Neutron star solutions in perturbative quadratic gravity

We study the structure of neutron stars in R+\beta\ R^{\mu \nu} R_{\mu \nu} gravity model with perturbative method. We obtain mass--radius relations for six representative equations of state (EoSs). We find that, for |\beta| ~ 10^11 cm^2, the results differ substantially from the results of general relativity. Some of the soft EoSs that are excluded within the framework of general relativity can be reconciled for certain values of \beta\ of this order with the 2 solar mass neutron star recently observed. For values of \beta\ greater than a few 10^11 cm^2 we find a new solution branch allowing highly massive neutron stars. By referring some recent observational constraints on the mass--radius relation we try to constrain the value of \beta\ for each EoS. The associated length scale \sqrt{\beta} ~ 10^6 cm is of the order of the typical radius of neutron stars implying that this is the smallest value we could find by using neutron stars as a probe. We thus conclude that the true value of \beta\ is most likely much smaller than 10^11 cm^2.Comment: 19 pages, 9 figures. v2: Analysis on validity of perturbative approach is added. References added. v3: Aesthetic improvement

On the evolution of anomalous X-ray pulsars and soft gamma ray repeaters with fallback disks

We show that the period clustering of anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs), their X-ray luminosities, ages and statistics can be explained with fallback disks with large initial specific angular momentum. The disk evolution models are developed by comparison to self-similar analytical models. The initial disk mass and angular momentum set the viscous timescale. An efficient torque, with (1 - w^2) dependence on the fastness parameter w leads to period clustering in the observed AXP-SGR period range under a wide range of initial conditions. The timescale t_0 for the early evolution of the fallback disk, and the final stages of fallback disk evolution, when the disk becomes passive, are the crucial determinants of the evolution. The disk becomes passive at temperatures around 100 K, which provides a natural cutoff for the X-ray luminosity and defines the end of evolution in the observable AXP and SGR phase. This low value for the minimum temperature for active disk turbulence indicates that the fallback disks are active up to a large radius greater than ~10^{12} cm. We find that transient AXPs and SGRs are likely to be older than their persistent cousins. A fallback disk with mass transfer rates corresponding to the low quiescent X-ray luminosities of the transient sources in early evolutionary phases would have a relatively lower initial mass, such that the mass-flow rate in the disk is not sufficient for the inner disk to penetrate into the light cylinder of the young neutron star, making mass accretion onto the neutron star impossible. The transient AXP phase therefore must start later. The model results imply that the transient AXP/SGRs, although older, are likely to be similar in number to persistent sources (abridged).Comment: 42 pages, 22 figures. Accepted for publication in the Astrophysical Journa

Where Are All The Fallback Disks? Constraints on Propeller Systems

Fallback disks are expected to form around new-born neutron stars following a supernova explosion. In almost all cases, the disk will pass through a propeller stage. If the neutron star is spinning rapidly (initial period $\sim 10$ ms) and has an ordinary magnetic moment ($\sim 10^{30}$ G cm$^3$), the rotational power transferred to the disk by the magnetic field of the neutron star will exceed the Eddington limit by many orders of magnitude, and the disk will be rapidly disrupted. Fallback disks can thus survive only around slow-born neutron stars and around black holes, assuming the latter do not torque their surrounding disks as strongly as do neutron stars. This might explain the apparent rarity of fallback disks around young compact objects.Comment: Submitted to Astrophysical Journal Letter

Relating the Kick Velocities of Young Pulsars with Magnetic Field Growth Timescales Inferred From Braking Indices

A nascent neutron star may be exposed to fallback accretion soon after the proto-neutron star stage. This high accretion episode can submerge the magnetic field deep in the crust. The diffusion of the magnetic field back to the surface will take hundreds to millions of years depending on the amount of mass accreted and the consequent depth the field is buried. Neutron stars with large kick velocities will accrete less amount of fallback material leading to shallower submergence of their fields and shorter time-scales for the growth of their fields. We obtain the relation $\tau_{\rm Ohm} \propto v^{-1}$ between the space velocity of the neutron star and Ohmic time-scale for the growth of the magnetic field. We compare this with the relation between the measured transverse velocities, $v_{\perp}$ and the field growth time-scales, $\mu/\dot{\mu}$, inferred from the measured braking indices. We find that the observational data is consistent with the theoretical prediction though the small number of data precludes a strong conclusion. Measurement of the transverse velocities of pulsars B1509$-$58, J1846$-$0258, J1119$-$6127 and J1734$-$3333 would increase the number of the data and strongly contribute to understanding whether pulsar fields grow following fallback accretion.Comment: Accepted to MNRAS Letters. Title and abstract are change

The reflares and outburst evolution in the accreting millisecond pulsar SAX J1808.4-3658: A disk truncated near co-rotation?

