M. A. Alpar

The anomalous X-ray pulsar 4U 0142+61: a neutron star with a gaseous fallback disk

The recent detection of the anomalous X-ray pulsar (AXP) 4U 0142+61 in the mid-infrared with the Spitzer observatory by Z. Wang and coworkers constitutes the first instance of a disk around an AXP. We show, by analyzing earlier optical and near-IR data together with the recent data, that the overall broadband data set can be reproduced by a single model of an irradiated and viscously heated disk.The anomalous X-ray pulsar 4U 0142+61 was recently detected in the mid infrared bands with the SPITZER Observatory (Wang, Chakrabarty & Kaplan 2006). This observation is the first instance for a disk around an AXP. From a reanalysis of optical and infrared data, we show that the observations indicate that the disk is likely to be an active disk rather than a passive dust disk beyond the light cylinder, as proposed in the discovering paper. Furthermore, we show that the irradiated accretion disk model can also account for all the optical and infrared observations of the anomalous X-ray pulsars in the persistent state.

SGR 0418+5729: how does a young neutron star spin down to a 9 s period with a dipole field less than 10 (13) G?

The period derivative bound for the soft gamma-ray repeater SGR 0418+5729 establishes the magnetic dipole moment to be distinctly lower than the magnetar range, placing the source beyond the regime of isolated pulsar activity in the diagram and giving a characteristic age >2 × 107 yr, much older than the 105 yr age range of SGRs and anomalous X-ray pulsars. So the spin-down must be produced by a mechanism other than dipole radiation in vacuum. A fallback disk will spin down a neutron star with surface dipole magnetic field in the 1012 G range and initial rotation period P 0 ~ 100 ms to the 9.1 s period of SGR 0418+5729 in a few 104 to ~105 yr. The current upper limit to the period derivative gives a lower limit of ~105 yr to the age that is not sensitive to the neutron stars initial conditions. The total magnetic field on the surface of SGR 0418+5729 could be significantly larger than its 1012 G dipole component.

The effect of quantized magnetic flux lines on the dynamics of superfluid neutron star cores

We investigate dynamical coupling time-scales of a neutron stars superfluid core, taking into account the interactions of quantized neutron vortices with quantized flux lines of the proton superconductor in addition to the previously considered scattering of the charged components against the spontaneous magnetization of the neutron vortex line. We compare the cases where vortex motion is constrained in different ways by the array of magnetic flux tubes associated with superconducting protons. This includes absolute pinning to and creeping across a uniform array of flux lines. The effect of a toroidal arrangement of flux lines is also considered. The inclusion of a uniform array of flux tubes in the neutron star core significantly decreases the time-scale of coupling between the neutron and proton fluid constituents in all cases. For the toroidal component, creep response similar to that of the inner crust superfluid is possible.

On The Evolution of The Radio Pulsar PSR J1734-3333

Recent measurements showed that the period derivative of the ‘hig h-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 ∼3×10^4 −2×10^5years. 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.

X-RAY AND INFRARED ENHANCEMENT OF ANOMALOUS X-RAY PULSAR 1E 2259+586

The long-term (~1.5 yr) X-ray enhancement and the accompanying infrared enhancement light curves of the anomalous X-ray pulsar 1E 2259+586 following the major bursting epoch can be accounted for by the relaxation of a fallback disk that has been pushed back by a gamma-ray flare. The required burst energy estimated from the results of our model fits is low enough for such a burst to have remained below the detection limits. We find that an irradiated disk model with a low irradiation efficiency is in good agreement with both X-ray and infrared data. Nonirradiated disk models also give a good fit to the X-ray light curve, but are not consistent with the infrared data for the first week of the enhancement.

Peculiar glitch of PSR J1119−6127 and extension of the vortex creep model

Glitches are sudden changes in rotation frequency and spin-down rate, observed from pulsars of all ages. Standard glitches are characterized by a positive step in angular velocity (Delta Omega > 0) and a negative step in the spin-down rate (Delta Omega < 0) of the pulsar. There are no glitch-associated changes in the electromagnetic signature of rotation-powered pulsars in all cases so far. For the first time, in the last glitch of PSR J1119-6127, there is clear evidence for changing emission properties coincident with the glitch. This glitch is also unusual in its signature. Further, the absolute value of the spin-down rate actually decreases in the long term. This is in contrast to usual glitch behaviour. In this paper we extend the vortex creep model in order to take into account these peculiarities. We propose that a starquake with crustal plate movement towards the rotational poles of the star induces inward vortex motion which causes the unusual glitch signature. The component of the magnetic field perpendicular to the rotation axis will decrease, giving rise to a permanent change in the pulsar external torque.

ROSAT OBSERVATIONS OF THE VELA PULSAR

The ROSAT HRI was used to monitor X-ray emission from the Vela pulsar. Six observations span 2.5 yr and three glitches. The summed data yield a determination of the pulse shape, and X-ray emission from the pulsar is found to be 12% pulsed with one broad and two narrow peaks. One observation occurred 15 days after a large glitch. No change in pulse structure was observed, and any change in X-ray luminosity, if present, was less than 3%. Implications for neutron star structure are discussed.

CONSTRAINTS ON A PUTATIVE PULSAR IN SN 1987A

We assume that all the energy loss of the putative pulsar in SN 1987A would contribute to the luminosity of the remnant, which acts like a bolometer. The bolometric luminosity of SN 1987A provides an upper bound on the pulsar’s rate of energy loss. An isolated pulsar spinning down by magnetic dipole radiation alone with initial rotation periods of 10–30 ms, as extrapolated for galactic young pulsars, can have a luminosity below the bolometric bound if either the magnetic field is weak, G, or if it is so strong that the pulsar 9 10 B ∼ 10 –10 luminosity decays rapidly, G. 16 B ∼ 10 Subject headings: pulsars: general — stars: neutron — supernovae: individual (SN 1987A)We assume that all the energy loss of the putative pulsar in SN 1987A would contribute to the luminosity of the remnant, which acts like a bolometer. The bolometric luminosity of SN 1987A provides an upper bound on the pulsars rate of energy loss. An isolated pulsar spinning down by magnetic dipole radiation alone with initial rotation periods of 10-30 ms, as extrapolated for galactic young pulsars, can have a luminosity below the bolometric bound if either the magnetic field is weak, B ~ 109-1010 G, or if it is so strong that the pulsar luminosity decays rapidly, B ~ 1016 G.

Hard X-Ray Flux Upper Limits of Central Compact Objects in Supernova Remnants

We searched for hard X-ray (20-300 keV) emission from nine central compact objects (CCOs) 1E 1207.4–5209, 1WGA J1713–3949, J082157.5–430017, J085201.4–461753, J160103.1–513353, J1613483–5055, J181852.0–150213, J185238.6+004020, and J232327.9+584843 with the International Gamma-Ray Astrophysics Laboratory observatory. We applied spectral imaging analysis and did not detect any of the sources with luminosity upper limits in the range of 1033-1034 erg s–1 in the 20-75 keV band. For nearby CCOs (less than 4 kpc), the upper-limit luminosities are an order of magnitude lower than the measured persistent hard X-ray luminosities of anomalous X-ray pulsars. This may indicate that the CCOs are low magnetic field systems with fallback disks around them.

Minimum glitch of the Crab pulsar and the crustquake as a trigger mechanism

We discuss the minimum glitch size of Crab observed by Espinoza et al. (2014). Modelling the crustquake as a trigger mechanism we estimate the size of the broken plate. The plate size obtained, D