Tidal Disruption of Magnetars as the Sources of Fast Radio Burst

Fast Radio Bursts (FRBs) are mysterious millisecond duration radio transients, and some FRBs repeat. FRBs are highly polarized, indicating that the source is within a strong magnetic field. I hypothesize their origin as Tidal Disruption Events (TDEs) of magnetars by black holes. The TDE releases energy stored in the magnetic field of the magnetar, producing an FRB. Occasionally the magnetar is disrupted into multiple debris chunks, and as individual chunks get consumed, multiple FRBs are produced, forming a repeating FRB. Another process of formation of repeating FRBs is starquakes on magnetars because of tidal forces from black holes. Due to the strong magnetic field of the accretion disk, the FRB is produced in jets. Sometimes the magnetar debris surrounding the magnetic field gets accelerated, producing Ultra-high energy cosmic rays, whose source remains unknown. The FRB signal produced gets polarized by the magnetosphere of the accretion disk and intergalactic magnetic fields.

1 Introduction 1 Fast radio bursts (FRBs) are radio frequency transient astronomical events with a 2 duration ranging from a fraction of a millisecond to a few milliseconds. Some FRBs are 3 found to repeat, like FRB 121102 and others. Around 40 FRBs are detected to date, 4 and their origin remains unknown. FRB signals are polarized, which suggests that the 5 source is within a strong magnetic field, which I take as a clue of its origin. FRBs have 6 very high dispersion (DM) such as 375 pc cm -3 ( [12]) for the Lorimer burst, much 7 greater than that of the Milky Way DM M = 45 pc cm −3 ( [11]), indicating that they 8 have an extragalactic origin. Measurements of various DM of FRBs can be found at the in (1) d is the distance traveled by the FRB signal and n e is the number density of 12 electrons along d. 13 In this paper, I hypothesise the origin of FRBs as Tidal Disruption Events (TDEs) 14 of magnetars, in which a magnetar is tidally disrupted by a Black hole. My hypothesis 15 1 http://frbcat.org/ 1/10 is for explaining the origin of a sub-population of FRBs (including repeating FRBs), if 16 not whole population of FRBs. Magnetars are stellar remnants formed by supernovae of 17 massive stars. Magnetars are highly magnetic Neutron stars, with magnetic fields 18 reaching strengths of 10 9 to 10 11 T (Tesla) and energy density 4.0 × 10 25 Jm −25 ( [1]). 19 These strong magnetic fields contribute to the magnetosphere of the FRB disk, giving 20 rise to highly polarised signals as observed. sky -1 by [7].

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Sometimes, an FRB signal is found to repeat, such as FRB 121102, FRB 180814 and 38 others. They can be explained by my hypothesis, refer 2.1 for more details into the 39 hypothesised process. In some FRBs, the debris from the magnetar gets accelerated by 40 the magnetic field of the disk and also by the jet ( [8]). These charged particles may be 41 Ultra-High Energy Cosmic rays according to my hypothesis.

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[4] has localised FRB 180916 to a nearby spiral galaxy. They found fast radio burst 43 in a star-forming region. This supports my hypothesis as magnetars are much more 44 common in these places, as many massive stars form and die here in a supernovae, 45 giving rise to magnetar(s) and stellar black hole(s), which in turn give rise to FRBs. of repeating FRBs to occur based on my hypothesis is fracturing of magnetar' s crust ( 66 unlike [18], in my idea the fracturing is due to tidal forces from the black hole) over time 67 due to tidal forces from the black hole, releasing huge amounts of radio energy stored in 68 the magnetar' s crust and giving rise to repeating FRBs. Observed properties of FRBs 69 can be also be explained by my hypothesis. The burst duration is discussed below in Based on [6], as N H is high in ISM and WHIM; and the electron (and positron) column 78 density is high around magnetars ( [2]), the DM is high (based on (2) and [6]; [2] ). This 79 explains the high DM of FRB observations (visit FRBCAT for data). Soon after the in (3), G is the universal gravitational constant, M BH is the mass of the black hole, c is 93 the speed of light, and R s is the schwarzschild radius. The separation d ( based on R isco 94 as discussed above) will hence be equal to R = 6 × R s . Solving (3) based on Kepler's 95 laws for orbital period gives: When (4)

