Local Circumnuclear Magnetar Solution to Extragalactic Fast Radio Bursts
We synthesize the known information about Fast Radio Bursts and radio magnetars, and describe an allowed origin near nuclei of external, but non-cosmological, galaxies. This places them at , within a few hundred megaparsecs. In this scenario, the high DM is dominated by the environment of the FRB, modelled on the known properties of the Milky Way Center, whose innermost 100pc provides 1000 pc/cm. A radio loud magnetar is known to exist in our galactic centre, within 2 arc seconds of Sgr A*. Based on the polarization, DM, and scattering properties of this known magnetar, we extrapolate its properties to those of Crab-like giant pulses and SGR flares and point out their consistency with observed Fast Radio Bursts. We conclude galactic center magnetars could be the source of FRB’s. This scenario is readily testable with VLBI measurements as well as with flux count statistics from large surveys such as CHIME or UTMOST.
Subject headings:FRB, magnetars, galactic center, giant pulse
222 Canadian Institute for Advanced Research, Program in Cosmology and Gravitation
333 Department of Astronomy and Astrophysics, University of Toronto, M5S 3H8 Ontario, Canada
444 Dunlap Institute for Astronomy and Astrophysics, University of Toronto, M5S 3H8 Ontario, Canada
The phenomenon of Fast Radio Bursts (FRB’s) has generated excitement in the astronomical community as well as speculation regarding the origin of the events (Lorimer et al., 2007; Thornton, 2013). FRB’s are millisecond radio transients with flux densities between 0.21.5 Jy. They are also highly dispersed, with DM’s far exceeding the expected contribution from our own galaxy in their direction (DM5001200 pc/cm). Theories of their distance vary from atmospheric to solar system, galactic and cosmological, however the high DM’s have lead a number of people to believe they are extragalactic. This is also partly due to their location on the sky, since the high galactic latitudes cast doubt on a galactic or solar system origin (Thornton, 2013). If the extragalactic dispersion is caused by the intergalactic medium (IGM) then the sources would be cosmological, found between redshifts 0.451. However it is possible that the dispersion could be due to dense regions in more nearby galaxies, as noted by Thornton (2013) and Luan & Goldreich (2014). These galaxies would be within a few hundred megaparsecs, which we will consider non-cosmological.
Perhaps more mysterious than their location are their progenitors and emission mechanism. A wide range of ideas have been proposed, from Blitzars (Falcke & Rezzolla, 2014) to superconducting cosmic strings (Yu et al., 2014), compact object mergers (Kashiyama et al., 2013)) to nearby flaring main sequence stars (Loeb et al., 2014).
In this letter we provide yet one more allowed interpretation consistent with current data: giant pulses or outbursts from magnetars in the nuclear regions of external galaxies. The idea that FRB’s could be radio-emitting magnetars was explored by Lyubarsky (2014) and Kulkarni et al. (2014), where the high brightness temperatures are explained by shock-induced maser emission. In our letter we do not focus on the emission mechanism nor do we favour pulsar-like emission (giant pulses) vs. SGR flares; we are simply putting forth an explanation for FRB’s based on magnetars near the centers of external galaxies that is consistent with the existing data, which makes falsifiable predictions. The non-cosmological (i.e. local) extragalactic nuclear FRB can naturally explain the observed large dispersion measures (DM) and scattering (SM) and could help explain their polarization properties.
2. Galactic Center Pulsars
2.1. Nuclear Properties
Our own galactic center region has a high measured electron density, and the recently discovered pulsar and magnetar SGR J1745-2900 has a measured DM=1778 (Eatough, 2013), most of which is thought to originate from the inner few parsecs of the galaxy. Seen from a typical extragalactic viewing angle, this magnetar would have a DM 1000. It is scattered by a few seconds at GHz, which is a thousand times longer than the observed scattering time scales of FRB’s. However VLBI measurements indicate that this scattering is dominated by a screen closer to our sun than the galactic center (Bower et al., 2014), in which case a typical extragalactic line of sight would see a much smaller scattering time, perhaps a few ms.
