21 research outputs found
Vacuum birefringence and X-ray polarimetry in transient magnetars
Recent optical polarimetry observations of an X-ray dim isolated neutron star, RX J1856.5-3754, showed a first evidence for QED vacuum birefringence induced by a strong magnetic field. This important result can be confirmed by performing systematically polarimetry observations in the X-ray band for other strongly magnetized neutron stars, such as transient or persistent magnetars. We computed the phase averaged polarization fraction (PF) and polarization angle (PA) expected in the thermal emission from transient magnetars in the soft X-ray energy band. We found that the detection of a PF higher than 60% is a strong evidence for vacuum birefringence. We also found that a steady change in the PA measured from transient magnetars during their outburst decay (up to 23 degrees for a magnetospheric untwisting of 0.5 rad) is a strong signature of vacuum birefringence. This latter detection would also provide an independent check of the magnetospheric untwisting model for these sources. Simulations show that these measurements are achievable by future polarimetric missions such as XIPE and IXPE with 20-380 ks of observational time, and with eXTP with 3-60 ks
Evidence for vacuum birefringence from the first optical-polarimetry measurement of the isolated neutron star RX J1856.5-3754
The "Magnificent Seven" (M7) are a group of radio-quiet Isolated Neutron
Stars (INSs) discovered in the soft X-rays through their purely thermal surface
emission. Owing to the large inferred magnetic fields ( G),
radiation from these sources is expected to be substantially polarised,
independently on the mechanism actually responsible for the thermal emission. A
large observed polarisation degree is, however, expected only if
quantum-electrodynamics (QED) polarisation effects are present in the
magnetised vacuum around the star. The detection of a strongly linearly
polarised signal would therefore provide the first observational evidence of
QED effects in the strong-field regime. While polarisation measurements in the
soft X-rays are not feasible yet, optical polarisation measurements are within
reach also for quite faint targets, like the M7 which have optical counterparts
with magnitudes --. Here, we report on the measurement of
optical linear polarisation for the prototype, and brightest member, of the
class, RX\, J1856.53754 (), the first ever for one of the M7,
obtained with the Very Large Telescope. We measured a polarisation degree
and a polarisation position angle
\mathrm{P.A.}=145\fdg39\pm9\fdg44, computed east of the North Celestial
Meridian. The that we derive is large enough to support the
presence of vacuum birefringence, as predicted by QED.Comment: 9 pages, 7 figures, accepted for publication on MNRA
Constraining the magnetic field geometry of the millisecond pulsar PSRJ0030+0451 from joint radio, thermal X-ray, and Îł-ray emission
Context. With the advent of multi-wavelength electromagnetic observations of neutron stars - spanning many decades in photon energies - from radio wavelengths up to X-rays and Îł-rays, it has become possible to significantly constrain the geometry and the location of the associated emission regions. Aims. In this work, we use results from the modelling of thermal X-ray observations of PSR J0030+0451 from the Neutron Star Interior Composition Explorer (NICER) mission and phase-aligned radio and Îł-ray pulse profiles to constrain the geometry of an off-centred dipole that is able to reproduce the light curves in these respective bands simultaneously. Methods. To this aim, we deduced a configuration with a simple dipole off-centred from the location of the centre of the thermal X-ray hot spots. We show that the geometry is compatible with independent constraints from radio and -ray pulsations only, leading to a fixed magnetic obliquity of α â 75° and a line-of-sight inclination angle of ζ â 54°. Results. We demonstrate that an off-centred dipole cannot be rejected by accounting for the thermal X-ray pulse profiles. Moreover, the crescent shape of one spot is interpreted as the consequence of a small-scale surface dipole on top of the large-scale off-centred dipole
Complex variations in X-ray polarization in the X-ray pulsar LS V +44 17/RX J0440.9+4431
We report on Imaging X-ray polarimetry explorer (IXPE) observations of the Be-transient X-ray pulsar LS V +44 17/RX J0440.9+4431 made at two luminosity levels during the giant outburst in January- February 2023. Considering the observed spectral variability and changes in the pulse profiles, the source was likely caught in supercritical and subcritical states with significantly different emission-region geometry, associated with the presence of accretion columns and hot spots, respectively. We focus here on the pulse-phase-resolved polarimetric analysis and find that the observed dependencies of the polarization degree and polarization angle (PA) on the pulse phase are indeed drastically different for the two observations. The observed differences, if interpreted within the framework of the rotating vector model (RVM), imply dramatic variations in the spin axis inclination, the position angle, and the magnetic colatitude by tens of degrees within the space of just a few days. We suggest that the apparent changes in the observed PA phase dependence are predominantly related to the presence of an unpulsed polarized component in addition to the polarized radiation associated with the pulsar itself. We then show that the observed PA phase dependence in both observations can be explained with a single set of RVM parameters defining the pulsar s geometry. We also suggest that the additional polarized component is likely produced by scattering of the pulsar radiation in the equatorial disk wind
