant age is about 60-200 kyr of Γ ≈ 2 means that the electron power law index is (Tian et al. 2007; Castro et al. 2013). The uncertainties in p = 2Γ − 1 ≈ 3. A Fermi type shock acceleration will the measured age of the supernova remnant may be a fac- generally result in a particle power law index of p ≈ 2 tor of several. The age of Swift J1834.9−0846 in the wind (Rosswog & Brüggen 2007). The corresponding photon p+1 braking model may be regarded as consistent with the age index of synchrotron emission is Γ = 2 ≈ 1.5. This of the supernova remnant. Therefore, by applying the wind may explain the photon index in the inner region of the braking model to Swift J1834.9−0846, the age discrep- wind nebula: ΓInn = 1.3 ± 0.3 (Younes et al. 2016). ancy between the characteristic age of the magnetar and Synchrotron cooling and related mechanisms will result in the corresponding supernova remnant age can be solved. a steeper particle spectrum. This may correspond to the The requirement is that the particle luminosity of Swift steeper photon index in the outer region of the wind neb- J1834.9−0846 is about 1036 ergs−1 to 1038 ergs−1. It is ula. At a distance of 4kpc, the nebula X-ray luminosity also possible that the particle luminosity has a finite duty is about L(0.5 − 10 keV) ≈ 2.5 × 1033 ergs−1 (Younes cycle and is temporarily relatively high. This will result in et al. 2016). The synchrotron power of a single electron is On the possible wind nebula of magnetar Swift J1834.9−0846 143–3

4 2 Ps = 3 σTcUBγ , where σT is the Thomson cross section 4 EMPIRICAL UPPER BOUND ON NEBULA and UB is the magnetic energy density in the emission re- X-RAY LUMINOSITY gion (Ghisellini 2013). The nebula X-ray luminosity can be approximated as The wind nebula luminosity is constant over a time span of nine years (Younes et al. 2016). For a particle lu- 36 −1 38 −1 Lnebula = PsN(γ)γVnebula, (5) minosity of 10 erg s to 10 erg s , the total energy released is about 1044 erg to 1046 erg. This is where Vnebula is the volume of the nebula. From this equa- equivalent to the energy released during magnetar gi- tion, the normalization K is found to be about K ≈ ant flares (Mereghetti 2008). However, no giant flare has 0.33cm−3. The energy density of the high energy elec- been observed from Swift J1834.9−0846. The spin-down trons is about 6.4 × 10−15 erg cm−3. rate of Swift J1834.9−0846 is relatively stable (Esposito The magnetic energy density in the emission region is et al. 2013). Therefore, the particle wind from Swift 2 B ≈ × −9 −3 J1834.9−0846 should have been relatively stable during about UB = 8π 2.3 10 erg cm . Assuming pressure balance between the magnetic field and the ambient the past years. The particle luminosity may also contain a material, the ambient pressure of the nebula should also be burst component (Harding et al. 1999; Tong et al. 2013). − −9 −3 37 1 about Pamb ∼ 10 erg cm . The particle wind becomes For a bursting particle luminosity of 10 ergs , with nearly isotropic far away from the magnetar (Tong et al. a duty cycle of 0.1, it may have the same effect of a 36 −1 2013). The termination shock radius of the wind nebula is persistent particle luminosity of 10 ergs (Tong et al. (Gaensler & Slane 2006) 2013). An empirical upper bound on the wind nebula X- ray luminosity is log LX < 1.6log E˙ − 24.2 (Kargaltsev ˙ Lp 1/2 −1/2 & Pavlov 2008). Here E is the particle luminosity. It is Rw = =0.02 Lp,36Pamb,−9 pc, (6) the rotational energy loss rate in the case of normal pul- r4πcPamb sars. For a particle luminosity of 1036 ergs−1, the em- where Lp,36 is the particle luminosity in units of pirical upper bound on the wind nebula X-ray luminos- 36 −1 33 −1 10 ergs and Pamb,−9 is the ambient pressure in ity is about 2.5 × 10 erg s . The observed wind neb- units of 10−9 erg cm−3. For a particle luminosity of ula X-ray luminosity around Swift J1834.9−0846 is also 1036 ergs−1, the termination shock radius is about 0.02pc. about 2.5 × 1033 ergs−1. Therefore, empirically, the par- If the particle luminosity is as high as 1038 ergs−1, the ter- ticle wind luminosity of Swift J1834.9−0846 should be mination shock radius is about 0.2pc. The wind nebula 1036 ergs−1 or higher. This is consistent with the above radius can be ten times larger than the termination shock analysis. radius (table 2 in Kargaltsev & Pavlov 2008). Therefore, the wind nebula can be produced if the particle lumi- 5 DISCUSSION AND CONCLUSIONS nosity is about 1036 ergs−1 to 1038 ergs−1. This is consistent with the above spin-down analysis of the central There have been possible non-detections of a wind neb- magnetar. A wind luminosity of 1036 ergs−1 or higher is ula associated with magnetar 1E 1547.0−5408 (Olausen much larger than the central magnetar’s rotational energy et al. 2011) and magnetar SGR 1806−20 (Viganòet al. loss rate (about 2.1 × 1034 ergs−1). Therefore, the pos- 2014). Possible signatures of a wind nebula have been sible wind nebula around Swift J1834.9−0846 cannot be reported in the high magnetic field rotating radio tran- rotation-powered. It should be a wind nebula powered by sients RRAT J1819−1458 (Camero-Arranz et al. 2013) the magnetic energy released from the central magnetar, and magnetar SGR J1935+2154 (Israel et al. 2016). Both i.e. a magnetar wind nebula. For this possible magnetar cases of non-detection and possible detection need further −5 wind nebula, its X-ray efficiency is η = LX/Lp ∼ 10 - study. After some timing events of magnetars have been 10−3. It is similar to the X-ray efficiency of a pulsar wind recorded, upper limits on possible extended emission have nebula (Kargaltsev & Pavlov 2008). also been reported (Archibald et al. 2013; Scholz et al. The supernova remnants associated with magnetars 2014). Typically, the supposed extended emission has been are similar to those of normal pulsars (Vink & Kuiper less than 2% of the magnetar’s X-ray luminosity. However, 2006; Martin et al. 2014). For a typical supernovaremnant, the X-ray efficiency of known wind nebulae is about 10−4. the reverse shock will reach the supernova remnant cen- Therefore, a 2% upper limit is not a definite constraint ter after about 7kyr (Reynolds & Chevalier 1984). At this (Tong 2015). time, the reverse shock will crush the previous wind nebula The possible wind nebula around Swift J1834.9−0846 (Gaensler & Slane 2006). The wind nebula radius will de- also needs further confirmation. Its association with su- crease and reexpand later (Gelfand et al. 2009). The wind pernova remnant W41 is questionable (Tian et al. 2007; nebula around Swift J1834.9−0846 may be older than Kargaltsev et al. 2012; Castro et al. 2013). For the current 20 kyr for particle luminosity higher than 1036 ergs−1. observations (Younes et al. 2016), a wind luminosity of The corresponding supernova remnant may have already 1036 erg s−1 to 1038 erg s−1 and a nebula magnetic field entered into the radiative phase. Therefore, it may be a of ∼ 10−4 G is required. The particle wind may be due to wind nebula expanding in a radiative phase supernova rem- persistent magnetic activities induced by the strong inter- nant. nal magnetic field (Thompson & Duncan 1995; Tong et al. 143–4 H. Tong

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