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Register a new userPost fall-back evolution of multipolar magnetic fields and radio pulsar activation
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Andrei Igoshev Petrovich
Publication date
2016
fields
Physics
and research's language is
English
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It has long been unclear if the small-scale magnetic structures on the neutron star (NS) surface could survive the fall-back episode. The study of the Hall cascade (Cumming, Arras and Zweibel 2004; Wareing and Hollerbach 2009) hinted that energy in small scales structures should dissipate on short timescales. Our new 2D magneto-thermal simulations suggest the opposite. For the first $sim$10 kyrs after the fall-back episode with accreted mass $10^{-3} M_odot$, the observed NS magnetic field appears dipolar, which is insensitive to the initial magnetic topology. In framework of the Ruderman & Sutherland (1975) vacuum gap model during this interval, non-thermal radiation is strongly suppressed. After this time the initial (i.e. multipolar) structure begins to re-emerge through the NS crust. We distinguish three evolutionary epochs for the re-emergence process: the growth of internal toroidal field, the advection of buried poloidal field, and slow Ohmic diffusion. The efficiency of the first two stages can be enhanced when small-scale magnetic structure is present. The efficient re-emergence of high order harmonics might significantly affect the curvature of the magnetospheric field lines in the emission zone. So, only after few $10^4$ yrs would the NS starts shining as a pulsar again, which is in correspondence with radio silence of central compact objects (CCOs). In addition, these results can explain the absence of good candidates for thermally emitting NSs with freshly re-emerged field among radio pulsars (Bogdanov, Ng and Kaspi 2014), as NSs have time to cool down, and supernova remnants can already dissipate.
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We report on the analysis of a deep Chandra observation of the high-magnetic field pulsar (PSR) J1119-6127 and its compact pulsar wind nebula (PWN) taken in October 2019, three years after the source went into outburst. The 0.5-7 keV post-outburst (2019) spectrum of the pulsar is best described by a two-component blackbody plus powerlaw model with a temperature of 0.2pm0.1 keV, photon index of 1.8pm0.4 and X-ray luminosity of ~1.9e33 erg s^{-1}, consistent with its pre-burst quiescent phase. We find that the pulsar has gone back to quiescence. The compact nebula shows a jet-like morphology elongated in the north-south direction, similar to the pre-burst phase. The post-outburst PWN spectrum is best fit by an absorbed powerlaw with a photon index of 2.3pm0.5 and flux of ~3.2e-14 erg cm^{-2} s^{-1} (0.5-7 keV). The PWN spectrum shows evidence of spectral softening in the post-outburst phase, with the pre-burst photon index of 1.2pm0.4 changing to 2.3pm0.5, and pre-burst luminosity of ~1.5e32 erg s^{-1} changing to 2.7e32 erg s^{-1} in the 0.5-7 keV band, suggesting magnetar outbursts can impact PWNe. The observed timescale for returning to quiescence, of just a few years, implies a rather fast cooling process and favors a scenario where J1119 is temporarily powered by magnetic energy following the magnetar outburst, in addition to its spin-down energy.
We use the Bayesian approach to write the posterior probability density for the three-dimensional velocity of a pulsar and for its kinematic age. As a prior, we use the bimodal velocity distribution found in a recent article by Verbunt, Igoshev & Cator (2017). When we compare the kinematic ages with spin-down ages, we find that in general, they agree with each other. In particular, maximum likelihood analysis sets the lower limit for the exponential magnetic field decay timescale at $8$ Myr with a slight preference of $t_mathrm{dec} approx 12$ Myr and compatible with no decay at all. One of the objects in the study, pulsar B0950+08 has kinematic and cooling ages $approx 2$ Myr which is in strong contradiction with its spin-down age $tauapprox 17$ Myr. The 68 per cent credible range for the kinematic age is 1.2--8.0 Myr. We conclude that the most probable explanation for this contradiction is a combination of magnetic field decay and long initial period. Further timing, UV and X-ray observations of B0950+08 are required to constrain its origin and evolution better.
Modeling of the NICER X-ray waveform of the pulsar PSR J0030+0451, aimed to constrain the neutron star mass and radius, has inferred surface hot-spots (the magnetic polar caps) that imply significantly non-dipolar magnetic fields. To this end, we investigate magnetic field configurations that comprise offset dipole plus quadrupole components using static vacuum field and force-free global magnetosphere models. Taking into account the compactness and observer angle values provided by Miller et al. (2019) and Riley et al. (2019), we compute geodesics from the observer plane to the polar caps to compute the resulting X-ray light curve. We explore, through Markov chain Monte Carlo techniques, the detailed magnetic field configurations that can reproduce the observed X-ray light curve and have discovered degeneracies, i.e., diverse field configurations, which can provide sufficient descriptions to the NICER X-ray waveforms. Having obtained the force-free field structures, we then compute the corresponding synchronous gamma-ray light curves following Kalapotharakos et al. (2014) these we compare to those obtained by Fermi-LAT, to provide models consistent with both the X-ray and the gamma-ray data, thereby restricting further the multipole field parameters. An essential aspect of this approach is the proper computation of the relative phase between the synchronous X- and gamma-ray light curves. We conclude with a discussion of the broader implications of our study.
Intense flares that occur at late times relative to the prompt phase have been observed by the $Swift$ satellite in the X-ray afterglows of gamma-ray bursts (GRBs). Here, we present a detailed analysis on the fall back accretion process to explain the intense flare phase in the very early X-ray afterglow light curves. To reproduce the afterglow at late times, we resort to the external shock by engaging energy injections. By applying our model to GRBs 080810, 081028 and 091029, we show that their X-ray afterglow light curves can be reproduced well. We then apply our model to the ultra-long $Swift$ GRB 111209A, which is the longest burst ever observed. The very early X-ray afterglow of GRB 111209A showed many interesting features, such as a significant bump observed at around 2000 s after the $Swift$/BAT trigger. We assume two constant energy injection processes in our model. These can explain the observed plateau at X-ray wavelength in the relatively early stage ($8.0times10^{3}$ s) and a second X-ray plateau and optical rebrightening at about $10^{5}$ s. Our analysis supports the scenario that a significant amount of material may fall back toward the central engine after the prompt phase, causing an enhanced and long lived mass accretion rate powering a Poynting-flux-dominated outflow.
Neutron stars are natural physical laboratories allowing us to study a plethora of phenomena in extreme conditions. In particular, these compact objects can have very strong magnetic fields with non-trivial origin and evolution. In many respects its magnetic field determines the appearance of a neutron star. Thus, understanding the field properties is important for interpretation of observational data. Complementing this, observations of diverse kinds of neutron stars enable us to probe parameters of electro-dynamical processes at scales unavailable in terrestrial laboratories. In this review we first briefly describe theoretical models of formation and evolution of magnetic field of neutron stars, paying special attention to field decay processes. Then we present important observational results related to field properties of different types of compact objects: magnetars, cooling neutron stars, radio pulsars, sources in binary systems. After that, we discuss which observations can shed light on obscure characteristics of neutron star magnetic fields and their behaviour. We end the review with a subjective list of open problems.
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