X-ray polarimetry of the accreting pulsar GX 301-2
The phase- and energy-resolved polarization measurements of accreting X-ray
pulsars (XRPs) allow us to test different theoretical models of their emission,
as well as to provide an avenue to determine the emission region geometry. We
present the results of the observations of the XRP GX 301-2 performed with the
Imaging X-ray Polarimetry Explorer (IXPE). GX 301-2 is a persistent XRP with
one of the longest known spin periods of ~680 s. A massive hyper-giant
companion star Wray 977 supplies mass to the neutron star via powerful stellar
winds. We do not detect significant polarization in the phase-averaged data
using spectro-polarimetric analysis, with the upper limit on the polarization
degree (PD) of 2.3% (99% confidence level). Using the phase-resolved
spectro-polarimetric analysis we get a significant detection of polarization
(above 99% c.l.) in two out of nine phase bins and marginal detection in three
bins, with a PD ranging between ~3% and ~10%, and a polarization angle varying
in a very wide range from ~0 deg to ~160 deg. Using the rotating vector model
we obtain constraints on the pulsar geometry using both phase-binned and
unbinned analysis getting excellent agreement. Finally, we discuss possible
reasons for a low observed polarization in GX 301-2.Comment: 10 pages, 10 figures, submitted to A&
X-ray polarimetry of X-ray pulsar X Persei: another orthogonal rotator?
X Persei is a persistent low-luminosity X-ray pulsar of period of ââ835âs in a Be binary system. The field strength at the neutron star surface is not known precisely, but indirect signs indicate a magnetic field above 1013âG, which makes the object one of the most magnetized known X-ray pulsars. Here we present the results of observations X Persei performed with the Imaging X-ray Polarimetry Explorer (IXPE). The X-ray polarization signal was found to be strongly dependent on the spin phase of the pulsar. The energy-averaged polarization degree in 3â8âkeV band varied from several to âŒ20 per cent over the pulse with a phase dependence resembling the pulse profile. The polarization angle shows significant variation and makes two complete revolutions during the pulse period, resulting in nearly nil pulse-phase averaged polarization. Applying the rotating vector model to the IXPE data we obtain the estimates for the rotation axis inclination and its position angle on the sky, as well as for the magnetic obliquity. The derived inclination is close to the orbital inclination, reported earlier for X Persei. The polarimetric data imply a large angle between the rotation and magnetic dipole axes, which is similar to the result reported recently for the X-ray pulsar GRO J1008â57. After eliminating the effect of polarization angle rotation over the pulsar phase using the best-fitting rotating vector model, the strong dependence of the polarization degree with energy was discovered, with its value increasing from 0 at âŒ2 keV to 30per cent at 8 keV
Constraining the magnetic field geometry of the millisecond pulsar PSR~J0030+0451 from joint radio, thermal X-ray and -ray emission
International audienceWith the advent of multi-wavelength electromagnetic observations of neutron stars, spanning many decades in photon energies, from radio wavelengths up to X-rays and -rays, it becomes possible to significantly constrain the geometry and the location of the associated emission regions. In this work, we use results from the modelling of thermal X-ray observations of PSR~J0030+0451 from the NICER mission and phase-aligned radio and -ray pulse profiles to constrain the geometry of an off-centred dipole able to reproduce the light-curves in these respective bands simultaneously. To this aim, we deduce a configuration with a simple dipole off-centred from the location of the centre of the thermal X-ray hot spots and show that the geometry is compatible with independent constraints from radio and -ray pulsations only, leading to a fixed magnetic obliquity of and a line of sight inclination angle of . We demonstrate that an off-centred dipole cannot be rejected by accounting for the thermal X-ray pulse profiles. Moreover, the crescent shape of one spot is interpreted as the consequence of a small scale surface dipole on top of the large scale off-centred dipole