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The tidal disruption process begins with scattering of the magnetar into the loss cone of 107 the black hole ( [5]). This may occur as a result of another body influencing the orbit; 108 or by emission of gravitational waves. Once inside, the magnetar heads towards the 109 black hole. The tidal disruption occurs only if the pericenter distance R p is less than 110 the tidal disruption radius R t . R t can be represented mathematically as: In (5) sphere is given by the penetration factor β: The tidal disruption radius marks the boundary beyond which the tidal forces F t 115 from the black hole exceed the self-gravitation forces F s . Hence, tidal disruption events 116 can occur only if R p < R t , which from (6) in (7) c is the speed of light in a vacuum , G is the Universal Gravitational Constant 121 and M BH is the mass of the black hole. Therefore R t ¿ R s in order for the magnetar 122 TDE to be detectable. The strength of the tidal disruption η by [17] is a dimensionless 123 parameter given by: 4/10 Whenever η 1, the magnetar will be disrupted in a single flyby ( [17]). During 125 pericenter passage, the magnetar deforms(1) under tidal influence from the black hole. 126 As per [5] the deformation leads to a spread in orbital energy ∆ :

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This is the next phase of the tidal disruption event of the magnetar. After pericenter 129 passage and tidal deformation, the tidal forces disrupt the magnetar into long 130 stream-like structures (2). The stream has a linear density λ (by [5]): where l in (10) By solving (13), I obtained:

(14)
Then the density of the stream ρ according to (14) is: by simplifying (15) and substituting λ from (10): Or by simply assuming that the stream is in the form of a cylinder, Sometimes the stream may form magnetar debris chunks due to self gravitation 144 being greater than the tidal forces, which may happen when the TDE doesn't deep 145 within the Tidal Disruption Sphere. Whenever these debris chunks get consumed by the 146 black hole. In some scenarios the tidal forces on the magnetar may cause starquakes, 147 which can also give rise to repeating FRBs. [5] has applied chain rule to (18). The result is as follows: After fallback the debris forms an accretion disk. The accretion disk then feeds the 154 black hole, which gives rise to an FRB. Using the Thin Disk Approximation that height 155 H of the disk relates with radius of the disk R as H << R and the disk is axisymmetric, 156 and also the mass of the disk is negligible compared to the black hole. Hence the 157 angular velocity Ω(R) will be in Keplerian Form: 158 As a result of combination of loss of orbital energy, magnetar binding energy, and 159 magnetar magnetic field decay , a short burst (due to compactness of magnetar 160 material) in order of milliseconds (4) in radio wavelengths is produced, along with some 161 multiwavelength emissions, which is observed as a Fast Radio Burst on Earth.

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The accretion disk has very strong magnetic field, as a result being composed of intergalactic magnetic fields also contribute to the polarization of the FRB signal.

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Sometimes, as discussed in section 2.4, some parts of the stream clump together to 176 form relatively small 'chunks' of debris. The reason behind this is self-gravitation 177 overcoming the tidal forces from the black hole, which may happen when the pericenter 178 distance is relatively far, but within the tidal disruption radius. As each of these chunks 179 get consumed by the black hole, a huge burst of energy is released, which is observed as 180 a repeating FRB. In some scenarios, the debris collides with Interstellar Material, which 181 causes the debris to decelerate and give rise to shocks. These shocks might accelerate 182 electrons to relativistic velocities, giving rise to non-thermal, synchrotron radiation 183 ( [13]). This further strengthens the radio emission from the FRB.In a few scenarios, 184 during the first phases of TDE the magnetar's crust is subject to enormous stresses 185 from tidal forces from the black hole. This results in breaking of the crust, which 186 releases the huge amount of energy stored in the magnetic fields embedded in the crust, 187 giving rise to a starquake. As multiple starquakes occur, multiple bursts are produced, 188 which is observed as a repeating FRB. shocks found near FRB 140514. The radio source found by GMRT can be thought of as 204 radio emissions from the other FRB jet, although this fact also depends on the 205 orientation of the jets with respect to the observer. If the jet points directly towards us 206 we observe only one, bright source. If we observe the jet at an angle, we see 2 sources. 207 The two x-ray sources detected by Swift might correspond to those two points created 208 by internal shocks in the FRB jets. In radio wavelengths, whenever a FRB jet is 209 observed at an angle, two radio sources are observed. The jet closely oriented toward us 210 is interpreted as the burst, while the second jet is observed as another radio source.

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That second radio source might correspond to the radio source detected by GMRT.
[14] 212 mentioned that any theory explaining the progenitor of FRBs must explain the 213 polarization associated with them, and my hypothesis readily explains the origin of the 214 polarization; The polarization is due to the magnetic field of the accretion disk and the 215 jet, along with contributions from other sources and processes during the journey of the 216 signal to Earth, such WHIM magnetic fields. A few FRBs have been localised to their sources. FRB 180916 has been localised to a 240 star-forming region in a spiral galaxy by [4]. This supports my hypothesis, as magnetars 241 and black holes are more common in star-forming regions.

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In conclusion, I hypothesize that Fast Radio Bursts are caused by tidal disruption of 243 magnetars by black holes. The energy is released in the form of twin jets. Repeating