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RE: FRB 121102
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Title: FRB 121102 is coincident with a star forming region in its host galaxy
Author: C.G. Bassa (ASTRON), S.P. Tendulkar (McGill), E.A.K. Adams (ASTRON), N. Maddox (ASTRON), S. Bogdanov (Columbia), G.C. Bower (ASIAA), S. Burke-Spolaor (NRAO/WVU), B.J. Butler (NRAO), S. Chatterjee (Cornell), J.M. Cordes (Cornell), J.W.T. Hessels (ASTRON/Amsterdam), V.M. Kaspi (McGill), C.J. Law (Berkeley), B. Marcote (JIVE), Z. Paragi (JIVE), S.M. Ransom (NRAO), P. Scholz (DRAO), L.G. Spitler (MPIfR), H.J. van Langevelde (JIVE/Leiden)

We present optical, near- and mid-infrared imaging of the host galaxy of FRB 121102 with the Gemini North telescope, the Hubble Space Telescope and the Spitzer Space Telescope. The FRB 121102 host galaxy is resolved, revealing a bright star forming region located in the outskirts of the irregular, low-metallicity dwarf galaxy. The star forming region has a half-light radius of 0.68 kpc (0.20 arcsec), encompassing the projected location of the compact (<0.7 pc), persistent radio source that is associated with FRB 121102. The half-light diameter of the dwarf galaxy is 5 to 7 kpc, and broadband spectral energy distribution fitting indicates that it has a total stellar mass of M*~10^8 Msun. The metallicity of the host galaxy is low, 12+log10 ([O/H])=8.0±0.1. The properties of the host galaxy of FRB 121102 are comparable to those of extreme emission line galaxies, also known to host hydrogen-poor superluminous supernovae and long-duration gamma-ray bursts. The projected location of FRB 121102 within the star forming region supports the proposed connection of FRBs with newly born neutron stars or magnetars.

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Title: A model for the repeating FRB 121102 in the AGN scenario
Author: Florencia L. Vieyro, Gustavo E. Romero, Valentí Bosch-Ramon, Benito Marcote, María V. del Valle

Fast radio bursts, or FRBs, are transient sources of unknown origin. Recent radio and optical observations have provided strong evidence for an extragalactic origin of the phenomenon and the precise localization of the repeating FRB 121102. Observations using the Karl G. Jansky Very Large Array (VLA) and very-long-baseline interferometry (VLBI) have revealed the existence of a continuum non-thermal radio source consistent with the location of the bursts in a dwarf galaxy. All these new data rule out several models that were previously proposed, and impose stringent constraints to new models. We aim to model FRB 121102 in light of the new observational results in the active galactic nucleus (AGN) scenario. We propose a model for repeating FRBs in which a non-steady relativistic e^±-beam, accelerated by an impulsive magnetohydrodynamic (MHD)-driven mechanism, interacts with a cloud at the centre of a star-forming dwarf galaxy. The interaction generates regions of high electrostatic field called cavitons in the plasma cloud. Turbulence is also produced in the beam. These processes, plus particle isotropization, the interaction scale, and light retardation effects, provide the necessary ingredients for short-lived, bright coherent radiation bursts. The mechanism studied in this work explains the general properties of FRB 121102, and may also be applied to other repetitive FRBs. Coherent emission from electrons and positrons accelerated in cavitons provides a plausible explanation of FRBs.

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Title: A flaring magnetar in FRB 121102?
Author: Andrei M. Beloborodov (Columbia University)

The persistent radio counterpart of FRB 121102 is estimated to have N~10^52 particles, energy EN~10^48 erg, and size R~10^17 cm. The source can be a nebula inflated and heated by an intermittent outflow from a magnetar -- a neutron star powered by its magnetic (rather than rotational) energy. The object is young and frequently liberating energy in magnetic flares driven by accelerated ambipolar diffusion in the neutron star core, feeding the nebula and producing bright millisecond bursts. The particle number in the nebula is consistent with ion ejecta from giant flares. The nebula should also contain the freeze-out of electron-positron pairs N±~1051 created months after the neutron star birth; the same mechanism offers an explanation for N± in the Crab nebula. The persistent source around FRB 121102 is likely heated by magnetic dissipation and internal waves excited by the magnetar ejecta. The volumetric heating by waves explains the nebula's enormous efficiency in producing radio emission. The repeating radio bursts are suggested to occur much closer to the magnetar, whose flaring magnetosphere drives ultrarelativistic internal shocks into the magnetar wind. The shocks are mediated by Larmor rotation that forms a GHz maser of observed duration <1 ms. Furthermore, the flare ejecta can become charge-starved and then convert to electromagnetic waves.

