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Register a new userBraking PSR J1734-3333 with a possible fall-back disk
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The very small braking index of PSR J1734-3333, $n=0.9pm0.2$, challenges the current theories of braking mechanisms in pulsars. We present a possible interpretation that this pulsar is surrounded by a fall-back disk and braked by it. A modified braking torque is proposed based on the competition between the magnetic energy density of a pulsar and the kinetic energy density of a fall-back disk. With this torque, a self-similar disk can fit all the observed parameters of PSR J1734-3333 with natural initial parameters. In this regime, the star will evolve to the region having anomalous X-ray pulsars and soft gamma repeaters in the $P-dot{P}$ diagram in about 20000 years and stay there for a very long time. The mass of the disk around PSR J1734-3333 in our model is about $10M_{oplus}$, similar to the observed mass of the disk around AXP 4U 0142+61.
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The low braking-index pulsar PSR J1734$-$3333 could be born with superhigh internal magnetic fields $B_{rm in}sim10^{15}-10^{16}$ G, and undergo a supercritical accretion soon after its formation in a supernova explosion. The buried multipole magnetic fields will merger into a dipole magnetic field. Since the magnetic flow transfers from the core to the crust of the pulsar, its surface dipole field grows quickly at a power-law form assumed until it saturates at the level of internal dipole field. The increase in surface dipole magnetic field results in the observed low braking index of $n=0.9(2)$. Keeping an average field growth index $varepsilon=1.34(6)$, this pulsar will become a magnetar with surface dipole magnetic field at the equator $B_{rm d}sim 2.6(1)times 10^{14}$,G and $sim 5.3(2)times 10^{14}$,G after next 50,kyrs and 100,kys, respectively.
Recent measurements showed that the period derivative of the high-B radio pulsar PSR J1734-3333 is increasing with time. For neutron stars evolving with fallback disks, this rotational behavior is expected in certain phases of the long-term evolution. Using the same model as employed earlier to explain the evolution of anomalous X-ray pulsars and soft gamma-ray repeaters, we show that the period, the first and second period derivatives and the X-ray luminosity of this source can simultaneously acquire the observed values for a neutron star evolving with a fallback disk. We find that the required strength of the dipole field that can produce the source properties is in the range of 10^{12} - 10^{13} G on the pole of the neutron star. When the model source reaches the current state properties of PSR J1734-3333, accretion onto the star has not started yet, allowing the source to operate as a regular radio pulsar. Our results imply that PSR J1734-3333 is at an age of ~ 3 x 10^4 - 2 x 10^5 years. Such sources will have properties like the X-ray dim isolated neutron stars or transient AXPs at a later epoch of weak accretion from the diminished fallback disk.
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.
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.
PSR J1846-0258 is an object which straddles the boundary between magnetars and rotation powered pulsars. Though behaving for many years as a rotation-powered pulsar, in 2006, it exhibited distinctly magnetar-like behavior - emitting several short hard X-ray bursts, and a flux increase. Here we report on 7 years of post-outburst timing observations of PSR J1846-0258 using the Rossi X-ray Timing Explorer and the Swift X-ray Telescope. We measure the braking index over the post-magnetar outburst period to be $n=2.19pm0.03$. This represents a change of $Delta n=-0.46pm0.03$ or a 14.5$;sigma$ difference from the pre-outburst braking index of $n=2.65pm0.01$, which itself was measured over a span of 6.5 yr. So large and long-lived a change to a pulsar braking index is unprecedented and poses a significant challenge to models of pulsar spin-down.
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