First X-ray polarimetric measurement of Her X-1 in the anomalous low state

ATel #17986; Sergey S. Tsygankov, Sofia V. Forsblom, Juri Poutanen, Alexandra Veledina (U. of Turku), Sergey Molkov, Alexander Lutovinov, Igor Lapshov, Alexey Tkachenko (IKI RAS), Herman L. Marshall (MIT)
on 19 Aug 2026; 06:11 UT
Credential Certification: Sergey Tsygankov (tsygankov@iki.rssi.ru)

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The accreting X-ray pulsar Her X-1 (HZ Her) is currently undergoing a rare anomalous low state (ALS), during which the normal 35-day X-ray cycle is strongly suppressed. The current event was identified following a progressive decrease of the X-ray flux in early 2026 (ATel #17607). The ALS provides a unique opportunity to study reprocessed X-ray emission. During normal bright states, the strong pulsar contribution and relatively stable reprocessed emission make their polarization properties difficult to disentangle. In contrast, suppression of the direct pulsar emission during an ALS allows the polarization of the reprocessed component to be probed more directly. Previous observations have shown that the ALS spectrum can be dominated by Compton-reflected emission (e.g. Still et al. 2001).

We observed Her X-1 with the Imaging X-ray Polarimetry Explorer (IXPE) during three consecutive binary orbital cycles, covering the Main-on phase of the 35-day superorbital cycle (MJD 61223.2-61227.4). Simultaneous observations with SRG/ART-XC covered four consecutive binary orbital cycles (MJD 61221-61227.6), while NuSTAR observed the source during the first IXPE orbit (MJD 61222.8-61224.2).

No significant pulsations were detected in the IXPE data. In simultaneous ART-XC and NuSTAR observations, however, we detected weak pulsations with a period of 1.237716(3) s and pulsed fractions of approximately 4% in the 4-12 keV ART-XC band and 5% in the 4-30 keV NuSTAR band. The low pulsed fraction is consistent with strong suppression of the direct pulsar emission during the ALS.

No significant X-ray polarization was detected in the IXPE data. For the phase-averaged emission, the 99% minimum detectable polarization (MDP99) is approximately 14% in the 2-8 keV band. Using the most conservative model-independent analysis, the measured normalized Stokes parameters are q = 1.6 +/- 4.6% and u = 1.0 +/- 4.6%. A pulse-phase-resolved analysis using the ART-XC/NuSTAR period also revealed no significant polarization, with an MDP99 of approximately 40% per phase bin.

The simultaneous NuSTAR observation reveals a broadband spectrum with a pronounced reflection-dominated shape, well described by a Compton-reflection component of a cutoff power-law continuum, together with a strong iron line complex with an equivalent width of approximately 0.25 keV (pexrav + gaussian model in xspec). This spectral shape is characteristic of a scenario in which the direct continuum from the pulsar is strongly suppressed. The resulting observed 3-50 keV flux is 2.2x10^-10 erg/cm²/s, corresponding to an X-ray luminosity of 10³⁶ erg/s, assuming a distance of 6.1 kpc (Leahy & Abdallah 2014).

The unexpectedly low polarization constraint is noteworthy given that polarization of tens of percent can be expected for X-rays reflected from an optically thick surface at appropriate scattering geometries (Matt 1993, Poutanen et al. 1996), particularly for potentially obscured systems such as Cyg X-3 (Veledina et al. 2024). Several effects may reduce the observed polarization. For instance, the ALS spectrum may not consist of a single reflection component: residual emission from the accretion-disk corona or other extended regions can dilute the polarization of the reflected component.

The present IXPE observation therefore places the first direct X-ray polarimetric constraint on Her X-1 during an ALS. A more thorough analysis combining the IXPE polarization measurements with broadband spectral and timing information, including detailed modeling of the reflection geometry and reprocessing components, will be presented in a separate paper.