arXiv subjects

Ü. Ertan

Publications and source records attributed to Ü. Ertan.

5 recordsLinked to original sources

arXiv2026

Rotating Radio Transient Phases of Isolated Neutron Star Systems and Descendants of Central Compact Objects

Rotating radio transients (RRATs) form an isolated neutron star (INS) population characterized by their sporadic and short radio bursts. In the fallback disc model, it was suggested that most INS populations pass through an RRAT phase during their evolution, and the mechanism producing these bursts could be the enhancement of the flux of open magnetic field lines through the poles of dead pulsars due to penetration of the inner disc into the light cylinder. To test this idea, we determine the RRAT phases that could be produced by this mechanism along the evolutionary curves of the INS families on the period-period derivative ($P$--$\dot{P}$ diagram). These RRAT phases form an RRAT region in the $P$--$\dot{P}$ diagram, which is in agreement with the known RRAT distribution. Using our results, we also investigate the descendants of central compact objects (CCOs). We estimate that the CCOs similar to the three CCOs with measured $P$ and $\dot{P}$ are not likely to become radio pulsars (RPs). The situation is rather different for Calvera, a recently identified CCO. In the model, Calvera-like sources become RPs with lifetimes $ > 10^{8}$ yr. Indeed, there is a cluster of RPs in the region of \PPdot~diagram where descendants of Calvera-like CCOs evolve in our model. We also estimate that they have a very low birth rate, and therefore their immediate descendants may not currently exist in our Galaxy.

astro-ph.HE↗

arXiv2018

Discovery of extended infrared emission around the neutron star RX J0806.4-4123

Following up on a faint detection of a near-infrared (NIR) source at the position of the X-ray thermal isolated neutron star RX J0806.4-4123, we present new Hubble Space Telescope observations in the H-band. The NIR source is unambiguously detected with a Vega magnitude of 23.7 +/- 0.2 (flux density of 0.40 +/- 0.06 microJy at lambda =1.54 microm. The source position is coincident with the neutron star position, and the implied NIR flux is strongly in excess of what one would expect from an extrapolation of the optical-UV spectrum of RX J0806.4-4123. The NIR source is extended with a size of at least 0.8arcsec and shows some asymmetry. The conservative upper limit on the flux contribution of a point source is 50%. Emission from gas and dust in the ambient diffuse interstellar medium can be excluded as cause for the extended emission. The source parameters are consistent with an interpretation as either the first NIR-only detected pulsar wind nebula or the first resolved disk around an isolated neutron star.

astro-ph.HE↗

arXiv2012

An accretion model for the anomalous X-ray pulsar 4U 0142+61

We propose that the quiescent emission of AXPs/SGRs is powered by accretion from a fallback disk, requiring magnetic dipole fields in the range 10^{12}-10^{13} 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.

astro-ph.HE↗

arXiv2010

The spectral and beaming characteristics of the anomalous X-ray pulsar 4U 0142+61

AXPs and SGRs constitute a special population of young neutron stars, which are thought to be magnetars, i.e., neutron stars with super-strong magnetic fields (10^14 - 10^15 G). Assuming that AXPs and SGRs accrete matter from a fallback disk, we attempt to explain the energy-dependent pulse profiles of AXP 4U 0142+61, as well as its phase-dependent energy spectra. In the fallback disk model, the Thomson optical depth along the accretion funnel is significant and bulk-motion Comptonization operates efficiently. This is enhanced by resonant cyclotron scattering. The thus scattered photons escape mainly sideways and produce a fan beam, which is detected as a main pulse up to energies of ~160 keV. The approximately isotropic emission from the stellar surface (soft thermal photons and reflected hard X-ray ones) is detected as a secondary pulse. This secondary pulse shows a bump at an energy of ~60 keV, which may be interpreted as resonant cyclotron scattering of fan-beam photons at the neutron-star surface, implying a dipole magnetic field strength B ~7 x 10^12 (1+z) G, where z is the gravitational redshift. Our model explains the soft and hard X-ray spectra of 4U 0142+61 and its energy dependent pulse profiles of the quiescent emission, while the short bursts are due to magnetar-type processes taking place in superstrong multiple fields.

astro-ph.HE↗

arXiv2010

The Energy Spectrum of Anomalous X-ray Pulsars and Soft Gamma-ray Repeaters

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. 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. 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. 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.

astro-ph.HE↗