Dr Alexander Mushtukov

Abstract:

In this paper we present the science potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission, in its new configuration, for studies of strongly magnetized compact objects. We discuss the scientific potential of eXTP for quantum electrodynamic (QED) studies, especially leveraging the recent observations made with the NASA IXPE mission. Given eXTP’s unique combination of timing, spectroscopy, and polarimetry, we focus on the perspectives for physics and astrophysics studies of strongly magnetized compact objects, such as magnetars and accreting X-ray pulsars. Developed by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Sciences, the eXTP mission is expected to launch in early 2030.

Abstract:

ABSTRACT Accretion on to neutron stars (NSs) in X-ray pulsars (XRPs) results in intense X-ray emission, and under specific conditions, high-energy nuclear interactions that produce gamma-ray photons at discrete energies. These interactions are enabled by the high free-fall velocities of accreting nuclei near the NS surface and give rise to characteristic gamma-ray lines, notably at 2.2, 5.5, and 67.5 MeV. We investigate the production mechanisms of these lines and estimate the resulting gamma-ray luminosities, accounting for the suppression effects of radiative deceleration in bright XRPs and the creation of electron–positron pairs in strong magnetic fields. The resulting annihilation of these pairs leads to a secondary emission line at ${\sim} 511$ keV. We also discuss the possibility that non-stationary pair creation in the polar cap region could drive coherent radio emission, though its detectability in accreting systems remains uncertain. Using a numerical framework incorporating general relativistic light bending and magnetic absorption, we compute the escape fraction of photons and distinguish between actual and apparent gamma-ray luminosities. Our results identify the parameter space – defined by magnetic field strength, accretion luminosity, and NS compactness – where these gamma-ray signatures may be observable by upcoming MeV gamma-ray missions. In particular, we highlight the diagnostic potential of detecting gravitationally redshifted gamma-ray lines and annihilation features for probing the mass–radius relation and magnetospheric structure of NSs.

Abstract:

Accretion onto strongly magnetized neutron stars in X-ray pulsars (XRPs) produces intense X-ray emission and gamma-ray photons, the latter arising from nuclear reactions and high-energy particle collisions in the stellar atmosphere. These gamma-rays interact with the magnetic field via one- and two-photon pair creation processes, generating electron-positron pairs. We investigate one-photon pair production in sub-critical XRPs, with a focus on how surface magnetic field strength affects gamma-ray absorption in the magnetosphere. Using general relativistic photon trajectory simulations, we map the spatial distribution of pair creation sites and quantify absorption efficiencies. We find that XRPs with surface fields B ≲ 10 12 G are largely transparent to 2.2MeV gamma-rays, while fields B ≳ 3 × 10 12 G lead to efficient absorption within a few tens of centimeters from the surface. For lower field strengths, absorption can occur at larger distances and outside the accretion column, offering a potential channel for radio emission. Our results provide new insight into the interplay between nuclear processes, magnetospheric structure, and multiwavelength radiation in XRPs.

Abstract:

We present observations of the accretion-powered X-ray pulsar 4U 1907+09 conducted with the Imaging X-ray Polarimetry Explorer, which has delivered the first high-quality polarization measurements of this source. 4U 1907+09 was observed twice during its brightest periods, close to the periastron. We observe a stronger polarization in the first observation, with a phase-averaged polarization degree (PD) of 6.0 ± 1.6% and a polarization angle (PA) of 69° ±8°. The second observation provides weaker constraints on the polarimetric properties, PD = 2.2 ± 1.6% and PA = 46° ±23°, as determined from the spectro-polarimetric analysis. Combining the data from the two observations results in PD = 3.7 ± 1.1% and PA = 63° ±9°. We detect an energy-dependent PA in the phase-averaged analyses with a significance of 1.7 σ . In the phase-resolved analyses, we observe a potential PA rotation of approximately 90° between adjacent energy bands (4–5 and 5–6 keV) within the single phase bin of 0.25–0.375. We also investigate the influence of short flares on the polarization properties of this source. The results suggest that flares do not significantly affect the energy-phase-dependent PA, implying that the pulsar’s geometry remains stable during flare events.

Abstract:

The Imaging X-ray Polarimetry Explorer (IXPE) observations of accreting X-ray pulsars (XRPs) continue to provide novel insights into the physics and geometry of these sources. We present the first X-ray polarimetric study of the persistent wind-fed XRP 4U 1538−52, based on five IXPE observations totaling 360 ks, conducted in March and October 2024. We detect marginally significant polarization in the combined data set in the full 2–8 keV energy band, with a polarization degree (PD) of 3.0 ± 1.1% and polarization angle (PA) of −18° ±11°. The energy-resolved analysis shows a clear energy dependence of the polarization properties, with a remarkable ∼70° switch in PA between low and high energies. Similarly, the pulse phase-resolved spectro-polarimetric analysis reveals different signatures at low and high energies. At low energies (2–3 keV), the PD spans from ∼2% up to ∼18%, accompanied by large-amplitude swings in the PA. At higher energies (4–8 keV), the PD varies between ∼3% and ∼12%, and the PA not only is similarly highly variable but also exhibits a markedly different phase dependence. Fitting the rotating vector model to the pulse phase dependence of the PA at the lower energies, we constrain the geometric configuration of the pulsar. The analysis favors a high spin-axis inclination of > 50°, which agrees with both previous pulse-phase-dependent spectral fitting of the cyclotron line region and the known high orbital inclination of the binary system. The magnetic obliquity is estimated to be 30° and the spin position angle to be 19°. A sharp switch in PA around 3 keV presents a particular theoretical challenge, as it is not consistent with the right-angle switch that was only seen in one other pulsar, Vela X-1.