What causes the absence of pulsations in Central Compact Objects in supernova remnants?

Most young neutron stars belonging to the class of Central Compact Objects (CCOs) in supernova remnants do not have known periodicities. We investigated seven such CCOs to understand the common reasons for the absence of detected pulsations. Making use of XMM-Newton, Chandra, and NICER observations, we perform a systematic timing and spectral analysis to derive updated sensitivity limits for both periodic signals and multi-temperature spectral components that could be associated with radiation from hotspots on the neutron star surface. Based on these limits, we then investigated for each target the allowed viewing geometry that could explain the lack of pulsations. We find that it is unlikely (< 10−6) to attribute that we do not see pulsations to an unfavorable viewing geometry for five considered sources. Alternatively, the carbon atmosphere model, which assumes homogeneous temperature distribution on the surface, describes the spectra equally well and provides a reasonable interpretation for the absence of detected periodicities within current limits. The unusual properties of CCOs with respect to other young neutron stars could suggest a different evolutionary path, as that proposed for sources experiencing episodes of significant fallback accretion after the supernova event.

NICER Study of Pulsed Thermal X-Rays from Calvera: A Neutron Star Born in the Galactic Halo?

Calvera (1RXS J141256.0+792204) is an isolated neutron star detected only through its thermal X-ray emission. Its location at high Galactic latitude (b = +37°) is unusual if Calvera is a relatively young pulsar, as suggested by its spin period (59 ms) and period derivative (3.2 × 10−15 s s−1). Using the Neutron Star Interior Composition Explorer, we obtained a phase-connected timing solution spanning four years, which allowed us to measure the second derivative of the frequency ν ̈ = − 2.5 × 10 − 23 Hz s−2 and to reveal timing noise consistent with that of normal radio pulsars. A magnetized hydrogen atmosphere model, covering the entire star surface, provides a good description of the phase-resolved spectra and energy-dependent pulsed fraction. However, we found that a temperature map more anisotropic than that produced by a dipole field is required, with a hotter zone concentrated toward the poles. By adding two small polar caps, we found that the surface effective temperature and that of the caps are ∼0.1 and ∼0.36 keV, respectively. The inferred distance is ∼3.3 kpc. We confirmed the presence of an absorption line at 0.7 keV associated with the emission from the whole star surface, difficult to interpret as a cyclotron feature and more likely originating from atomic transitions. We searched for pulsed γ-ray emission by folding seven years of Fermi-LAT data using the X-ray ephemeris, but no evidence for pulsations was found. Our results favor the hypothesis that Calvera is a normal rotation-powered pulsar, with the only peculiarity of being born at a large height above the Galactic disk.

The influence of small-scale magnetic field on the heating of J0250+5854 polar cap

The influence of surface small-scale magnetic field on the heating of PSR J0250+5854 polar cap is considered. It is assumed that the polar cap is heated only by reverse positrons accelerated in pulsar diode. It is supposed that pulsar diode is located near the star surface (polar cap model) and operates in the steady state space charge-limited flow regime. The reverse positron current is calculated in the framework of two models: rapid and gradually screening. To calculate the production rate of electron-positron pairs we take into account only the curvature radiation of primary electrons and its absorption in magnetic field. It is assumed that some fraction of electron-positron pairs may be created in bound state that can later be photoionized by thermal photons from star surface.

The X-ray evolution and geometry of the 2018 outburst of XTE J1810–197

After 15 years, in late 2018, the magnetar XTE J1810–197 underwent a second recorded X-ray outburst event and reactivated as a radio pulsar. We initiated an X-ray monitoring campaign to follow the timing and spectral evolution of the magnetar as its flux decays using Swift, XMM–Newton, NuSTAR, and NICER observations. During the year-long campaign, the magnetar reproduced similar behaviour to that found for the first outburst, with a factor of two change in its spin-down rate from ∼7.2 × 10−12 s s−1 to ∼1.5 × 10−11 s s−1 after two months. Unique to this outburst, we confirm the peculiar energy-dependent phase shift of the pulse profile. Following the initial outburst, the spectrum of XTE J1810–197 is well-modelled by multiple blackbody components corresponding to a pair of non-concentric, hot thermal caps surrounded by a cooler one, superposed to the colder star surface. We model the energy-dependent pulse profile to constrain the viewing and surface emission geometry and find that the overall geometry of XTE J1810–197 has likely evolved relative to that found for the 2003 event.

