AX J1749.1-2733 and AX J1749.2-2725 - the close pair of X-ray pulsars behind the Galactic Center: an optical identification

Two faint X-ray pulsars, AX J1749.2-2725 and AX J1749.1-2733, located in the direction to the Galactic Center, were studied in detail using data of INTEGRAL, XMM-Newton and Chandra observatories in X-rays, the SOFI/NTT instrument in infrared and the RTT150 telescope in optics. X-ray positions of both sources were determined with the uncertainty better than ~1 arcsec, that allowed us to identify their infrared counterparts. From the subsequent analysis of infrared and optical data we conclude that counterparts of both pulsars are likely massive stars of B0-B3 classes located behind the Galactic Center at distances of 12-20 kpc, depending on the type, probably in further parts of galactic spiral arms. In addition, we investigated the extinction law towards the galactic bulge and found that it is significantly different from standard one.

💡 Research Summary

This paper presents a comprehensive multi‑wavelength study of two faint, heavily absorbed X‑ray pulsars, AX J1749.2‑2725 and AX J1749.1‑2733, located in the direction of the Galactic Center. The authors combine archival and new observations from INTEGRAL, XMM‑Newton, and Chandra in the X‑ray band with near‑infrared (NIR) imaging from the SOFI instrument on the ESO NTT telescope and optical imaging from the RTT150 telescope.

First, the X‑ray positions of both sources are refined to sub‑arcsecond accuracy (≈0.7″ for AX J1749.2‑2725 and ≈0.4″ for AX J1749.1‑2733) using XMM‑Newton MOS2 data (∼6 ks exposure) and Chandra pipeline analysis. The systematic uncertainties are constrained to ≈2″, and the final error circles are ≤1″ in radius. This precise astrometry enables an unambiguous identification of NIR counterparts within the error regions.

For AX J1749.2‑2725, the X‑ray timing analysis reveals a coherent pulsation at 216.86 ± 0.14 s with a single‑peaked profile and a pulse fraction of ∼70 % in the 2–10 keV band. The spectrum is well described by an absorbed power‑law (photon index Γ = 1.41 +0.75/‑1.06) with a very high column density N_H = (1.41 +0.61/‑0.80) × 10^23 cm⁻², indicating strong intrinsic absorption beyond the line‑of‑sight Galactic value. The unabsorbed 2–10 keV flux is 2.6 × 10⁻¹² erg cm⁻² s⁻¹, about an order of magnitude lower than the ASCA measurement in 1995, but the spectral shape remains consistent.

AX J1749.1‑2733 shows a previously reported 132 s pulsation (Karasev et al. 2007) and was also detected in the same XMM‑Newton observation. Its NIR counterpart is distinct from the one proposed by Zurita Heras & Chaty (2008); the new candidate is consistently detected in all ∼400 SOFI exposures in the K_s band.

Photometric measurements of the two NIR counterparts are presented (Table 1). Both objects are detected in J, H, and K_s with magnitudes: AX J1749.2‑2725 (J > 20.5, H = 16.57 ± 0.07, K_s = 14.95 ± 0.05) and AX J1749.1‑2733 (J > 18.7, H = 17.43 ± 0.14, K_s = 15.18 ± 0.03). Neither source is detected in the optical i′ band (i′ > 20.5).

To assess the extinction toward the Galactic bulge, the authors construct H‑K_s color–magnitude diagrams (CMDs) for 1′ × 2′ fields around each pulsar. The red clump giants (RCG) are clearly identified, allowing the determination of the average extinction to the bulge: A_H = 2.1 ± 0.1 for the AX J1749.2‑2725 field and A_H = 1.42 ± 0.25 for the AX J1749.1‑2733 field. Importantly, the measured extinction law, A_H/E(H‑K) = 1.67 ± 0.12, is significantly flatter than the standard Cardelli et al. (1989) value of 2.75, confirming previous findings that the bulge exhibits a non‑standard reddening law. A similar deviation is found in the J‑K CMD (A_J/E(J‑K) = 1.29 ± 0.14 versus the canonical 1.69).

Using these extinction parameters, the authors estimate the intrinsic colors and absolute magnitudes of the candidate counterparts. Assuming the sources lie at the distance of the Galactic Center (8.4 kpc) and suffer the measured A_H, the observed H‑K ≈ 1.6 implies an intrinsic extinction A_H ≈ 2.7. Comparing the dereddened magnitudes with standard main‑sequence calibrations shows that only early‑type B stars (B0–B3) can reproduce the observed brightness at distances of 12–20 kpc, i.e., behind the Galactic Center. Later‑type stars would require unrealistically low extinction or would be placed in front of the bulge, contradicting the measured reddening.

Consequently, both pulsars are identified as high‑mass X‑ray binaries (HMXBs) with massive B‑type companions. The inferred distances are ≈16 kpc for AX J1749.2‑2725 and ≈18 kpc for AX J1749.1‑2733, placing them in the far side of the Milky Way’s spiral arms beyond the Galactic Center. The authors also estimate the X‑ray luminosities: L_X(2–10 keV) ≈ 7 × 10^34 erg s⁻¹ for AX J1749.2‑2725, consistent with a typical HMXB powered by wind accretion.

In summary, the paper demonstrates that precise X‑ray astrometry combined with deep NIR imaging can successfully identify heavily obscured HMXB counterparts even in the crowded, highly extincted region toward the Galactic Center. Additionally, the work provides an empirical measurement of the non‑standard extinction law in this direction, which is essential for future studies of Galactic bulge populations.