It had been thought that the GC harboured a large number of pulsars but that they were difficult to observe at low frequencies due to a scattering screen within 200 pc of Sgr A* (Wharton et al., 2012; Lazio & Cordes, 1998). However after the discovery of the radio-loud magnetar J1745-2900 just 2 arcseconds from Sgr A* it now seems there really is a dearth of regular pulsars and an over-representation of magnetars. Though hundreds to thousands of ordinary pulsars were predicted to exists within pc of the galactic centre, none has yet to to be found (Pfahl & Loeb, 2004). Dexter & O’Leary (2014) show that this implies the region is an effective environment for magnetar formation, whose short lives could explain the lack of such radio-loud objects in the central parsec. The GC would then be a graveyard of highly-magnetized massive stars, some of which became magnetars and emitted in the radio for years before spinning down sufficiently to cross the death line.
It is worth pointing out that J1745-2900 is one of just four known radio-loud magnetars. Within 2.2 arcseconds of Sgr A* it occupies volume that is of our galaxy’s volume and 5 of its mass, and where there is an anomalous absence of ordinary pulsars. This suggests to us that radio-loud magnetars not only form in such environments, but preferentially do. External nuclear regions could therefore also harbour magnetars and could provide both the dispersion and the scattering observed in FRB’s.
In the cosmological picture it is difficult to explain the scattering tails seen in several bursts from the IGM. Luan & Goldreich (2014) point out that if it is due to turbulence then the length scale of plasma scattering in the IGM at a distance of 1 Gpc for ms tails is impossibly small. In other models the IGM is an equally unlikely place for the scattering to occur (Pen & Levin, 2014). However we do point out that McQuinn (2014) have shown cosmological FRB’s could be scattered at ms by intervening galactic disks if their electron distribution were more extended than is currently believed.
2.2. Possible Sources
A dozen or so pulsars are known to exhibit giant pulses (GP’s), which are of very short duration and can be many orders of magnitude brighter than their average pulse flux (Mickaliger et al., 2012). A rare tail of supergiant pulses (Cordes et al., 2004) has also been identified, with brightness temperatures reaching up to K (Hankins et al., 2003). They tend to be short enough (16 ns) that their pulse is consistent with a pure scattering profile, which is also the case for the observed FRB’s. It is worth noting that the only FRB for which there is polarization information is FRB 140514, which was found to have 20% circular polarization and very little linear polarization (Petroff, 2014). Considering the rotation measure of the GC magnetar, J1745-2900, is RM=-6.710 rad m, if other galactic centers were like our own then nuclear pulsars and magnetars could become linearly depolarized due to multi-path Faraday rotation from a scattering screen (Petroff, 2014). It is also possible that the sources themselves are circularly polarized. At 2 GHz, J1745-2900 is also observed to be 20% circularly polarized, with no detected linear polarization. At higher frequencies, this magnetar is strongly linearly polarized. Giant pulses are known to often be highly circularly polarized, for example over half of the peaks from B1937+21 are in a pure Stokes V state.
Though none of the known pulsars that exhibits GP’s is a radio loud magnetar, the energetics of FRB’s are not difficult to accommodate and one could imagine high luminosity radio outbursts from such objects; some magnetars are soft gamma ray repeaters (SGR), which have episodic outbursts emitting erg in a fraction of a second. At distances of 100 Mpc, the inferred energy of an FRB is erg, a tiny fraction of known SGR burst energies. Magnetars that emit in the radio can also have non-negligible circular polarization and the GC object J1745-2900 seems to have a typical circular fraction of , though this increases when the pulsar flares up (Lynch et al., 2014).
Only a tiny fraction of the burst energy needs to come out to power a fast radio burst. In order to explain the common large DM of FRB’s, these GP’s would have to be preferential properties of circumnuclear magnetars. Events would be expected to repeat after years, making a direct search challenging. An all sky search with a telescope such as CHIME (Bandura, 2014) over a year could discover events, of which would repeat in a year and a few would be lucky enough to be caught in the same CHIME beam a second time. The long integrations at known FRB locations have not resulted in repeat events, which is consistent with this picture.