Rapid Modification of Neutron Star Surface Magnetic Field: A proposed mechanism for explaining Radio Emission State Changes in Pulsars

The radio emission in many pulsars show sudden changes, usually within a period, that cannot be related to the steady state processes within the inner acceleration region (IAR) above the polar cap. These changes are often quasi-periodic in nature, where regular transitions between two or more stable emission states are seen. The durations of these states show a wide variety ranging from several seconds to hours at a time. There are strong, small scale magnetic field structures and huge temperature gradients present at the polar cap surface. We have considered several processes that can cause temporal modifications of the local magnetic field structure and strength at the surface of the polar cap. Using different magnetic field strengths and scales, and also assuming realistic scales of the temperature gradients, the evolutionary timescales of different phenomena affecting the surface magnetic field was estimated. We find that the Hall drift results in faster changes in comparison to both Ohmic decay and thermoelectric effects. A mechanism based on the Partially Screened Gap (PSG) model of the IAR has been proposed, where the Hall and thermoelectric oscillations perturb the polar cap magnetic field to alter the sparking process in the PSG. This is likely to affect the observed radio emission resulting in the observed state changes.

PSR B0656+14: the unified outlook from the infrared to X-rays

ABSTRACT We report detection of PSR B0656+14 with the Gran Telescopio Canarias in narrow optical F657, F754, F802, and F902 and near-infrared JHKs bands. The pulsar detection in the Ks band extends its spectrum to 2.2 $\mu$m and confirms its flux increase towards the infrared. We also present a thorough analysis of the optical spectrum obtained by us with the VLT. For a consistency check, we revised the pulsar near-infrared and narrow-band photometry obtained with the HST. We find no narrow spectral lines in the optical spectrum. We compile available near-infrared-optical-UV and archival 0.3–20 keV X-ray data and perform a self-consistent analysis of the rotation phase-integrated spectrum of the pulsar using unified spectral models. The spectrum is best fitted by the four-component model including two blackbodies, describing the thermal emission from the neutron star surface and its hot polar cap, the broken power law, originating from the pulsar magnetosphere, and an absorption line near ∼0.5 keV detected previously. The fit provides better constraints on the model parameters than using only a single spectral domain. The derived surface temperature is $T_{NS}^{\infty } = 7.9(3)\times 10^5$ K. The intrinsic radius (7.8–9.9 km) of the emitting region is smaller than a typical neutron star radius (13 km) and suggests a non-uniform temperature distribution over the star surface. In contrast, the derived radius of the hot polar cap is about twice as large as the ‘canonical’ one. The spectrum of the non-thermal emission steepens from the optical to X-rays and has a break near 0.1 keV. The X-ray data suggest the presence of another absorption line near 0.3 keV.

Polarized radiation transfer in neutron star surface layers

ABSTRACT The study of polarized radiation transfer in the highly magnetized surface locales of neutron stars is of great interest to the understanding of accreting X-ray pulsars, rotation-powered pulsars, and magnetars. This paper explores scattering transport in the classical magnetic Thomson domain that is of broad applicability to these neutron star classes. The development of a Monte Carlo simulation for the polarized radiative transfer is detailed: it employs an electric field vector formalism to enable a breadth of utility in relating linear, circular, and elliptical polarizations. The simulation can be applied to any neutron star surface locale, and is adaptable to accretion column and magnetospheric problems. Validation of the code for both intensity and Stokes parameter determination is illustrated in a variety of ways. Representative results for emergent polarization signals from surface layers are presented for both polar and equatorial magnetic locales, exhibiting contrasting signatures between the two regions. There is also a strong dependence of these characteristics on the ratio of the frequency $\, \omega \,$ of a photon to the cyclotron frequency $\, \omega _{\mathrm{B}}=eB/mc\,$. Polarization signatures for high-opacity domains are presented, highlighting compact analytical approximations for the Stokes parameters and anisotropy relative to the local field direction for an extended range of frequencies. These are very useful in defining injection conditions deep in the simulation slab geometries, expediting the generation of emission signals from highly opaque stellar atmospheres. The results are interpreted throughout using the polarization characteristics of the magnetic Thomson differential cross-section.

