X-ray polarimetry is a unique way to probe the geometrical configuration of highly magnetized accreting neutron stars (X-ray pulsars).GRO J1008−57 is the first transient X-ray pulsar observed at two ...different flux levels by the Imaging X-ray Polarimetry Explorer (IXPE) duringits outburst in November 2022. We find the polarization properties of GRO J1008−57 to be independent of its luminosity, with the polarization degree varying between nondetection and about 15% over the pulse phase. Fitting the phase-resolved spectro-polarimetric data with the rotatingvector model allowed us to estimate the pulsar inclination (130◦, which is in good agreement with the orbital inclination), the position angle (75◦)of the pulsar spin axis, and the magnetic obliquity (∼74◦). This makes GRO J1008−57 the first confidently identified nearly orthogonal rotator among X-ray pulsars. We discuss our results in the context of the neutron star atmosphere models and theories of the axis alignment of accreting pulsars.
Abstract
The Imaging X-ray Polarimetry Explorer (IXPE) is a NASA Small Explorer mission—in partnership with the Italian Space Agency—dedicated to X-ray polarimetry in the 2–8 keV energy band. The ...IXPE telescope comprises three grazing incidence mirror modules coupled to three detector units hosting each one a Gas Pixel Detector, a gas detector that allows measuring the polarization degree by using the photoelectric effect. A wide and accurate ground calibration was carried out on the IXPE Detector Units at INAF-IAPS, in Italy, where a dedicated facility was setup at this aim. In this paper, we present the results obtained from this calibration campaign to study the IXPE focal plane detector response to polarized radiation. In particular, we report on the modulation factor, which is the main parameter to estimate the sensitivity of a polarimeter.
Abstract
We present the Fermi-LAT observations of the behind-the-limb (BTL) flare of 2021 July 17 and the joint detection of this flare by STIX on board the Solar Orbiter. The separation between ...Earth and the Solar Orbiter was 99.°2 at 05:00 UT, allowing STIX to have a front view of the flare. The location of the flare was S20E140 in Stonyhurst heliographic coordinates, making this the most distant behind-the-limb flare ever detected in >100 MeV gamma-rays. The LAT detection lasted for ∼16 minutes, the peak flux was 3.6 ± 0.8 (10
−5
) ph cm
−2
s
−1
with a significance >15
σ
. A coronal wave was observed from both STEREO-A and SDO in extreme ultraviolet (EUV), with an onset on the visible disk in coincidence with the LAT onset. A complex type II radio burst was observed by GLOSS also in coincidence with the onset of the LAT emission, indicating the presence of a shock wave. We discuss the relation between the time derivative of the EUV wave intensity profile at 193 Å as observed by STEREO-A and the LAT flux to show that the appearance of the coronal wave at the visible disk and the acceleration of protons as traced by the observed >100 MeV gamma-ray emission are coupled. We also report how this coupling is present in the data from three other BTL flares detected by Fermi-LAT, suggesting that the protons driving the gamma-ray emission of BTL solar flares and the coronal wave share a common origin.
A black hole x-ray binary (XRB) system forms when gas is stripped from a normal star and accretes onto a black hole, which heats the gas sufficiently to emit x-rays. We report a polarimetric ...observation of the XRB Cygnus X-1 using the Imaging X-ray Polarimetry Explorer. The electric field position angle aligns with the outflowing jet, indicating that the jet is launched from the inner x-ray–emitting region. The polarization degree is 4.01 ± 0.20% at 2 to 8 kiloelectronvolts, implying that the accretion disk is viewed closer to edge-on than the binary orbit. These observations reveal that hot x-ray–emitting plasma is spatially extended in a plane perpendicular to, not parallel to, the jet axis.
x-ray polarization of Cygnus X-1
A black hole in a binary system can rip material off of its companion star, which heats up and forms an accretion disk. The disc emits light in the optical and x-ray bands, forming an x-ray binary (XRB) system. Some XRBs also launch a jet of fast-moving material that is visible at radio wavelengths. Krawczynski
et al
. observed the x-ray polarization of Cygnus X-1, a black hole XRB with a radio jet. By comparing the measured polarization properties with several competing XRB models, they eliminated some hypothesized geometries and determined that the x-ray–emitting region extends parallel to the accretion disc. —KTS
x-ray polarization measurements determine the geometric arrangement of hot material accreting onto a black hole.
Context.
After about 16 years since its first outburst, the transient neutron star low-mass X-ray binary XTE J1701−462 turned on again in September 2022, allowing for the first study of its X-ray ...polarimetric characteristics by a dedicated observing program with the Imaging X-ray Polarimeter Explorer (IXPE).
Aims.
Polarimetric studies of XTE J1701−462 have been expected to improve our understanding of accreting weakly magnetized neutron stars, in particular, the physics and the geometry of the hot inner regions close to the compact object.
Methods.
The IXPE data of two triggered observations were analyzed using time-resolved spectroscopic and polarimetric techniques, following the source along its
Z
-track of the color–color diagram.
Results.