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Fast Radio Burst 121102
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Title: FRB 121102 Casts New Light on the Photon Mass
Author: Luca Bonetti, John Ellis, Nikolaos E. Mavromatos, Alexander S. Sakharov, Edward K. Sarkisyan-Grinbaum, Alessandro D.A.M. Spallicci

The photon mass, m_\gamma, can in principle be constrained using measurements of the dispersion measures (DMs) of fast radio bursts (FRBs), once the FRB redshifts are known. The DM of the repeating FRB 121102 is known to <1%, a host galaxy has now been identified with high confidence,and its redshift, z, has now been determined with high accuracy: z=0.19273(8). Taking into account the plasma contributions to the DM from the Intergalactic medium (IGM) and the Milky Way, we use the data on FRB 121102 to derive the constraint m_\gamma \lesssim 2.2 x 10^-14 eV c^-2 (3.9 x 10^-50 kg). Since the plasma and photon mass contributions to DMs have different redshift dependences, they could in principle be distinguished by measurements of more FRB redshifts, enabling the sensitivity to m to be improved. 

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Title: Millisecond Magnetar Birth Connects FRB 121102 to Superluminous Supernovae and Long Duration Gamma-ray Bursts
Author: Brian D. Metzger, Edo Berger, Ben Margalit

Sub-arcsecond localization of the repeating fast radio burst FRB 121102 revealed its coincidence with a dwarf host galaxy and a steady (`quiescent') non-thermal radio source. We show that the properties of the host galaxy are consistent with those of long-duration gamma-ray bursts (LGRB) and hydrogen-poor superluminous supernovae (SLSNe-I). Both LGRBs and SLSNe-I were previously hypothesized to be powered by the electromagnetic spin-down of newly-formed, strongly-magnetized neutron stars with millisecond birth rotation periods (`millisecond magnetars'). This motivates considering a scenario whereby the repeated bursts from FRB 121102 originate from a young magnetar remnant embedded within a young hydrogen-poor supernova remnant. Requirements on the GHz free-free optical depth through the expanding supernova ejecta (accounting for photo-ionization by the rotationally-powered magnetar nebula), energetic constraints on the bursts, and constraints on the size of the quiescent source all point to an age of less than a few decades. The quiescent radio source can be attributed to radio synchrotron emission from the shock interaction between the fast outer layer of the supernova ejecta with the surrounding wind of the progenitor star, or from deeper within the magnetar wind nebula. Alternatively, the radio emission could be an orphan radio afterglow from an initially off-axis LGRB jet, though this might require the source to be too young. We propose future tests of the SLSNe-I/LGRB/FRB connection, such as searches for FRBs from nearby SLSNe-I/LGRB on timescales of decades after their explosions.

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Fast Radio Burst 121102
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Title: The Repeating Fast Radio Burst FRB 121102 as Seen on Milliarcsecond Angular Scales
Author: B. Marcote (1), Z. Paragi (1), J. W. T. Hessels (2,3), A. Keimpema (1), H. J. van Langevelde (1,4), Y. Huang (5,1), C. G. Bassa (2), S. Bogdanov (6), G. C. Bower (7), S. Burke-Spolaor (8,9,10), B. J. Butler (8), R. M. Campbell (1), S. Chatterjee (11), J. M. Cordes (11), P. Demorest (8), M. A. Garrett (12,4,2), T. Ghosh (13), V. M. Kaspi (14), C. J. Law (15), T. J. W. Lazio (16), M. A. McLaughlin (9,10), S. M. Ransom (17), C. J. Salter (13), P. Scholz (18), A. Seymour (13), A. Siemion (15,2,19), L. G. Spitler (20), S. P. Tendulkar (14), R. S. Wharton (11) ((1) JIVE, (2) ASTRON, (3) U. of Amsterdam, (4) U. of Leiden, (5) Carleton College, (6) Columbia U., (7) ASIAA, (8) NRAO Socorro, (9) U. of West Virginia Dept. of Physics and Astronomy, (10) U. of West Virginia Center for GWs and Cosmology, (11) Cornell U., (12) U. of Manchester, (13) NAIC Arecibo, (14) McGill U., (15) UC Berkeley, (16) NASA JPL, (17) NRAO CV, (18) DRAO, (19) U. of Nijmegen, (20) MPIfR)

The millisecond-duration radio flashes known as Fast Radio Bursts (FRBs) represent an enigmatic astrophysical phenomenon. Recently, the sub-arcsecond localization (~ 100mas precision) of FRB121102 using the VLA has led to its unambiguous association with persistent radio and optical counterparts, and to the identification of its host galaxy. However, an even more precise localization is needed in order to probe the direct physical relationship between the millisecond bursts themselves and the associated persistent emission. Here we report very-long-baseline radio interferometric observations using the European VLBI Network and the 305-m Arecibo telescope, which simultaneously detect both the bursts and the persistent radio emission at milliarcsecond angular scales and show that they are co-located to within a projected linear separation of < 40pc (< 12mas angular separation, at 95% confidence). We detect consistent angular broadening of the bursts and persistent radio source (~ 2-4mas at 1.7GHz), which are both similar to the expected Milky Way scattering contribution. The persistent radio source has a projected size constrained to be < 0.7pc (< 0.2mas angular extent at 5.0GHz) and a lower limit for the brightness temperature of T_b > 5 x 10^7K. Together, these observations provide strong evidence for a direct physical link between FRB121102 and the compact persistent radio source. We argue that a burst source associated with a low-luminosity active galactic nucleus or a young neutron star energizing a supernova remnant are the two scenarios for FRB121102 that best match the observed data.