Detection of millihertz quasi-periodic oscillations in the X-Ray binary 1RXS J180408.9−342058

ABSTRACT Millihertz quasi-periodic oscillations (mHz QPOs) observed in neutron-star low-mass X-ray binaries (NS LMXBs) are generally explained as marginally stable thermonuclear burning on the neutron star surface. We report the discovery of mHz QPOs in an XMM–Newton observation of the transient 1RXS J180408.9−342058, during a regular bursting phase of its 2015 outburst. We found significant periodic signals in the March observation, with frequencies in the range 5–8 $\, \mathrm{mHz}$, superimposed on a strong ∼1/f power-law noise continuum. Neither the QPO signals nor the power-law noise were present during the April observation, which exhibited a 2.5 × higher luminosity and had correspondingly more frequent bursts. When present, the QPO signal power decreases during bursts and disappears afterwards, similar to the behaviour in other sources. 1RXS J180408.9−342058 is the eighth source known to date that exhibits such QPOs driven by thermonuclear burning. We examine the range of properties of the QPO signals in different sources. Whereas the observed oscillation profile is similar to that predicted by numerical models, the amplitudes are significantly higher, challenging their explanation as originating from marginally stable burning.

Laboratory evidence for an asymmetric accretion structure upon slanted matter impact in young stars

Aims. Investigating the process of matter accretion onto forming stars through scaled experiments in the laboratory is important in order to better understand star and planetary system formation and evolution. Such experiments can indeed complement observations by providing access to the processes with spatial and temporal resolution. A previous investigation revealed the existence of a two-component stream: a hot shell surrounding a cooler inner stream. The shell was formed by matter laterally ejected upon impact and refocused by the local magnetic field. That laboratory investigation was limited to normal incidence impacts. However, in young stellar objects, the complex structure of magnetic fields causes variability of the incident angles of the accretion columns. This led us to undertake an investigation, using laboratory plasmas, of the consequence of having a slanted accretion impacting a young star. Methods. Here, we used high power laser interactions and strong magnetic field generation in the laboratory, complemented by numerical simulations, to study the asymmetry induced upon accretion structures when columns of matter impact the surface of young stars with an oblique angle. Results. Compared to the scenario where matter accretes perpendicularly to the star surface, we observe a strongly asymmetric plasma structure, strong lateral ejecta of matter, poor confinement of the accreted material, and reduced heating compared to the normal incidence case. Thus, slanted accretion is a configuration that seems to be capable of inducing perturbations of the chromosphere and hence possibly influencing the level of activity of the corona.

Redshifted absorption lines from an isolated neutron star

The solid crust constituting the outer layers of a hot neutron star is wrapped by an mm-to-cm thin atmosphere. Even if the atmosphere is so thin, it substantially affects the blackbody spectrum emitted by the surface, resulting in an overall hardening of the emitted spectrum. The composition of the atmosphere has so far remained elusive. Several narrow absorption features have been detected and interpreted as arising from proton (or electron) resonant cyclotron absorption in the neutron star magnetic field. Apart from these, for a Hydrogen atmosphere no spectral features are expected, whereas when it is polluted with metals, absorption features start appearing in soft X-ray spectra. Absorption edges and features have been possibly observed during thermonuclear explosions onto the neutron star surface. Isolated neutron stars represent a breeding ground where to look for absorption features, thanks to their simple X-ray spectra. Here we report on the detection of redshifted Nitrogen and Oxygen absorption features from the closest and brightest isolated neutron star. The lines are ~50 eV wide and their intensity is incompatible from originating in the interstellar path to the neutron star. Lines are redshifted by a common gravitational redshift of z_g=0.216±0.004.