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Register a new userConstraining the GRB-magnetar model by means of the Galactic pulsar population
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A large fraction of Gamma Ray Bursts (GRBs) displays an X-ray plateau phase within <10^{5} s from the prompt emission, proposed to be powered by the spin-down energy of a rapidly spinning newly born magnetar. In this work we use the properties of the Galactic neutron star population to constrain the GRB-magnetar scenario. We re-analyze the X-ray plateaus of all Swift GRBs with known redshift, between January 2005 and August 2014. From the derived initial magnetic field distribution for the possible magnetars left behind by the GRBs, we study the evolution and properties of a simulated GRB-magnetar population using numerical simulations of magnetic field evolution, coupled with Monte Carlo simulations of Pulsar Population Synthesis in our Galaxy. We find that if the GRB X-ray plateaus are powered by the rotational energy of a newly formed magnetar, the current observational properties of the Galactic magnetar population are not compatible with being formed within the GRB scenario (regardless of the GRB type or rate at z=0). Direct consequences would be that we should allow the existence of magnetars and super-magnetars having different progenitors, and that Type Ib/c SNe related to Long GRBs form systematically neutron stars with higher initial magnetic fields. We put an upper limit of <16 super-magnetars formed by a GRB in our Galaxy in the past Myr (at 99% c.l.). This limit is somewhat smaller than what roughly expected from Long GRB rates, although the very large uncertainties do not allow us to draw strong conclusion in this respect.
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With the large sample of young gamma-ray pulsars discovered by the Fermi Large Area Telescope (LAT), population synthesis has become a powerful tool for comparing their collective properties with model predictions. We synthesised a pulsar population based on a radio emission model and four gamma-ray gap models (Polar Cap, Slot Gap, Outer Gap, and One Pole Caustic) normalizing to the number of detected radio pulsars in select group of surveys. The luminosity and the wide beams from the outer gaps can easily account for the number of Fermi detections in 2 years of observations. The wide slot-gap beams requires an increase by a factor of ~10 of the predicted luminosity to produce a reasonable number of gamma-ray pulsars. Such large increases in the luminosity may be accommodated by implementing offset polar caps. The narrow polar-cap beams contribute at most only a handful of LAT pulsars. Standard distributions in birth location and pulsar spin-down power (Edot) fail to reproduce the LAT findings: all models under-predict the number of LAT pulsars with high Edot, and they cannot explain the high probability of detecting both the radio and gamma-ray beams at high Edot. The beaming factor remains close to 1 over 4 decades in Edot evolution for the slot gap whereas it significantly decreases with increasing age for the outer gaps. The evolution of the slot-gap luminosity with Edot is compatible with the large dispersion of gamma-ray luminosity seen in the LAT data. The stronger evolution predicted for the outer gap, which is linked to the polar cap heating by the return current, is apparently not supported by the LAT data. The LAT sample of gamma-ray pulsars therefore provides a fresh perspective on the early evolution of the luminosity and beam width of the gamma-ray emission from young pulsars, calling for thin and more luminous gaps.
The nine-year H.E.S.S. Galactic Plane Survey (HGPS) yielded the most uniform observation scan of the inner Milky Way in the TeV gamma-ray band to date. The sky maps and source catalogue of the HGPS allow for a systematic study of the population of TeV pulsar wind nebulae found throughout the last decade. To investigate the nature and evolution of pulsar wind nebulae, for the first time we also present several upper limits for regions around pulsars without a detected TeV wind nebula. Our data exhibit a correlation of TeV surface brightness with pulsar spin-down power $dot{E}$. This seems to be caused both by an increase of extension with decreasing $dot{E}$, and hence with time, compatible with a power law $R_mathrm{PWN}(dot{E}) sim dot{E}^{-0.65 pm 0.20}$, and by a mild decrease of TeV gamma-ray luminosity with decreasing $dot{E}$, compatible with $L_{1-10,mathrm{TeV}} sim dot{E}^{0.59 pm 0.21}$. We also find that the offsets of pulsars with respect to the wind nebula centres with ages around 10 kyr are frequently larger than can be plausibly explained by pulsar proper motion and could be due to an asymmetric environment. In the present data, it seems that a large pulsar offset is correlated with a high apparent TeV efficiency $L_{1-10,mathrm{TeV}}/dot{E}$. In addition to 14 HGPS sources considered as firmly identified pulsar wind nebulae and 5 additional pulsar wind nebulae taken from literature, we find 10 HGPS sources that form likely TeV pulsar wind nebula candidates. Using a model that subsumes the present common understanding of the very high-energy radiative evolution of pulsar wind nebulae, we find that the trends and variations of the TeV observables and limits can be reproduced to a good level, drawing a consistent picture of present-day TeV data and theory.
