J. Astrophys. Astr. (2018) 39:14 © Indian Academy of Sciences https://doi.org/10.1007/s12036-017-9499-9 Review Collapsing supra-massive magnetars: FRBs, the repeating FRB121102 and GRBs PATRICK DAS GUPTA∗ and NIDHI SAINI Department of Physics and Astrophysics, University of Delhi, Delhi 110007, India. ∗Corresponding author. E-mail: [email protected], [email protected] MS received 30 August 2017; accepted 3 October 2017; published online 10 February 2018 Abstract. Fast Radio Bursts (FRBs) last for ∼ few milli-seconds and, hence, are likely to arise from the gravitational collapse of supra-massive, spinning neutron stars after they lose the centrifugal support (Falcke & Rezzolla 2014). In this paper, we provide arguments to show that the repeating burst, FRB 121102, can also be modeled in the collapse framework provided the supra-massive object implodes either into a Kerr black hole surrounded by highly magnetized plasma or into a strange quark star. Since the estimated rates of FRBs and SN Ib/c are comparable, we put forward a common progenitor scenario for FRBs and long GRBs in which only those compact remnants entail prompt γ -emission whose kick velocities are almost aligned or anti-aligned with the stellar spin axes. In such a scenario, emission of detectable gravitational radiation and, possibly, of neutrinos are expected to occur during the SN Ib/c explosion as well as, later, at the time of magnetar implosion. Keywords. FRBs—FRB 121102—Kerr black holes—Blandford–Znajek process—strange stars—GRBs— pre-natal kicks. 1. Introduction 43% linear polarization and 3% circular polarization (Petroff et al. 2015; Ravi & Lasky 2016; Petroff et al. Ever since the serendipitous discovery of FRB 010724, 2017). the very first Fast Radio Burst (FRB) gleaned from Have FRB counterparts been seen in other wave- archival pulsar survey data by Lorimer and his team bands? FRB 131104, the first one to be detected in members (Lorimer et al. 2007), about 24 FRBs have a targeted search using Parkes radio telescope, lies in been detected so far whose physical nature still con- the direction of Carina dwarf spheroidal galaxy (Ravi tinue to confound astrophysicists (Katz 2016; Zhang et al. 2015). According to DeLaunay et al. (2016), the 2017). FRBs are bright radio transients with very gamma ray transient Swift J0644.55111 is a counterpart high brightness temperatures, lasting for ∼ few mil- of FRB 131104 at 3.2σ confidence level. The estimated liseconds with peak flux densities ranging from ∼0.1 DM for this FRB is about 779 pc cm−3 implying a Jy to ∼10 Jy at frequencies of about ∼700 MHz–2 redshift of z =∼ 0.55 while Swift J0644.55111 has a GHz. fluence and duration of about 4 × 10−6 erg cm−2 and The associated large dispersion measure (DM) 377 s, respectively (Ravi et al. 2015; DeLaunay et al. ∼500–1200 pc cm−3 and high galactic latitudes stron- 2016; Murase et al. 2017). However, Shannon and Ravi gly suggest that FRBs are extragalactic events with (2017) reported that the absence of radio afterglow in redshifts z in the range ∼0.3to∼1, implying that they the direction of Swift J0644.55111 strongly constrains lie at distances 1 Gpc, if DM is largely due to elec- the energetics indicating that this gamma ray transient trons in the IGM. As of now, polarization data exist is unlikely to be a standard long-duration Gamma Ray only for few FRBs like FRB 110523 (z < 0.5),FRB Burst (GRB). 140514 (z 0.5) and FRB 150215 (luminosity dis- If FRBs result from such catastrophic events then tance <3.3 Gpc), with the first showing 44% linear a natural model that can account for their millisecond polarization, second less than 10% linear polarization duration radio emission at ∼1 GHz is that of initially but ∼20% circular polarization and the third, about rapidly rotating supra-massive neutron stars collapsing 14 Page 2 of 9 J. Astrophys. Astr. (2018) 39:14 into black holes (BHs) due to loss of centrifugal support It is plausible that if the core of a rotating, mas- as they spin down due to magnetic braking (Falcke & sive (M∗ 35M) Wolf–Rayet-like star collapses Rezzolla 2014; Zhang 2013). eventually to form a rapidly spinning (P ∼ 1ms), However, a catastrophic event like a collapsing neu- supra-massive magnetar (M0 3M), then as this rem- tron star is unlikely to produce a FRB candidate nant spins down due to magnetic braking, it can further that exhibit recurring outbursts. There certainly is one collapse after it loses its centrifugal support (CS). In par- such source from which sporadic radio transients are ticular, since the observed radio transients from FRBs observed – FRB 121102 (Spitler et al. 2016; Scholz have millisecond duration, the model posited by Falcke et al. 