Given the small number of radio loud magnetars in the Milky Way, one cannot comment on their distribution in other galaxies. However in this picture there could be a sizeable fraction of sources that exist outside of their galaxy’s nuclear regions, in which case there should be a commensurate number of FRB’s with modest dispersion measures, perhaps 70100 pc cm for an object at 100 Mpc. The apparent lack of sources with such DMs could be explained by a selection effect: radio bursts whose dispersion measures are not extraordinary may simply not get identified as FRB’s. These may be missed or ignored given the large ensemble of radio transients with an apparent sweep, including RRAT’s and perytons (Bagchi et al., 2012).
This scenario is readily testable: At redshifts less than unity , the flux distribution is given by a Euclidean universe, with , only weakly dependent on DM, assuming the bursts are standard candle-like. This is not necessarily expected for high redshift objects, where cosmological expansion and source population evolution are expected to change.
A VLBI detection would find a spatial coincidence to within a few parsecs of a galactic nucleus, which is milli arcseconds at distances of 100 Mpc. The current non-coincidence with nearyby galaxies constrains the typical distance to be larger than 100 Mpc. This is still an order of magnitude closer than if the DM is primarily accounted for by the intergalactic medium.
The galactic center magnetar is linearly depolarized at frequencies below 4 GHz, consistent with multi-path Faraday depolarization from the scattering screen (Petroff, 2014). Circular polarization is not affected, and has indeed been observed in FRB’s.
Substantial interest has developed for cosmology, should FRB be at cosmological distances. These are summarized in (McQuinn, 2014). Should the DM be dominated by the host galaxy, these applications would be difficult to materialize. The expected scattering size of such events would be micro arcseconds, which could be detectable with galactic scintillation(Pen et al., 2014).
In a large survey, such as CHIME, the closest event could be at Mpcs distances. Continuous monitoring of neighboring galactic centers, e.g. M31, for years, could detect pulses many kJy bright, requiring only a small receiver to monitor. Similarly, long term continuous monitoring of the GC magnetar may uncover rare super-giant pulses. All-sky telescopes, such as the FFTT (Tegmark & Zaldarriaga, 2009), may be well suited for finding close, bright, sources.
Extrapolating from the one known nuclear magnetar, a sample of as might be found by CHIME, could result in the closest projected impact angle of 7 mas. This would place it near the Einstein Ring radius, within Schwarzschild radii, such that it could be gravitationally lensed by the central black hole. Assuming its projected proximity to the black hole does not increase the FRB’s DM or SM too significantly, this would be seen as an echo separated by the black hole Schwarzschild time, seconds. The echo would be fainter, and the combination of delay and flux constrains the central black hole mass.
We have described a FRB scenario based on circumnuclear magnetar phenomena. In this scenario FRB’s are bright bursts or giant pulses from magnetars at the centers of nearby external galaxies, within a few hundred Mpc. The dominant DM contribution is due to the nuclear medium, which is sufficient for galaxies similar to the Milky Way whose innermost 100 pc provides 1000 pc cm.
Though we do not know to what extent magnetars preferentially form at the GC, the fact that one of just four radio loud magnetars is within 2.2” of Sgr A* tells us such objects are over represented in these environments. There are also physical arguments that could explain the lack of pulsars and the apparent tendency to form magnetars: Dexter & O’Leary (2014) suggest efficient formation could be due to highly magnetized progenitors or a top-heavy initial mass function. Given the large energy released in the episodic outbursts of SGR magnetars and the tendency for some pulsars to emit giant pulses, we have shown that FRB’s could be nuclear events. This picture also alleviates the difficulty of producing 1 ms scattering tails from the diffuse IGM, which has been shown to be problematic by Luan & Goldreich (2014) and Macquart & Koay (2013). Though we do not quantify scattering from galactic nuclei, we think temporal broadening from such regions at ms is reasonable. Our explanation is also consistent with the polarization properties of FRB 140514, which had no detectable linear polarization and 20 circular polarization. This could be caused by linear depolarization at low frequencies due to phase randomization from multiple paths through a scattering screen. Such polarization properties are seen in the galactic center magnetar and giant pulses from other pulsars.
