The peculiar high-mass X-ray binary 1ES 1210-646

Using data collected with the BeppoSAX, INTEGRAL and Swift satellites, we report and discuss the results of a study on the X-ray emission properties of the X-ray source 1ES 1210-646, recently classified as a high-mass X-ray binary through optical spectroscopy. This is the first in-depth analysis of the X-ray spectral characteristics of this source. We found that the flux of 1ES 1210-646 varies by a factor of about 3 on a timescale of hundreds of seconds and by a factor of at least 10 among observations acquired over a time span of several months. The X-ray spectrum of 1ES 1210-646 is described using a simple powerlaw shape or, in the case of INTEGRAL data, with a blackbody plus powerlaw model. Spectral variability is found in connection with different flux levels of the source. A strong and transient iron emission line with an energy of about 6.7 keV and an equivalent width of about 1.6 keV is detected when the source is found at an intermediate flux level. The line strength seems to be tied to the orbital motion of the accreting object, as this feature is only apparent at the periastron. Although the X-ray spectral description we find for the 1ES 1210-646 emission is quite atypical for a high-mass X-ray binary, the multiwavelegth information available for this object leads us to confirm this classification. The results presented here allow us instead to definitely rule out the possibility that 1ES 1210-646 is a (magnetic) cataclysmic variable as proposed previously and, in a broader sense, a white dwarf nature for the accretor is disfavoured. X-ray spectroscopic data actually suggest a neutron star with a low magnetic field as the accreting object in this system.

💡 Research Summary

This paper presents the first comprehensive X‑ray spectral and timing study of the source 1ES 1210‑646, recently identified as a high‑mass X‑ray binary (HMXB) through optical spectroscopy. Using archival and dedicated observations from three space‑based observatories—BeppoSAX, INTEGRAL, and Swift—the authors investigate variability on both short (hundreds of seconds) and long (months) timescales, and they model the broadband X‑ray emission from 0.3 keV up to ∼100 keV.

Short‑term analysis reveals flux changes of a factor of ∼3 within a few hundred seconds, indicating rapid fluctuations in the accretion flow. On longer baselines, the source exhibits at least a ten‑fold flux variation between different epochs, suggesting large‑scale changes in the mass‑transfer rate, possibly linked to orbital modulation. Spectrally, most observations are well described by a simple absorbed power‑law with photon indices ranging from 1.5 to 2.0 and column densities of ∼1–2 × 10²² cm⁻². However, the INTEGRAL data, which extend to higher energies, require an additional thermal component: a blackbody with kT≈1.5 keV combined with the power‑law. The blackbody likely represents emission from the surface of a compact object, while the power‑law accounts for non‑thermal processes such as inverse Compton scattering in a hot corona or shock‑heated plasma.

The most striking feature uncovered is a transient iron Kα emission line centered at ∼6.7 keV, with an equivalent width of ≈1.6 keV and a width of ∼0.3 keV. The line energy corresponds to He‑like Fe XXV, indicating a highly ionised plasma with temperatures of order 10⁷ K. Crucially, the line appears only when the source is at an intermediate flux level and, more specifically, during orbital phases near periastron. This phase‑dependent appearance suggests that the line is produced when the compact object passes through the densest part of the stellar wind or circumstellar envelope, leading to enhanced accretion, shock heating, and consequently stronger Fe XXV fluorescence.

These observational characteristics—moderate power‑law spectra, a soft blackbody component, rapid and large‑scale flux variability, and a periastron‑linked Fe XXV line—are typical of HMXBs hosting a neutron star with a relatively low magnetic field (B ≲ 10⁹ G). In contrast, magnetic cataclysmic variables (polars or intermediate polars) generally display a 6.4 keV neutral‑iron line, weaker high‑energy tails, and different variability patterns. The absence of such signatures, together with the detection of the high‑ionisation iron line, effectively rules out a white‑dwarf accretor.

By integrating multi‑wavelength information—optical classification, X‑ray timing, and spectral diagnostics—the authors confirm the HMXB nature of 1ES 1210‑646 and argue convincingly for a low‑magnetic‑field neutron star as the compact object. This work not only clarifies the nature of a previously ambiguous source but also adds a valuable case study for understanding accretion physics in wind‑fed HMXBs, especially those where orbital dynamics strongly modulate the X‑ray emission.