During the first pointing on 2022 September 29, an average 2–8 keV polarization degree of (4.6 ± 0.4)% was measured, the highest value found up to now for this class of sources. Conversely, only a ∼0.6% average degree was obtained during the second pointing ten days later.
Conclusions.
The polarimetric signal appears to be strictly related to the higher energy blackbody component associated with the boundary layer (BL) emission and its reflection from the inner accretion disk, and it is as strong as 6.1% and 1.2% (> 95% significant) above 3–4 keV for the two measurements, respectively. The variable polarimetric signal is apparently related to the spectral characteristics of XTE J1701−462, which is the strongest when the source was in the horizontal branch of its
Z
-track and the weakest in the normal branch. These IXPE results provide new important observational constraints on the physical models and geometry of the
Z
-sources. Here, we discuss the possible reasons for the presence of strong and variable polarization among these sources.
Abstract
The launch of the Imaging X-ray Polarimetry Explorer (IXPE) on 2021 December 9 has opened a new window in X-ray astronomy. We report here the results of the first IXPE observation of a ...weakly magnetized neutron star, GS 1826−238, performed on 2022 March 29–31 when the source was in a high soft state. An upper limit (99.73% confidence level) of 1.3% for the linear polarization degree is obtained over the IXPE 2–8 keV energy range. Coordinated INTEGRAL and NICER observations were carried out simultaneously with IXPE. The spectral parameters obtained from the fits to the broadband spectrum were used as inputs for Monte Carlo simulations considering different possible geometries of the X-ray emitting region. Comparing the IXPE upper limit with these simulations, we can put constraints on the geometry and inclination angle of GS 1826–238.
Polarized x-rays from a magnetar Taverna, Roberto; Turolla, Roberto; Muleri, Fabio ...
Science (American Association for the Advancement of Science),
11/2022, Volume:
378, Issue:
6620
Journal Article
Peer reviewed
Open access
Magnetars are neutron stars with ultrastrong magnetic fields, which can be observed in x-rays. Polarization measurements could provide information on their magnetic fields and surface properties. We ...observed polarized x-rays from the magnetar 4U 0142+61 using the Imaging X-ray Polarimetry Explorer and found a linear polarization degree of 13.5 ± 0.8% averaged over the 2– to 8–kilo–electron volt band. The polarization changes with energy: The degree is 15.0 ± 1.0% at 2 to 4 kilo–electron volts, drops below the instrumental sensitivity ~4 to 5 kilo–electron volts, and rises to 35.2 ± 7.1% at 5.5 to 8 kilo–electron volts. The polarization angle also changes by 90° at ~4 to 5 kilo–electron volts. These results are consistent with a model in which thermal radiation from the magnetar surface is reprocessed by scattering off charged particles in the magnetosphere.
Polarization constrains magnetar emission
Magnetars are young neutron stars with high magnetic fields that are usually observed at x-ray wavelengths. The emission mechanism and geometry of the emitting region have been unclear. Taverna
et al
. measured the x-ray polarization of the magnetar 4U 0142+61. The polarization degree and angle change as a function of x-ray energy, indicating two different emission regions. The authors preferred a model in which most of the x-rays are emitted by an equatorial band on the surface of the neutron star, with some of the photons then being scattered to higher energies by collisions with electrons in the surrounding magnetic field. —KTS
Measurements of a magnetar’s x-ray polarization constrain models of the emission mechanism.
Pulsar wind nebulae are formed when outflows of relativistic electrons and positrons hit the surrounding supernova remnant or interstellar medium at a shock front. The Vela pulsar wind nebula is ...powered by a young pulsar (B0833-45, aged 11,000 years)
and located inside an extended structure called Vela X, which is itself inside the supernova remnant
. Previous X-ray observations revealed two prominent arcs that are bisected by a jet and counter jet
. Radio maps have shown high linear polarization of 60% in the outer regions of the nebula
. Here we report an X-ray observation of the inner part of the nebula, where polarization can exceed 60% at the leading edge-approaching the theoretical limit of what can be produced by synchrotron emission. We infer that, in contrast with the case of the supernova remnant, the electrons in the pulsar wind nebula are accelerated with little or no turbulence in a highly uniform magnetic field.
The centre of the Milky Way Galaxy hosts a black hole with a solar mass of about 4 million (Sagittarius A
(Sgr A)) that is very quiescent at present with a luminosity many orders of magnitude below ...those of active galactic nuclei
. Reflection of X-rays from Sgr A
by dense gas in the Galactic Centre region offers a means to study its past flaring activity on timescales of hundreds and thousands of years
. The shape of the X-ray continuum and the strong fluorescent iron line observed from giant molecular clouds in the vicinity of Sgr A
are consistent with the reflection scenario
. If this interpretation is correct, the reflected continuum emission should be polarized
. Here we report observations of polarized X-ray emission in the direction of the molecular clouds in the Galactic Centre using the Imaging X-ray Polarimetry Explorer. We measure a polarization degree of 31% ± 11%, and a polarization angle of -48° ± 11°. The polarization angle is consistent with Sgr A
being the primary source of the emission, and the polarization degree implies that some 200 years ago, the X-ray luminosity of Sgr A
was briefly comparable to that of a Seyfert galaxy.
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