We thank NSERC for support.
- Bagchi et al. (2012) Bagchi, M., Nieves, A. C., & McLaughlin, M. 2012, MNRAS, 425, 2501
- Bandura (2014) Bandura, K. e. a. 2014, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 9145, Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, 22
- Bower et al. (2014) Bower, G. C., Deller, A., Demorest, P., et al. 2014, ApJ, 780, L2
- Cordes et al. (2004) Cordes, J. M., Bhat, N. D. R., Hankins, T. H., McLaughlin, M. A., & Kern, J. 2004, ApJ, 612, 375
- Dexter & O’Leary (2014) Dexter, J., & O’Leary, R. M. 2014, ApJ, 783, L7
- Eatough (2013) Eatough, R. P. e. a. 2013, Nature, 501, 391
- Falcke & Rezzolla (2014) Falcke, H., & Rezzolla, L. 2014, A&A, 562, A137
- Hankins et al. (2003) Hankins, T. H., Kern, J. S., Weatherall, J. C., & Eilek, J. A. 2003, Nature, 422, 141
- Kashiyama et al. (2013) Kashiyama, K., Ioka, K., & Mészáros, P. 2013, ApJ, 776, L39
- Kulkarni et al. (2014) Kulkarni, S. R., Ofek, E. O., Neill, J. D., Zheng, Z., & Juric, M. 2014, ApJ, 797, 70
- Lazio & Cordes (1998) Lazio, T. J. W., & Cordes, J. M. 1998, ApJ, 505, 715
- Loeb et al. (2014) Loeb, A., Shvartzvald, Y., & Maoz, D. 2014, MNRAS, 439, L46
- Lorimer et al. (2007) Lorimer, D. R., Bailes, M., McLaughlin, M. A., Narkevic, D. J., & Crawford, F. 2007, Science, 318, 777
- Luan & Goldreich (2014) Luan, J., & Goldreich, P. 2014, ApJ, 785, L26
- Lynch et al. (2014) Lynch, R. S., Archibald, R. F., Kaspi, V. M., & Scholz, P. 2014, ArXiv e-prints 1412.0610, arXiv:1412.0610
- Lyubarsky (2014) Lyubarsky, Y. 2014, MNRAS, 442, L9
- Macquart & Koay (2013) Macquart, J.-P., & Koay, J. Y. 2013, ApJ, 776, 125
- McQuinn (2014) McQuinn, M. 2014, ApJ, 780, L33
- Mickaliger et al. (2012) Mickaliger, M. B., McLaughlin, M. A., Lorimer, D. R., et al. 2012, ApJ, 760, 64
- Pen & Levin (2014) Pen, U.-L., & Levin, Y. 2014, MNRAS, 442, 3338
- Pen et al. (2014) Pen, U.-L., Macquart, J.-P., Deller, A. T., & Brisken, W. 2014, MNRAS, 440, L36
- Petroff (2014) Petroff, E. e. a. 2014, ArXiv e-prints 1412.0342, arXiv:1412.0342
- Pfahl & Loeb (2004) Pfahl, E., & Loeb, A. 2004, ApJ, 615, 253
- Tegmark & Zaldarriaga (2009) Tegmark, M., & Zaldarriaga, M. 2009, Phys. Rev. D, 79, 083530
- Thornton (2013) Thornton, D. e. a. 2013, Science, 341, 53
- Wharton et al. (2012) Wharton, R. S., Chatterjee, S., Cordes, J. M., Deneva, J. S., & Lazio, T. J. W. 2012, ApJ, 753, 108
- Yu et al. (2014) Yu, Y.-W., Cheng, K.-S., Shiu, G., & Tye, H. 2014, J. Cosmology Astropart. Phys, 11, 40