Discovery of a large and bright bow shock nebula associated with low mass X-ray binary SAX J1712.6-3739

In a multiwavelength program dedicated to identifying optical counterparts of faint persistent X-ray sources in the Galactic Bulge, we find an accurate X-ray position of SAX J1712.6-3739 through Chandra observations, and discover its faint optical counterpart using our data from EFOSC2 on the ESO 3.6m telescope. We find this source to be a highly extincted neutron star LMXB with blue optical colours. We serendipitously discover a relatively bright and large bow shock shaped nebula in our deep narrowband H alpha imaging, most likely associated with the X-ray binary. A nebula like this has never been observed before in association with a LMXB, and as such provides a unique laboratory to study the energetics of accretion and jets. We put forward different models to explain the possible ways the LMXB may form this nebulosity, and outline how they can be confirmed observationally.

💡 Research Summary

In this paper the authors present a multi‑wavelength study of the faint, persistent low‑mass X‑ray binary (LMXB) SAX J1712.6‑3739, located in the Galactic bulge. Using a short (1.176 ks) Chandra HRC‑I observation they obtain a precise X‑ray position (RA = 258.15319°, Dec = ‑37.64473°, 90 % confidence radius ≈ 0.6″). The source is clearly detected with a count rate of 5.16 cts s⁻¹ and its position matches the previously known ROSAT and BeppoSAX error circles. The authors also identify the source in two XMM‑Newton slew observations (2006 Mar 8 and 2008 Feb 27) where it appears very bright (23–64 cts s⁻¹). Spectral fitting of the slew data with an absorbed power‑law (photon index fixed at Γ = 2.2) yields column densities N_H ≈ 1.9–2.4 × 10²² cm⁻², confirming the high extinction previously reported for this object.

Optical imaging was carried out with the ESO 3.6 m telescope using EFOSC2 in Bessel V, R, Gunn i and a narrow‑band Hα filter. After standard reduction, astrometric calibration against 2MASS (rms ≈ 0.13″) and PSF photometry, two candidate counterparts (named S1 and S2) are found within the X‑ray error circle. S1 lies only 1.0″ from the Chandra position, has V = 23.93 ± 0.26, R = 22.61 ± 0.12 and I = 21.45 ± 0.06, and appears intrinsically blue after correcting for the large line‑of‑sight extinction (A_V ≈ 7.3, derived from N_H). S2 is farther (2.1″), redder, and not detected in V. Assuming a distance of 6–8 kpc (based on type‑I X‑ray bursts) the absolute magnitude of S1 is M_V ≈ +2.4, consistent with a typical short‑period LMXB and somewhat brighter than expected for an ultra‑compact X‑ray binary (UCXB). The authors therefore favour S1 as the true optical counterpart, although spectroscopic confirmation is required.

The most striking result is the discovery of a large, bow‑shock‑shaped nebula in the Hα image. The nebula consists of two roughly parallel, linear Hα‑bright “trails” that appear to emanate from the X‑ray position, together with a diffuse, semi‑circular bright region located ≈30″ (≈3 × 10¹⁸ cm at 7 kpc) ahead of the source toward lower Galactic latitude. The trails widen from ≈9″ near the source to ≈33″ at 100″ distance, with peak surface brightness at ≈20–40″. No counterpart is seen in the broadband R image except for the central part, indicating that the emission is dominated by the Hα line.

The authors discuss two plausible mechanisms for the nebula’s origin, both assuming a physical link to the LMXB. (1) Jet‑powered nebula: a relativistic jet launched by the accreting neutron star interacts with the surrounding interstellar medium (ISM), producing a bow shock and compressing gas that radiates in Hα. The required jet power would be of order 10³⁶ erg s⁻¹, comparable to the observed X‑ray luminosity. (2) Bow‑shock from space motion: the binary may be moving supersonically through the ISM (perhaps after a natal kick), and the leading edge of the shock creates the semi‑circular bright region while the trailing wakes form the linear features. The inferred direction of motion (toward lower Galactic latitude) is consistent with a trajectory from the Galactic plane.

To discriminate between these scenarios, the authors propose several follow‑up observations: high‑resolution spectroscopy of the Hα line to measure velocity gradients and line widths (which would reveal shock speeds), deep radio imaging to search for synchrotron emission from a jet, and time‑resolved optical photometry or spectroscopy to confirm the binary period and nature (UCXB vs. normal LMXB). Additionally, proper motion measurements could directly test the space‑velocity hypothesis.

In summary, the paper reports the first known case of a large Hα nebula physically associated with a low‑mass X‑ray binary. The discovery provides a unique laboratory for studying how accretion‑driven outflows or high‑velocity binary motion feed energy back into the ISM. It opens a new avenue for probing jet–ISM interactions at modest X‑ray luminosities and for understanding the role of LMXBs in Galactic ecology. Future multi‑wavelength, high‑resolution observations will be essential to unravel the nebula’s origin and to exploit this system as a benchmark for feedback processes in compact binaries.