Ünal Ertan

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.

On the evolution of anomalous x-ray pulsars and soft gamma-ray repeaters with fall back 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 fall back 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 – ω2 *) dependence on the fastness parameter ω*, 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 fall back disk, and the final stages of fall back 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 fall back disks are active up to a large radius, 1012 cm. We find that transient AXPs and SGRs are likely to be older than their persistent cousins. A fall back 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. This is because the X-ray luminosities of AXPs and SGRs are found to decrease faster at the end of their evolution, and the X-ray luminosities of transient AXP and SGRs in quiescence lie in the luminosity range of X-ray cutoff phase. Taking the range of quiescent X-ray luminosities of transient AXPs and SGRs ~1033-1034 erg s–1, our simulations imply that the duration of the cutoff phase is comparable to the lifetime in the persistent phase for a large range of initial conditions.

AN ACCRETION MODEL FOR THE ANOMALOUS X-RAY PULSAR 4U 0142+61

We propose that the quiescent emission of anomalous X-ray pulsars/soft gamma-ray repeaters (AXPs/SGRs) is powered by accretion from a fallback disk, requiring magnetic dipole fields in the range 1012-1013 G, and that the luminous hard tails of their X-ray spectra are produced by bulk-motion Comptonization in the radiative shock near the bottom of the accretion column. This radiation escapes as a fan beam, which is partly absorbed by the polar cap photosphere, heating it up to relatively high temperatures. The scattered component and the thermal emission from the polar cap form a polar beam. We test our model on the well-studied AXP 4U 0142+61, whose energy-dependent pulse profiles show double peaks, which we ascribe to the fan and polar beams. The temperature of the photosphere (kT ~ 0.4 keV) is explained by the heating effect. The scattered part forms a hard component in the polar beam. We suggest that the observed high temperatures of the polar caps of AXPs/SGRs, compared with other young neutron stars, are due to the heating by the fan beam. Using beaming functions for the fan beam and the polar beam and taking gravitational bending into account, we fit the energy-dependent pulse profiles and obtain the inclination angle and the angle between the spin axis and the magnetic dipole axis, as well as the height of the radiative shock above the stellar surface. We do not explain the high-luminosity bursts, which may be produced by the classical magnetar mechanism operating in super-strong multipole fields.

The energy spectrum of anomalous X-ray pulsars and soft gamma-ray repeaters

Context. Anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) exhibit characteristic X-ray luminosities (both soft and hard) of around 10 35 erg s −1 and characteristic power-law, hard X-ray spectra extending to about 200 keV. Two AXPs also exhibit pulsed radio emission. Aims. Assuming that AXPs and SGRs accrete matter from a fallback disk, we attempt to explain both the soft and the hard X-ray emission as the result of the accretion process. We also attempt to explain their radio emission or the lack of it. Methods. We test the hypothesis that the power-law, hard X-ray spectra are produced in the accretion flow mainly by bulk-motion Comptonization of soft photons emitted at the neutron star surface. Fallback disk models invoke surface dipole magnetic fields of 10 12 −10 13 G, which is what we assume here. Results. Unlike normal X-ray pulsars, for which the accretion rate is highly super-Eddington, the accretion rate is approximately Eddington in AXPs and SGRs and thus the bulk-motion Comptonization operates efficiently. As an illustrative example we reproduce both the hard and the soft X-ray spectra of AXP 4U 0142+61 well using the XSPEC package compTB. Conclusions. Our model seems to explain both the hard and the soft X-ray spectra of AXPs and SGRs, as well as their radio emission or the lack of it, in a natural way. It might also explain the short bursts observed in these sources. On the other hand, it cannot explain the giant X-ray outbursts observed in SGRs, which may result from the conversion of magnetic energy in local multipole fields.

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.

ON THE INFRARED, OPTICAL, AND HIGH-ENERGY EMISSION FROM THE ANOMALOUS X-RAY PULSAR 4U 0142+61

We show that the observed pulsed optical emission of the anomalous X-ray pulsar 4U 0142+61 can be accounted for by both the magnetar outer gap models and the disk-star dynamo gap models; therefore, there is no evidence favoring only one of these models as its responsible mechanism. Nevertheless, the estimated high-energy gamma-ray spectra from these models have different power-law indices and can be tested by future observations with the Gamma-Ray Large Area Space Telescope (GLAST). Furthermore, we show by analytical estimations that the expectations of a standard disk model are in agreement with the observed unpulsed optical and infrared luminosities of the AXP 4U 0142+61.

On the X-ray light curve, pulsed-radio emission, and spin frequency evolution of the transient anomalous X-ray pulsar XTE J1810-197 during its X-ray outburst

Weshow that (1) the long-term X-ray outburst light curve of the transient AXP XTE J1810� 197 can beaccounted for by a fallback disk that is evolving toward quiescence through a disk instability after having been heated by a soft gamma-ray burst, (2) the spin-frequency evolution of this source in the same period can also be explained by the disk torque acting on the magnetosphere of the neutron star, and (3) most significantly, recently observed pulsed-radio emissionfromthissourcecoincideswiththeepochof minimumX-rayluminosity.Thisisnaturalintermsof afallbackdisk model, as the accretion power becomes so low that it is not sufficient to suppress the beamed radio emission from XTE J1810� 197. Subject headingg accretion, accretion disks — pulsars: individual (AXPs) — stars: neutron — X-rays: bursts

Optical and Infrared Emission from the Anomalous X-Ray Pulsars and Soft Gamma-Ray Repeaters

We show that the irradiated accretion disk model can account for all the optical and infrared observations of the anomalous X-ray pulsars in the persistent state. While placing an upper limit on the inner disk radii, and thus on the strength of the dipole component of the stellar magnetic field, the model fits do not constrain the outer disk radii. And while magnetar fields (B* > 1014 G) in higher multipoles are compatible with the irradiated disk model, magnetic dipole components of magnetar strength are not consistent with optical data.We show that the irradiated accretion disk model can account for all the optical and infrared observations of the anomalous X-ray pulsars in the persistent state. Model fits do not constrain the outer disk radii, while placing an upper limit to the inner disk radii, and thus to the strength of the dipole component of the stellar magnetic field. While magnetar fields (B_*>10^{14} G) in higher multipoles are compatible with the irradiated disk model, magnetic dipole components of magnetar strength are not consistent with optical data.

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.