We report on Bayesian parameter estimation of the mass and equatorial radius of the millisecond pulsar PSRJ0030 +0451, conditional on pulse-profile modeling of Neutron Star Interior Composition Explorer X-ray spectraltiming event data. We perform relativistic ray-tracing of thermal emission from hot regions of the pulsar's surface. We assume two distinct hot regions based on two clear pulsed components in the phase-folded pulse-profile data; we explore a number of forms (morphologies and topologies) for each hot region, inferring their parameters in addition to the stellar mass and radius. For the family of models considered, the evidence (prior predictive probability of the data) strongly favors a model that permits both hot regions to be located in the same rotational hemisphere. Models wherein both hot regions are assumed to be simply connected circular single-temperature spots, in particular those where the spots are assumed to be reflection-symmetric with respect to the stellar origin, are strongly disfavored. For the inferred configuration, one hot region subtends an angular extent of only a few degrees (in spherical coordinates with origin at the stellar center) and we are insensitive to other structural details; the second hot region is far more azimuthally extended in the form of a narrow arc, thus requiring a larger number of parameters to describe. The inferred mass M and equatorial radius R eq are, respectively,-+ eq 2 0.010 0.008 is more tightly constrained; the credible interval bounds reported here are approximately the 16% and 84% quantiles in marginal posterior mass.

Neutron stars are not only of astrophysical interest, but are also of great interest to nuclear physicists because their attributes can be used to determine the properties of the dense matter in their cores. One of the most informative approaches for determining the equation of state (EoS) of this dense matter is to measure both a star's equatorial circumferential radius R e and its gravitational mass M . Here we report estimates of the mass and radius of the isolated 205.53 Hz millisecond pulsar PSR J0030+0451 obtained using a Bayesian inference approach to analyze its energy-dependent thermal X-ray waveform, which was observed using the Neutron Star Interior Composition Ex-Corresponding author: M. C. Miller miller@astro.umd.edu a Einstein Fellow arXiv:1912.05705v1 [astro-ph.HE] 12 Dec 2019 Miller, Lamb, Dittmann, et al. plorer (NICER). This approach is thought to be less subject to systematic errors than other approaches for estimating neutron star radii. We explored a variety of emission patterns on the stellar surface. Our best-fit model has three oval, uniform-temperature emitting spots and provides an excellent description of the pulse waveform observed using NICER. The radius and mass estimates given by this model are R e = 13.02 +1.24 −1.06 km and M = 1.44 +0.15 −0.14 M (68%). The independent analysis reported in the companion paper by Riley et al. explores different emitting spot models, but finds spot shapes and locations and estimates of R e and M that are consistent with those found in this work. We show that our measurements of R e and M for PSR J0030+0451 improve the astrophysical constraints on the EoS of cold, catalyzed matter above nuclear saturation density.

