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.

An expanding radio jet from a destroyed star: If a star gets too close to a supermassive black hole, it gets ripped apart in a tidal disruption event (TDE). Mattilaet al.discovered a transient source in the merging galaxy pair Arp 299, which they interpret as a TDE. The optical light is hidden by dust, but the TDE generated copious infrared emission. Radio observations reveal that a relativistic jet was produced as material fell onto the black hole, with the jet expanding over several years. The results elucidate how jets form around supermassive black holes and suggest that many TDEs may be missed by optical surveys.Science, this issue p.482

Most of the light from blazars, active galactic nuclei with jets of magnetized plasma that point nearly along the line of sight, is produced by high-energy particles, up to around 1 TeV. Although the jets are known to be ultimately powered by a supermassive black hole, how the particles are accelerated to such high energies has been an unanswered question. The process must be related to the magnetic field, which can be probed by observations of the polarization of light from the jets. Measurements of the radio to optical polarization—the only range available until now—probe extended regions of the jet containing particles that left the acceleration site days to years earlier^1–3, and hence do not directly explore the acceleration mechanism, as could X-ray measurements. Here we report the detection of X-ray polarization from the blazar Markarian 501 (Mrk 501). We measure an X-ray linear polarization degree Π_X of around 10%, which is a factor of around 2 higher than the value at optical wavelengths, with a polarization angle parallel to the radio jet. This points to a shock front as the source of particle acceleration and also implies that the plasma becomes increasingly turbulent with distance from the shock.

We present here the first convincing observational manifestation of a magnetar-like magnetic field in an accreting neutron star in binary system – the first pulsating ultraluminous X-ray source X−2 in the galaxy M82. Using the Chandra X-ray observatory data, we show that the source exhibit the bimodal distribution of the luminosity with two well-defined peaks separated by a factor of 40. This behaviour can be interpreted as the action of the ‘propeller regime’ of accretion. The onset of the propeller in a 1.37 s pulsar at luminosity of ∼10⁴⁰ erg s⁻¹ implies the dipole component of the neutron star magnetic field of ∼10¹⁴ G.

We present an X-ray spectro-polarimetric analysis of the bright Seyfert galaxy NGC 4151. The source has been observed with the Imaging X-ray Polarimetry Explorer (IXPE) for 700 ks, complemented with simultaneous XMM-Newton (50 ks) and NuSTAR (100 ks) pointings. A polarization degree Π = 4.9 ± 1.1% and angle Ψ = 86^○ ± 7^○ east of north (68% confidence level) are measured in the 2–8 keV energy range. The spectro-polarimetric analysis shows that the polarization could be entirely due to reflection. Given the low reflection flux in the IXPE band, this requires however a reflection with a very large (>38%) polarization degree. Assuming more reasonable values, a polarization degree of the hot corona ranging from ∼4 to ∼8% is found. The observed polarization degree excludes a ‘spherical’ lamppost geometry for the corona, suggesting instead a slab-like geometry, possibly a wedge, as determined via Monte Carlo simulations. This is further confirmed by the X-ray polarization angle, which coincides with the direction of the extended radio emission in this source, supposed to match the disc axis. NGC 4151 is the first AGN with an X-ray polarization measure for the corona, illustrating the capabilities of X-ray polarimetry and IXPE in unveiling its geometry.