2016). About 200 such intermittent outbursts in and Rezzolla (2014) attains a special status, in which radio have already been detected from it so far. This FRBs result from gravitational collapse of spun down repeater is unlikely to be an active magnetar since recent supra-massive NSs on dynamical time scales, simultaneous observations of FRB 121102 with the help of Chandra Observatory and XMM-Newton place strin- R3 −11 −2 −3 gent upper limits, 3 × 10 erg cm on X-ray fluence τcoll ∼ ∼ 1 Gρnuc ∼ 10 s, (1) GM during the episodic radio-bursts and 3 × 1041 erg s−1 on a possible persistent X-ray luminosity (Scholz et al. where M and R are the mass and radius, respectively, 2017). ∼ of the NS at the onset of the collapse, while ρnuc = Several models for such repeating FRBs have been 1012−1014 gm cm−3 is the nuclear density inside the posited ranging from intense plasma wind sweeping NS. across the magnetosphere of an extragalactic pulsar ± If the NS collapses to a final radius Rf , energy that (Zhang 2017), episodic relativistic e wind from an can be released is given by Active Galactic Nucleus (AGN) impinging on a plasma cloud nearby (Vieyro et al. 2017), active young remnant ∼ 2 1 1 of a neutron star or a magnetar (Kashiyama & Murase E = GM − . (2) Rf R 2017; Metzger et al. 2017), intense flares from young magnetars leading to shock-induced maser emission According to the above expression, in case the NS col- (Kulkarni et al. 2014; Lyubarsky 2014; Beloborodov lapses to form a black hole (BH), energy generated and 2017) to asteroids falling randomly on a neutron star shared among high energy particles like photons and (Dai et al. 2016; Bagchi 2017). neutrinos created during the implosion is of supernova In the present study, we explore two distinct scenar- explosion energy scale ios to show that a repeating FRB can also be brought within the ambit of a collapsing supra-massive mag- E ≈ 8 × 1053 erg. (3) netar framework. In the penultimate section, we also delve into the possibility of linking FRBs with long (One has substituted M = 1.4M, R = 12 km and 2 GRBs. Rf = 2GM/c = 4.2 km in equation (2), to obtain equation (3). It will be an order of magnitude higher if M ∼ 2.5M.) 2. Magnetar collapse and FRB 121102 However, several factors like short time scale of col- lapse, small mean free path due to density being >ρnuc Magnetars are highly magnetized neutron stars (NSs) and ever increasing space-time curvature prohibit the with surface |B| 1014 Gauss. However, they spin rela- hot, relativistic particles to escape out of the collaps- tively at slower rates (period P ≈ 2−12 s) compared to ing object. Under such conditions, only the radiation pulsars. They display episodic intense X-ray outbursts caused by physical processes that take place in the that are powered by internal magnetic field instead of ephemeral magnetosphere left behind outside is likely rotational kinetic energy (Thompson & Duncan 1993; to be detected, entailing an event akin to a FRB (Falcke Usov 1993). Magnetars could have originated initially & Rezzolla 2014). Since the deduced FRB brightness with high spin rates (P ≈ few milliseconds) so that temperatures are 1030 K, one needs to invoke coherent small seed magnetic fields could have been amplified to emission mechanisms to explain these observed bright, very high B by α − dynamo action, and then, because short duration radio transients. of magnetic dipole radiation depleting the rotational For many FRBs, flux density Sν shows power law α kinetic energy, their period increased steadily (Thomp- behavior, i.e. Sν ∝ ν . The isotropic luminosity L for son & Duncan 1993). FRBs can then be readily obtained from the expression J. Astrophys. Astr. (2018) 39:14 Page 3 of 9 14 1+α ν Sν sensitivity of Arecibo radio telescope that could pick up L = 6.76 × 1043 1.4 GHz 2Jy the intermittent radio transients from FRB 121102 (Lu 2 & Kumar 2016; Spitler et al. 2016). D − × L erg s 1, (4) On the other hand, for the currently available FRB 10 Gpc sample, Palaniswamy and Zhang (2017) studied the dis- where DL is the luminosity distance corresponding to tribution of ratios of adjacent peak flux densities and the the cosmological parameters m = 0.27, = 0.73 time intervals between corresponding successive bursts and H0 = 68 km/s/Mpc. Given millisecond duration, and concluded that either the repeater belongs to a dis- equation (4) implies that FRBs radiate energy ∼1038 − tinct class or that all FRBs physically originate from a 1041 erg in radio, about 12 orders of magnitude less common progenitor system with FRB 121102, however, compared to the total energy released in most stellar being extra-active. Espousing their latter conclusion, we core collapse events.