11 research outputs found
Discovery of X-Ray Polarization from the Black Hole Transient Swift J1727.8â1613
\ua9 2023. The Author(s). Published by the American Astronomical Society.We report the first detection of the X-ray polarization of the bright transient Swift J1727.8â1613 with the Imaging X-ray Polarimetry Explorer. The observation was performed at the beginning of the 2023 discovery outburst, when the source resided in the bright hard state. We find a time- and energy-averaged polarization degree of 4.1% \ub1 0.2% and a polarization angle of 2.\ub02 \ub1 1.\ub03 (errors at 68% confidence level; this translates to âŒ20Ï significance of the polarization detection). This finding suggests that the hot corona emitting the bulk of the detected X-rays is elongated, rather than spherical. The X-ray polarization angle is consistent with that found in submillimeter wavelengths. Since the submillimeter polarization was found to be aligned with the jet direction in other X-ray binaries, this indicates that the corona is elongated orthogonal to the jet
X-ray Polarization of the Eastern Lobe of SS 433
How astrophysical systems translate the kinetic energy of bulk motion into
the acceleration of particles to very high energies is a pressing question. SS
433 is a microquasar that emits TeV gamma-rays indicating the presence of
high-energy particles. A region of hard X-ray emission in the eastern lobe of
SS 433 was recently identified as an acceleration site. We observed this region
with the Imaging X-ray Polarimetry Explorer and measured a polarization degree
in the range 38% to 77%. The high polarization degree indicates the magnetic
field has a well ordered component if the X-rays are due to synchrotron
emission. The polarization angle is in the range -12 to +10 degrees (east of
north) which indicates that the magnetic field is parallel to the jet. Magnetic
fields parallel to the bulk flow have also been found in supernova remnants and
the jets of powerful radio galaxies. This may be caused by interaction of the
flow with the ambient medium.Comment: 8 pages, accepted in the Astrophysical Journal Letter
Magnetic Field Properties inside the Jet of Mrk 421: Multiwavelength Polarimetry Including the Imaging X-ray Polarimetry Explorer
We conducted a polarimetry campaign from radio to X-ray wavelengths of the
high-synchrotron-peak (HSP) blazar Mrk 421, including Imaging X-ray Polarimetry
Explorer (IXPE) measurements on 2022 December 6-8. We detected X-ray
polarization of Mrk 421 with a degree of =141 and an
electric-vector position angle =1073 in the 2-8
keV band. From the time variability analysis, we find a significant episodic
variation in . During 7 months from the first IXPE pointing of
Mrk 421 in 2022 May, varied across the range of 0 to
180, while maintained similar values within
10-15. Furthermore, a swing in in 2022 June was
accompanied by simultaneous spectral variations. The results of the
multiwavelength polarimetry show that the X-ray polarization degree was
generally 2-3 times greater than that at longer wavelengths, while the
polarization angle fluctuated. Additionally, based on radio, infrared, and
optical polarimetry, we find that rotation of occurred in the opposite
direction with respect to the rotation of over longer timescales
at similar epochs. The polarization behavior observed across multiple
wavelengths is consistent with previous IXPE findings for HSP blazars. This
result favors the energy-stratified shock model developed to explain variable
emission in relativistic jets. The accompanying spectral variation during the
rotation can be explained by a fluctuation in the physical
conditions, e.g., in the energy distribution of relativistic electrons. The
opposite rotation direction of between the X-ray and longer-wavelength
polarization accentuates the conclusion that the X-ray emitting region is
spatially separated from that at longer wavelengths.Comment: 17 pages, 13 figures, 4 tables; Accepted for publication in A&
Tracking the X-ray Polarization of the Black Hole Transient Swift J1727.8-1613 during a State Transition
We report on a campaign on the bright black hole X-ray binary Swift
J1727.81613 centered around five observations by the Imaging X-ray
Polarimetry Explorer (IXPE). This is the first time it has been possible to
trace the evolution of the X-ray polarization of a black hole X-ray binary
across a hard to soft state transition. The 2--8 keV polarization degree slowly
decreased from 4\% to 3\% across the five observations, but
remained in the North-South direction throughout. Using the Australia Telescope
Compact Array (ATCA), we measure the intrinsic 7.25 GHz radio polarization to
align in the same direction. Assuming the radio polarization aligns with the
jet direction (which can be tested in the future with resolved jet images),
this implies that the X-ray corona is extended in the disk plane, rather than
along the jet axis, for the entire hard intermediate state. This in turn
implies that the long (10 ms) soft lags that we measure with the
Neutron star Interior Composition ExploreR (NICER) are dominated by processes
other than pure light-crossing delays. Moreover, we find that the evolution of
the soft lag amplitude with spectral state differs from the common trend seen
for other sources, implying that Swift J1727.81613 is a member of a hitherto
under-sampled sub-population.Comment: Submitted to ApJ. 20 pages, 8 figure
Joint machine learning and analytic track reconstruction for X-ray polarimetry with gas pixel detectors
