Millisecond extragalactic radio bursts as magnetar flares
aa r X i v : . [ a s t r o - ph . H E ] J u l Millisecond extragalactic radio bursts as magnetarflares. Comment on “A population of fast radio burstsat cosmological distances” by Thornton et al.
Sergei B. Popov, ∗ Konstantin A. Postnov Sternberg Astronomical Institute, Lomonosov Moscow State University,Universitetski pr. 13, Moscow, 119991, Russia ∗ To whom correspondence should be addressed; E-mail: [email protected].
Properties of the population of millisecond extragalactic radio bursts discov-ered by Thornton et al. (2013) are in good correspondence with the hypothesisthat such events are related to hyperflares of magnetars, as was proposed byus after the first observation of an extragalactic millisecond radio burst byLorimer et al. (2007). We also point that some of multiple millisecond radiobursts from M31 discovered by Rubio-Herrera et al. (2013) also can be relatedto weaker magnetar bursts. Recently, a population of fast radio bursts (FRBs) of apparently extragalactic origin hasbeen discovered ( ). Their nature has been disputed since the first detection of a millisecondextragalactic radio burst ( ). Apparently non-thermal, these bursts can be formed in relativisticplasma which can be produced in a variety of astrophyical events (core-collapse supernovae,neutron star mergers, etc.). A natural explanation of FRBs can also be related to hyperflaresfrom soft gamma-ray repeaters (SGRs), as was suggested in ( ). This note is based on the Comment published on-line in the Science magazine:http://comments.sciencemag.org/content/10.1126/science.1236789 ). Energeticgiant flares can be due to relativistic magnetized explosions in SGR magnetospheres ( ), andthe appearence of non-thermal radio emission associated with SGR giant flares and hyperflaresseems natural. The possible physical mechanism for such radio bursts was suggested in ( ).Due to induced scattering, short radio bursts with high brightness temperature can propagateonly in relativistic plasmas ( ).Radio fluxes of FRBs agree with the scaling suggested by ( ). A hyperflare with L =10 erg s − (like the one detected from SGR 1806-20 on Dec. 27, 2004, see ( )) at adistance of 600 Mpc could produce a FRB with a flux ∼ Jy ( ). FRBs detected by ( ) aremore distant, so their radio fluxes should to be correspondingly lower, as observed.The rate of hyperflares for extragalactic SGRs, and so for millisecond extragalactic radiobursts, was estimated in ( ) to be 20-100 d − Gpc − . This is confirmed by new FRB obser-vations ( ) giving the rate ∼ d − from the whole sky up to a distance of ∼ Gpc. Thus,new observations ( ) support the model of FRB origin due to SGR hyperflares from the pointof view of predicted fluxes and statistics.Large dispersion measure, ∼ -1000, detected for FRBs ( ) is not unexpected in the caseof SGR flares, as these sources are associated with recent massive star formation ( ). Galaxiesat z ∼ . -1, in which FRBs were discovered, have significant massive star formation and areexpected to be rich in gas and dust, resulting in large dispersion measure.We also note that short millisecond radio burts with fluxes ∼ –4 Jy from M31 have recentlybeen reported ( ). Some of these events, especially multiple bursts with similar dispersionmeasures, can be connected with the repetitive activity of magnetars in M31. Candidates tohyperflares of magnetars in M31 have already been reported ( ). In the present case not hy-perflares, but weaker SGR bursts with luminosities L ∼ – erg s − can be responsible2or the observed radio bursts, assuming the SGR burst energy scaling from ( ).The rate of bursts obtained in ( ) is in agreement with that of X/gamma-ray bursts frommagnetars with required luminosity assuming one SGR in M31 to be in active phase — a fewevents per hour. Such a rate was observed by ( ) during the RXTE monitoring of a GalacticSGR during its active phase. X-ray (or/and gamma-ray) bursts with L ∼ – erg s − bythemselves can be easily missed from M31, because of their short duration. Despite potentiallydetectable fluxes, the number of photons from ∼ . s burst would be too low to be detectedand identified as a real source.However, if multiple radio bursts in M31 are related to magnetars activity, their rate isexpected to fluctuate significantly. Future observations can be used to check the expected radioburst energy distribution dN/dE ∼ E − . known for SGR bursts ( ). References and Notes
1. D. Thornton, et al. , Science , 53 (2013).2. D. R. Lorimer, M. Bailes, M. A. McLaughlin, D. J. Narkevic, F. Crawford,
Science ,777 (2007).3. S. B. Popov, K. A. Postnov,
ArXiv e-prints: 0710.2006 (2007).4. S. Mereghetti,
Astron. Astrophys. Review , 225 (2008).5. N. Rea, P. Esposito, High-Energy Emission from Pulsars and their Systems , D. F. Torres,N. Rea, eds. (2011), p. 247.6. M. Lyutikov,
Mon. Not. R. Astron. Soc. , 1594 (2006).7. M. Lyutikov,
Astrophys. J. Letters , L65 (2002).3. Y. Lyubarsky,
Astrophys. J. , 1443 (2008).9. J. Borkowski, et al. , GRB Coordinates Network , 1 (2004).10. K. Hurley, et al. , Nature , 1098 (2005).11. D. M. Palmer, et al. , Nature , 1107 (2005).12. S. B. Popov,
ArXiv Astrophysics e-prints: astro-ph/0502391 (2005).13. E. Rubio-Herrera, B. W. Stappers, J. W. T. Hessels, R. Braun,
Mon. Not. R. Astron. Soc. , 2857 (2013).14. E. P. Mazets, et al. , ApJ , 545 (2008).15. E. G¨oˇg¨us¸, et al. , Astrophys. J. Letters532