Fast Radio Bursts are millisecond-duration astronomical radio pulses of unknown physical origin that appear to come from extragalactic distances [1][2][3][4][5][6][7][8] . Previous follow-up observations have failed to find additional bursts at the same dispersion measures (i.e. integrated column density of free electrons between source and telescope) and sky position as the original detections 9 . The apparent non-repeating nature of the fast radio bursts has led several authors to hypothesise that they originate in cataclysmic astrophysical events 10 . Here we report the detection of ten additional bursts from the direction of FRB 121102, using the 305-m Arecibo telescope. These new bursts have dispersion measures and sky positions consistent with the original burst 4 . This unambiguously identifies FRB 121102 as repeating and demonstrates that its source survives the energetic events that cause the bursts. Additionally, the bursts from FRB 121102 show a wide range of spectral shapes that appear to be predominantly intrinsic to the source and which vary on timescales of minutes or shorter. While there may be multiple physical origins for the population of fast radio bursts, the repeat bursts with high dispersion measure and variable spectra specifically seen from FRB 121102 support models that propose an origin in a young, highly magnetised, extragalactic neutron star 11,12 .2 FRB 121102 was discovered 4 in the PALFA survey, a deep search of the Galactic plane at 1.4 GHz for radio pulsars and fast radio bursts (FRBs) using the 305-m William E. Gordon Telescope at the Arecibo Observatory and the 7-beam Arecibo L-band Feed Array (ALFA) 13,14 . The observed dispersion measure (DM) of the burst is roughly three times the maximum value expected along this line of sight in the NE2001 model 15 of Galactic electron density, i.e. β DM ≡ DM FRB /DM Gal Max ∼ 3, suggesting an extragalactic origin.Initial Arecibo follow-up observations were limited in both dwell time and sky coverage and resulted in no detection of additional bursts 4 . In 2015 May and June we carried out more extensive follow-up using Arecibo, covering a ∼ 9 radius with a grid of six ALFA pointings around the then-best sky position of FRB 121102 (Figure 1 and Extended Data Table 1 and 2). As described in the Methods, high-time-resolution, total intensity spectra were recorded, and the data were processed using standard radio-frequency interference (RFI) excision, dispersion removal, and single-pulse-search algorithms implemented in the PRESTO software suite and associated data reduction pipelines 14,16,17 .We detected 10 additional bursts from FRB 121102 in these observations. The burst properties, and those of the initial FRB 121102 burst, are listed in Table 1. The burst intensities are shown in Figure 2. No other periodic or single-pulse signals of a plausible astrophysical origin were detected at any other DM. Until the source's physical nature is clear, we continue to refer to it as FRB 121102 and label each burst chronologically starting with the o...

The precise localization of the repeating fast radio burst (FRB 121102) has provided the first unambiguous association (chance coincidence probability p3×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 [O III] emission lines. Based on the spectrum and emission line ratios, we classify the counterpart as a low-metallicity, star-forming, m r′ = 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 4 kpc and a stellar mass of M * ∼(4-7)×107 M e , assuming a mass-to-light ratio between 2 to 3 M e L e −1 . Based on the Hα flux, we estimate the star formation rate of the host to be 0.4 M e yr −1 and a substantial host dispersion measure (DM) depth 324 pc cm −3 . The net DM 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. 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.

Fast radio bursts 1,2 are astronomical radio flashes of unknown physical nature with durations of milliseconds. Their dispersive arrival times suggest an extragalactic origin and imply radio luminosities that are orders of magnitude larger than those of all known short-duration radio transients 3 . So far all fast radio bursts have been detected with large single-dish telescopes with arcminute localizations, and attempts to identify their counterparts (source or host galaxy) have relied on the contemporaneous variability of field sources 4 or the presence of peculiar field stars 5 or galaxies 4 . These attempts have not resulted in an unambiguous association 6,7 with a host or multi-wavelength counterpart. Here we report the subarcsecond localization of the fast radio burst FRB 121102, the only known repeating burst source 8-11 , using high-time-resolution radio interferometric observations that directly image the bursts. Our precise localization reveals that FRB 121102 originates within 100 milliarcseconds of a faint 180-microJansky persistent radio source with a continuum spectrum that is consistent with nonthermal emission, and a faint (twenty-fifth magnitude) optical counterpart. The flux density of the persistent radio source varies by around ten per cent on day timescales, and very long baseline radio interferometry yields an angular size of less than 1.7 milliarcseconds. 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. Localization and identification of a host or counterpart has been essential to understanding the origins and physics of other kinds of transient events, including gamma-ray bursts 12,13 and tidal disruption events 14 . However, if other fast radio bursts have similarly faint radio and optical counterparts, our findings imply that direct subarcsecond localizations may be the only way to provide reliable associations.The repetition of bursts from FRB 121102 9,10 enabled a targeted interferometric localization campaign with the Karl G. Jansky Very Large Array (VLA) in concert with single-dish observations using the 305-m William E. Gordon Telescope at the Arecibo Observatory. We searched for bursts in VLA data with 5-ms sampling using both beam-forming and imaging techniques 15 (see Methods). In over 83 h of VLA observations distributed over six months, we detected nine bursts from FRB 121102 in the 2.5-3.5-GHz band with signalto-noise ratios ranging from 10 to 150, all at a consistent sky position.