The observed delay of GRB 170817A relative to GW170817 carries significant information about gamma-ray burst (GRB) physics and is subject to intense debate. In this letter, we put forward an approach to discuss the major source of this time delay. First of all, we use the structured jet model to fit the X-ray/optical/radio afterglows of GRB 170817A together with superluminal motion measured by the Very Long Baseline Interferometry. Our structured jet is modelled with angle-dependent energy and baryon loading. It is found that our model can well fit the afterglows of GRB 170817A. After that, the baryon loading in the jet is inferred based on our fitting results. By comparing the baryon loading to the mass outflow in different stages, we infer that the time lag of the jet launch relative to the merger is less than hundreds or tens of milliseconds. It suggests that the time delay of GRB 170817A relative to GW170817 is defined mostly by the spreading time of the jet propagating to its dissipation radius.
Through high-precision radio timing observations, we show that five recycled pulsars in the direction of the Galactic Centre (GC) have anomalous spin period time derivative ($dot P$) measurements -- PSRs J1748$-$3009, J1753$-$2819, J1757$-$2745, and J1804$-$2858 show negative values of $dot P$ and PSR J1801$-$3210 is found to have an exceptionally small value of $dot P$. We attribute these observed $dot P$ measurements to acceleration of these pulsars along their lines-of-sight (LOSs) due to the Galactic gravitational field. Using models of the Galactic mass distribution and pulsar velocities, we constrain the distances to these pulsars, placing them on the far-side of the Galaxy, providing the first accurate distance measurements to pulsars located in this region and allowing us to consider the electron density along these LOSs. We find the new electron density model YMW16 to be more consistent with these observations than the previous model NE2001. The LOS dynamics further constrain the model-dependent intrinsic $dot P$ values for these pulsars and they are consistent with measurements for other known pulsars. In the future, the independent distance measurements to these and other pulsars near the GC would allow us to constrain the Galactic gravitational potential more accurately.
We present the analysis of the extraordinarily bright Gamma-Ray Burst (GRB) 130427A under the hypothesis that the GRB central engine is an accretion-powered magnetar. In this framework, initially proposed to explain GRBs with precursor activity, the prompt emission is produced by accretion of matter onto a newly-born magnetar, and the observed power is related to the accretion rate. The emission is eventually halted if the centrifugal forces are able to pause accretion. We show that the X-ray and optical afterglow is well explained as the forward shock emission with a jet break plus a contribution from the spin-down of the magnetar. Our modelling does not require any contribution from the reverse shock, that may still influence the afterglow light curve at radio and mm frequencies, or in the optical at early times. We derive the magnetic field ($Bsim 10^{16}$ G) and the spin period ($Psim 20$ ms) of the magnetar and obtain an independent estimate of the minimum luminosity for accretion. This minimum luminosity results well below the prompt emission luminosity of GRB 130427A, providing a strong consistency check for the scenario where the entire prompt emission is the result of continuous accretion onto the magnetar. This is in agreement with the relatively long spin period of the magnetar. GRB 130427A was a well monitored GRB showing a very standard behavior and, thus, is a well-suited benchmark to show that an accretion-powered magnetar gives a unique view of the properties of long GRBs.
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