© 2016. The American Astronomical Society. All rights reserved. The accreting millisecond X-ray pulsar SAX J1808.43658 shows peculiar low luminosity states known as "reflares" after the end of the main outburst. During this phase the X-ray luminosity of the source varies by up to three orders of magnitude in less than 12 days. The lowest X-ray luminosity observed reaches a value of ~1032 erg s-1, only a factor of a few brighter than its typical quiescent level. We investigate the 2008 and 2005 reflaring state of SAX J1808.43658 to determine whether there is any evidence for a change in the accretion flow with respect to the main outburst. We perform a multiwavelength photometric and spectral study of the 2005 and 2008 reflares with data collected during an observational campaign covering the near-infrared, optical, ultra-violet and X-ray band. We find that the NIR/optical/UV emission, expected to come from the outer accretion disk, shows variations in luminosity over an order of magnitude. The corresponding X-ray luminosity variations are instead much deeper, spanning about 23 orders of magnitude. The X-ray spectral state observed during the reflares does not change substantially with X-ray luminosity, indicating a rather stable configuration of the accretion flow. We investigate the most likely configuration of the innermost regions of the accretion flow and we infer an accretion disk truncated at or near the co-rotation radius. We interpret these findings as due to either a strong outflow (due to a propeller effect) or a trapped disk (with limited/no outflow) in the inner regions of the accretion flow

Intra-pulse variability induced by plasmoid formation in pulsar magnetospheres

Context. Pulsars show irregularities in their pulsed radio emission that originate from propagation effects and the intrinsic activity of the source. Aims. In this work, we investigate the role played by magnetic reconnection and the formation of plasmoids in the pulsar wind current sheet as a possible source of intrinsic pulse-to-pulse variability in the incoherent, high-energy emission pattern. Methods. We used a two-dimensional particle-in-cell simulation of an orthogonal pulsar magnetosphere restricted to the plane perpendicular to the star spin axis. We evolved the solution for several tens of pulsar periods to gather a statistically significant sample of synthetic pulse profiles. Results. The formation of plasmoids leads to strong pulse-to-pulse variability in the form of multiple short, bright subpulses, which appear only on the leading edge of each main pulse. These secondary peaks of emission are dominated by the dozen plasmoids that can grow up to macroscopic scales. They emerge from the high end of the hierarchical merging process occurring along the wind current layer. The flux of the subpulses is correlated with their width in phase. Although the full-scale separation is not realistic, we argue that the simulation correctly captures the demographics and the properties of the largest plasmoids, and therefore of the brightest subpulses. Conclusions. The prediction of subpulses at specific pulse phases provides a new observational test of the magnetic reconnection scenario as the origin of the pulsed incoherent emission. High-time-resolution observations of the Crab pulsar in the optical range may be the most promising source to target for this purpose

On the magnetic fields, beaming fractions, and fastness parameters of pulsating ultraluminous x-ray sources

The discovery of pulsating ultraluminous X-ray sources (PULX) suggests that neutron stars are presumably common within the ultraluminous X-ray source (ULX) population though the majority of the population members currently lack pulsations. These systems are likely to host neutron stars accreting mass at super-Eddington (supercritical) rates from their massive companion in high-mass X-ray binaries. Taking into account the spherization of the accretion flow in the supercritical regime, the beaming of X-ray emission, and the reduction of the scattering cross section in a strong magnetic field, we infer the ranges for the neutron-star surface magnetic dipole field strengths, beaming fractions, and fastness parameters in the PULX M82 X-2, ULX NGC 5907, ULX NGC 7793 P13, NGC 300 ULX1, M51 ULX-7, NGC 1313 X-2, and Swift J0243.6+6124 from a set of conditions based on a variety of combinations of different spin and luminosity states. Using the observed spin-up rates under the critical luminosity condition, we estimate the surface-field strengths in the ∼1011–1013 G range for all PULX. In general, the results of our analysis under the subcritical luminosity condition indicate surface-field strengths in the ∼1011–1015 G range. We argue that PULX do not require magnetar-strength surface dipole fields if beaming is taken into account, yet the fields are strong enough for the neutron stars in ULX to magnetically channel the accretion flow in supercritical accretion disks

The ultraluminous X-ray source NuSTAR J095551+6940.8: a magnetar in a high-mass X-ray binary

The recent detection of pulsations from the ultraluminous X-ray source (ULX) NuSTAR J095551+6940.8 in M82 by Bachetti et al. indicates that the object is an accreting neutron star in a high-mass X-ray binary (HMXB) system. The super-Eddington luminosity of the object implies that the magnetic field is sufficiently strong to suppress the scattering cross-section unless its beam is viewed at a favourable angle. We show that the torque equilibrium condition for the pulsar indicates that the dipole magnetic field of the neutron star is 6.7 x 10(13) G, two orders of magnitude higher than that estimated by Bachetti et al., and further point to the possibility that even stronger magnetic fields could well be in the higher multipoles. This supports the recent view that magnetars descent from HMXBs if the magnetic field decays an order of magnitude during the process of transition