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Title: The direct localization of a fast radio burst and its host
Author: S. Chatterjee, C. J. Law, R. S. Wharton, S. Burke-Spolaor, J. W. T. Hessels, G. C. Bower, J. M. Cordes, S. P. Tendulkar, C. G. Bassa, P. Demorest, B. J. Butler, A. Seymour, P. Scholz, M. W. Abruzzo, S. Bogdanov, V. M. Kaspi, A. Keimpema, T. J. W. Lazio, B. Marcote, M. A. McLaughlin, Z. Paragi, S. M. Ransom, M. Rupen, L. G. Spitler, H. J. van Langevelde

Fast radio bursts are astronomical radio flashes of unknown physical nature with durations of milliseconds. Their dispersive arrival times suggest an extragalactic origin and imply radio luminosities orders of magnitude larger than any other kind of known short-duration radio transient. Thus far, all FRBs have been detected with large single-dish telescopes with arcminute localizations, and attempts to identify their counterparts (source or host galaxy) have relied on contemporaneous variability of field sources or the presence of peculiar field stars or galaxies. These attempts have not resulted in an unambiguous association with a host or multi-wavelength counterpart. Here we report the sub-arcsecond localization of FRB 121102, the only known repeating burst source, using high-time-resolution radio interferometric observations that directly image the bursts themselves. Our precise localization reveals that FRB 121102 originates within 100 mas of a faint 180 uJy persistent radio source with a continuum spectrum that is consistent with non-thermal emission, and a faint (25th magnitude) optical counterpart. The flux density of the persistent radio source varies by tens of percent on day timescales, and very long baseline radio interferometry yields an angular size less than 1.7 mas. Our observations are inconsistent with the fast radio burst having a Galactic origin or its source being located within a prominent star-forming galaxy. Instead, the source appears to be co-located with a low-luminosity active galactic nucleus or a previously unknown type of extragalactic source. [Truncated] If other fast radio bursts have similarly faint radio and optical counterparts, our findings imply that direct sub-arcsecond localizations of FRBs may be the only way to provide reliable associations.

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Title: The Host Galaxy and Redshift of the Repeating Fast Radio Burst FRB 121102
Author: Shriharsh P. Tendulkar (McGill U.), Cees Bassa (ASTRON), James M. Cordes (Cornell U.), Geoffery C. Bower (ASIAA), Casey J. Law (UC Berkeley), Shamibrata Chatterjee (Cornell U.), Elizabeth A. K. Adams (ASTRON), Slavko Bogdanov (Columbia U.), Sarah Burke-Spolaor (NRAO, WVU), Bryan J. Butler (NRAO), Paul Demorest (NRAO), Jason W. T. Hessels (ASTRON, API), Victoria M. Kaspi (McGill U.), T. Joseph W. Lazio (JPL), Benito Marcote (JIVE), Maura A. McLaughlin (WVU), Zsolt Paragi (JIVE), Scott M. Ransom (NRAO), Paul Scholz (DRAO), Andrew Seymour (Arecibo), Laura G. Spitler (MPIfR), Huib J. van Langevelde (JIVE, Leiden), Robert S. Wharton (Cornell U.)

The precise localization of the repeating fast radio burst (FRB 121102) has provided the first unambiguous association (chance coincidence probability p \lesssim 3 x 10^-4) of an FRB with an optical and persistent radio counterpart. We report on optical imaging and spectroscopy of the counterpart and find that it is an extended (0.6" - 0.8") object displaying prominent Balmer and [OIII] emission lines. Based on the spectrum and emission line ratios, we classify the counterpart as a low-metallicity, star-forming, m_{r^\prime}=25.1 AB mag dwarf galaxy at a redshift of z=0.19273(8), corresponding to a luminosity distance of 972 Mpc. From the angular size, the redshift, and luminosity, we estimate the host galaxy to have a diameter \lesssim 4 kpc and a stellar mass of M^*~4-7 x 10^7 solar masses, assuming a mass-to-light ratio between 2 to 3 solar masses L^-1 solar. Based on the H alpha flux, we estimate the star formation rate of the host to be 0.4 solar masses yr^-1 and a substantial host dispersion measure depth \lesssim 324pccm^-3. The net dispersion measure contribution of the host galaxy to FRB 121102 is likely to be lower than this value depending on geometrical factors. We show that the persistent radio source at FRB 121102's location reported by Marcote et al (2017) is offset from the galaxy's center of light by ~200 mas and the host galaxy does not show optical signatures for AGN activity. If FRB 121102 is typical of the wider FRB population and if future interferometric localizations preferentially find them in dwarf galaxies with low metallicities and prominent emission lines, they would share such a preference with long gamma ray bursts and superluminous supernovae.

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