We present our study on the reconstruction of photoelectron tracks in gas pixel detectors used for astrophysical X-ray polarimetry. Our work aims to maximize the performance of convolutional neural networks (CNNs) to predict the impact point of incoming X-rays from the image of the photoelectron track. A very high precision in the reconstruction of the impact point position is achieved thanks to the introduction of an artificial sharpening process of the images. We find that providing the CNN-predicted impact point as input to the state-of-the-art analytic analysis improves the modulation factor (~1% at 3 keV and ~6% at 6 keV) and naturally mitigates a subtle effect appearing in polarization measurements of bright extended sources known as âpolarization leakageâ
Magnetic field properties inside the jet of Mrk 421. Multiwavelength polarimetry, including the Imaging X-ray Polarimetry Explorer
International audience Aims: We aim to probe the magnetic field geometry and particle acceleration mechanism in the relativistic jets of supermassive black holes. Methods: We conducted a polarimetry campaign from radio to X-ray wavelengths of the high-synchrotron-peak (HSP) blazar Mrk 421, including Imaging X-ray Polarimetry Explorer (IXPE) measurements from 2022 December 6-8. During the IXPE observation, we also monitored Mrk 421 using Swift-XRT and obtained a single observation with XMM-Newton to improve the X-ray spectral analysis. The time-averaged X-ray polarization was determined consistently using the event-by-event Stokes parameter analysis, spectropolarimetric fit, and maximum likelihood methods. We examined the polarization variability over both time and energy, the former via analysis of IXPE data obtained over a time span of 7 months. Results: We detected X-ray polarization of Mrk 421 with a degree of Î X = 14 ± 1% and an electric-vector position angle ÏX = 107 ± 3° in the 2-8 keV band. From the time variability analysis, we find a significant episodic variation in ÏX. During the 7 months from the first IXPE pointing of Mrk 421 in 2022 May, ÏX varied in the range 0° to 180°, while Î X remained relatively constant within âŒ10-15%. Furthermore, a swing in ÏX in 2022 June was accompanied by simultaneous spectral variations. The results of the multiwavelength polarimetry show that Î X was generally âŒ2-3 times greater than Î at longer wavelengths, while Ï fluctuated. Additionally, based on radio, infrared, and optical polarimetry, we find that the rotation of Ï occurred in the opposite direction with respect to the rotation of ÏX and over longer timescales at similar epochs. Conclusions: The polarization behavior observed across multiple wavelengths is consistent with previous IXPE findings for HSP blazars. This result favors the energy-stratified shock model developed to explain variable emission in relativistic jets. We considered two versions of the model, one with linear and the other with radial stratification geometry, to explain the rotation of ÏX. The accompanying spectral variation during the ÏX rotation can be explained by a fluctuation in the physical conditions, for example in the energy distribution of relativistic electrons. The opposite rotation direction of Ï between the X-ray and longer wavelength polarization accentuates the conclusion that the X-ray emitting region is spatially separated from that at longer wavelengths. Moreover, we identify a highly polarized knot of radio emission moving down the parsec-scale jet during the episode of ÏX rotation, although it is unclear whether there is any connection between the two events
X-Ray Polarization of the Eastern Lobe of SS 433
International audienceHow astrophysical systems translate the kinetic energy of bulk motion into the acceleration of particles to very high energies is a pressing question. SS 433 is a microquasar that emits TeV γ-rays indicating the presence of high-energy particles. A region of hard X-ray emission in the eastern lobe of SS 433 was recently identified as an acceleration site. We observed this region with the Imaging X-ray Polarimetry Explorer and measured a polarization degree in the range 38%-77%. The high polarization degree indicates the magnetic field has a well-ordered component if the X-rays are due to synchrotron emission. The polarization angle is in the range -12° to +10° (east of north), which indicates that the magnetic field is parallel to the jet. Magnetic fields parallel to the bulk flow have also been found in supernova remnants and the jets of powerful radio galaxies. This may be caused by interaction of the flow with the ambient medium
Detection of X-Ray Polarization from the Blazar 1ES 1959+650 with the Imaging X-Ray Polarimetry Explorer
Observations of linear polarization in the 2â8 keV energy range with the Imaging X-ray Polarimetry Explorer (IXPE) explore the magnetic field geometry and dynamics of the regions generating nonthermal radiation in relativistic jets of blazars. These jets, particularly in blazars whose spectral energy distribution peaks at X-ray energies, emit X-rays via synchrotron radiation from high-energy particles within the jet. IXPE observations of the X-ray-selected BL Lacâtype blazar 1ES 1959+650 on 2022 May 3â4 showed a significant linear polarization degree of Î _x = 8.0% ± 2.3% at an electric-vector position angle Ï _x = 123° ± 8°. However, on 2022 June 9â12, only an upper limit of Î _x †5.1% could be derived (at the 99% confidence level). The degree of optical polarization at that time, Î _O ⌠5%, is comparable to the X-ray measurement. We investigate possible scenarios for these findings, including temporal and geometrical depolarization effects. Unlike some other X-ray-selected BL Lac objects, there is no significant chromatic dependence of the measured polarization in 1ES 1959+650, and its low X-ray polarization may be attributed to turbulence in the jet flow with dynamical timescales shorter than 1 day