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The First X-Ray Polarimetry of an Eclipsing Low-Mass X-Ray Binary: Serendipitous IXPE Observation of AX J1745.6-2901

Romana Mikušincová, Lorenzo Marra, Hemanth Manikantan, Stefano Bianchi, Fiamma Capitanio, Sudip Chakraborty, Raul Ciancarella, Enrico Costa, Nicolas De Angelis, Melania Del Santo, Sergio Fabiani, Riccardo Ferrazzoli, Vittoria E. Gianolli, Andrea Gnarini, Adam Ingram, Shifra Mandel, Guglielmo Mastroserio, Giorgio Matt, Kaya Mori, Fabio Muleri, Simone Pagliarella, Maxime Parra, P. O. Petrucci, Jakub Podgorný, Juri Poutanen, Swati Ravi, Paolo Soffitta, James F. Steiner, Antonella Tarana, Roberto Taverna, Francesco Ursini, Alexandra Veledina, Federico M. Vincentelli, Anastasiya Yilmaz, Barbara De Marco, Maitrayee Gupta, Vladislav Loktev, Thomas D. Russell, Jiří Svoboda, Francesco Tombesi, Shuo Zhang

TL;DR

AX J1745.6-2901, a high-inclination NS-LMXB near the Galactic Center, is targeted here with IXPE to probe geometry through X-ray polarization. The phase-averaged polarization is $PD = 14.7\% \pm 4.0\%$ with $PA = 122\circ \pm 8\circ$, rising to $PD = 34.2\% \pm 8.7\%$ during the eclipse/dip phase, consistent with scattering in disk winds. Spectral fits using diskbb and power-law components are both acceptable, implying a soft state but leaving the dominant emission mechanism ambiguous within the 2–8 keV IXPE band. The results highlight IXPE's ability to constrain scattering geometries in high-inclination NS-LMXBs and underscore the importance of careful background treatment in crowded regions for polarization studies.

Abstract

We present the first X-ray polarimetric measurement of the neutron star low-mass X-ray binary system AX J1745.6-2901 conducted by the Imaging X-ray Polarimetry Explorer (IXPE) satellite. This transient source, located within $ \sim $1.5' of the Galactic center, was observed serendipitously during a MAXI J1744-294 observation with a duration of 150 ks. The complex nature of the region in which AX J1745.6-2901 is located poses a challenge for studying its polarization. By performing a detailed analysis of the contamination from MAXI J1744-294 and the Galactic center diffuse emission, we find the source polarization degree PD = 14.7$\% \pm$ 4.0$\%$ and polarization angle PA = 122$^\circ \pm 8^\circ$. The phase-resolved analysis shows increase in polarization during the eclipse phase, with PD = 34.2$\% \pm$ 8.7$\%$, suggesting that the polarization-inducing mechanisms are of scattering nature, probably originating from disk winds.

The First X-Ray Polarimetry of an Eclipsing Low-Mass X-Ray Binary: Serendipitous IXPE Observation of AX J1745.6-2901

TL;DR

AX J1745.6-2901, a high-inclination NS-LMXB near the Galactic Center, is targeted here with IXPE to probe geometry through X-ray polarization. The phase-averaged polarization is with , rising to during the eclipse/dip phase, consistent with scattering in disk winds. Spectral fits using diskbb and power-law components are both acceptable, implying a soft state but leaving the dominant emission mechanism ambiguous within the 2–8 keV IXPE band. The results highlight IXPE's ability to constrain scattering geometries in high-inclination NS-LMXBs and underscore the importance of careful background treatment in crowded regions for polarization studies.

Abstract

We present the first X-ray polarimetric measurement of the neutron star low-mass X-ray binary system AX J1745.6-2901 conducted by the Imaging X-ray Polarimetry Explorer (IXPE) satellite. This transient source, located within 1.5' of the Galactic center, was observed serendipitously during a MAXI J1744-294 observation with a duration of 150 ks. The complex nature of the region in which AX J1745.6-2901 is located poses a challenge for studying its polarization. By performing a detailed analysis of the contamination from MAXI J1744-294 and the Galactic center diffuse emission, we find the source polarization degree PD = 14.7 4.0 and polarization angle PA = 122. The phase-resolved analysis shows increase in polarization during the eclipse phase, with PD = 34.2 8.7, suggesting that the polarization-inducing mechanisms are of scattering nature, probably originating from disk winds.

Paper Structure

This paper contains 9 sections, 10 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Observation and Data reduction
  3. Results
  4. Source and background polarization
  5. Time-dependent analysis
  6. Phase-resolved polarimetry
  7. Spectral properties
  8. Discussion
  9. Conclusions

Figures (10)

  • Figure 1: IXPE image of MAXI J1744$-$294 (bright source in the center of the FOV) and AX J1745.6$-$2901 (SRC) in gray. The red star marks the position of Sgr A$^{*}$. The selected circular background regions are numbered 1 to 7. The dark green semicircular annulus represents the 8th region, chosen to be diametrically opposite of the source region, as highlighted by the white guidelines. The two guidelines intersect the center of MAXI J1744$-$294, defined as the pixel with the highest count rate.
  • Figure 2: Normalized Stokes parameters $U/I$ vs. $Q/I$ of the source with different background subtractions. The gray point labeled as "BKG none" refers to the polarization of the source region without background subtraction. The values of PD and PA from the combined data of the three DUs, for each background can be read using the gray grid. The points (plotted with 1 $\sigma$ errors) are clustered within an angular wedge of size $\approx 10\degr$, regardless of the background region subtracted. The clustering is represented by the cyan wedge. The gray grid represents the PD (in %) and PA (in degrees).
  • Figure 3: Normalized Stokes parameters $U/I$ vs. $Q/I$ of different background regions. The values of PD and PA of the sum of the three DUs, for each background, can be read using the gray grid. The only region with a PD above MDP is region "2" (in red), which is probably polarized. All other regions are well below the MDP and have scattered PA. The gray grid represents the PD (in %) and PA (in degrees). The red circle represents the MDP value for background region "2".
  • Figure 4: First and second sub-panels (respectively): normalized Stokes parameters $Q/I$ and $U/I$ as a function of time. The data are binned in intervals of 7.5 ks and are energy-averaged over the 2--8 keV band. The dark gray dashed line represents the constant fit, its 1$\sigma$ error is represented by the shaded area. Third sub-panel from top: time evolution of count rate in the 2--8 keV band. Gray lines represent expected eclipses. Fourth sub-panel: time evolution of hardness ratio defined as a ratio between 4--8 keV and 2--4 keV count rates. The count rate and hardness ratio show data binned in intervals of 300 s. Each data point in all subplots represents the sum of the 3 DUs.
  • Figure 5: Phase versus normalized intensity. The plot is divided into three parts separated by red dash-dotted lines: (i) before the dip + eclipse (phase 0--0.38), (ii) dip + eclipse (phase 0.38--0.70), (iii) after the dip + eclipse (phase 0.70--1.).
  • ...and 5 more figures