First detection of X-ray polarization in the symbiotic X-ray binary 4U 1954+319 with IXPE

ATel #17464; Alexander Salganik (U. of Turku), Lingda Kong (U. of Tuebingen), Sofia V. Forsblom (U. of Turku), Menglei Zhou (U. of Tuebingen), Honghui Liu (U. of Tuebingen), Sergey S. Tsygankov (U. of Turku), Andrea Santangelo (U. of Tuebingen), Juri Poutanen (U. of Turku)
on 31 Oct 2025; 15:14 UT
Credential Certification: Alexander Salganik (alsalganik@gmail.com)

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The symbiotic X-ray binary 4U 1954+319 hosts a slowly rotating pulsar with a spin period of about 5 h (ATel #797), one of the longest among known accreting neutron stars. The optical companion has been firmly identified as a red supergiant (RSG) at a distance of 3.3 kpc (Hinkle et al. 2020), making this system one of the very few RSG systems in the Galaxy.

The MAXI/GSC detected an X-ray brightening from 4U 1954+319 starting on 2025-08-25 (MJD 60912). The flux in the 4-10 keV band increased steadily, peaking at 99 mCrab on 2025-09-22 (MJD 60940; ATel #17422), and has since declined. The Imaging X-ray Polarimetry Explorer (IXPE) observed 4U 1954+319 during the late decay of the outburst, between 2025 Oct 16-20, with two simultaneous NuSTAR pointings on Oct 16 and Oct 18.

The NuSTAR spectrum from Oct 16 is well described by the XSPEC model tbabs(po*highecut + gaussian), an absorbed cutoff power law with a Gaussian Fe Kα line at 6.4 keV. The best-fit parameters are NH = (4.0 ± 0.5) × 10^22 cm^-2, Γ = 1.14 ± 0.04, Ecut = 6.0 ± 0.2 keV, and Efold = 17.1 ± 0.6 keV (errors at the 1σ confidence level). The corresponding unabsorbed fluxes are (4.2 ± 0.1) × 10^-10 erg cm^-2 s^-1 in the 2-8 keV band and (1.2 ± 0.1) × 10^-9 erg cm^-2 s^-1 in the 3-79 keV band, which corresponds to a luminosity of ~1.6 × 10^36 erg s^-1 in the 3-79 keV band for a distance of 3.3 kpc. The equivalent width (EW) of the Fe Kα line is 0.12 ± 0.02 keV.

The NuSTAR spectrum from Oct 18 is also well described by the same model, with best-fit parameters NH = (4.8 ± 0.4) × 10^22 cm^-2, Γ = 1.34 ± 0.03, Ecut = 5.8 ± 0.2 keV, and Efold = 20.3 ± 0.8 keV. The unabsorbed fluxes are (3.5 ± 0.1) × 10^-10 erg cm^-2 s^-1 in the 2-8 keV band and (8.8 ± 0.1) × 10^-10 erg cm^-2 s^-1 in the 3-79 keV band. The EW of the Fe Kα line is 0.05 ± 0.01 keV. IXPE revealed no significant polarization in the phase-averaged 2-8 keV data. A model-independent analysis performed with the ixpepolarization package (part of the HEASoft) yielded a polarization degree (PD) of 1.8 ± 1.3% and polarization angle (PA) = 80 ° ± 21 °, with the minimum detectable PD at 99% confidence level being MDP99 = 3.8%.

We then studied the pulse phase-resolved polarization properties. For that, the IXPE data were divided into eight equal phase bins based on the ephemeris parameters T0 = MJD(TDB) 60964.0 and Pspin = 5.33 h (obtained from the Swift/BAT light curve). At phases 0.250-0.375, coinciding with the maximum flux in the pulse profile, a significant polarization (>MDP99) in the 2-8 keV band was detected with PD = 12.8 ± 3.4% and PA = 61.7 ° ± 7.6 ° (MDP99 = 10.2%). The PD shows no significant dependence on energy. No statistically significant polarization was detected in the other phase intervals.

We thank the IXPE and NuSTAR teams for promptly scheduling and performing these observations.