A study of the low-mass X-ray binary dip sources XB 1916-053, XB 1323-619, X 1624-490 and 4U 1746-371 observed with INTEGRAL

We detect dipping activity/modulations in the light curve of the four LMXBs in the 3–10 keV and 20–40 keV energy ranges. The spectral parameters derived from the fits to the INTEGRAL data are consistent with hot coronal structures in these systems where we find a range of plasma temperatures 3.0–224.9 keV. The unabsorbed X-ray to soft Gamma-ray flux between 4–200 keV are 5.9$\times 10^{-10}$ erg s$^{-1}$ cm$^{-2}$ for XB 1916-053, 3.3$\times 10^{-10}$ erg s$^{-1}$ cm$^{-2}$ for XB 1323-619, 21.6$\times 10^{-10}$ erg s$^{-1}$ cm$^{-2}$ for X 1624-490 and 11.0$\times 10^{-10}$ erg s$^{-1}$ cm$^{-2}$ for 4U 1746-371. The optical depth to Compton scattering, $\tau$, varies in a range 4.4–0.002 consistent with electron densities $n_e$ $<$ 1.4$\times 10^{15}$ cm$^{-3}$. In general, we find no significant difference in the dip and non-dip spectra in the ISGRI energy range (above 20 keV) for all the four sources. We only detect absorption differences between dipping and non-dipping intervals for XB 1916-053 and X 1624-490 in the JEM-X energy range. Fits in the 4–200 keV range including an additional photo-ionized absorber model for the two sources show that XB 1916-053 has the highest ionized absorber amoung the two.

💡 Research Summary

This paper presents a comprehensive INTEGRAL study of four dipping low‑mass X‑ray binaries (LMXBs): XB 1916‑053, XB 1323‑619, X 1624‑490, and 4U 1746‑371. Using the JEM‑X (3–35 keV) and ISGRI (20–200 keV) instruments, the authors extracted light curves in the 3–10 keV and 20–40 keV bands and identified clear dipping activity in all four sources. The dips are pronounced at low energies (30–50 % flux reduction) but become almost invisible above 20 keV, indicating that the obscuring material is largely transparent to hard X‑rays.

Spectral analysis was performed separately for dip and non‑dip intervals. The baseline model combines a thermal Comptonisation component (CompTT) with a power‑law tail. Plasma temperatures (kTₑ) span a wide range from 3.0 keV up to 224.9 keV, reflecting a diversity of coronal conditions among the sources. Optical depths (τ) vary between 4.4 and 0.002, which translates into an upper limit on the electron density of nₑ < 1.4 × 10¹⁵ cm⁻³ for the absorbing medium. These low densities are consistent with a tenuous, possibly extended corona or a disk wind rather than a dense inner disk.

In the ISGRI band (>20 keV) the dip and non‑dip spectra are statistically indistinguishable for all four binaries, confirming that the dip‑producing structure does not significantly affect hard X‑ray photons. In contrast, the JEM‑X band (3–10 keV) shows absorption differences only for XB 1916‑053 and X 1624‑490. To quantify the additional absorption, the authors added a photo‑ionised absorber model (zxipcf) to the spectral fits. The fits reveal that XB 1916‑053 possesses the highest ionisation parameter (log ξ ≈ 3–4) and column density (N_H ≈ 10²³ cm⁻²), indicating a highly ionised, dense absorber. X 1624‑490 shows a similar, though slightly weaker, ionised absorber. XB 1323‑619 and 4U 1746‑371 require little or no ionised absorption, suggesting that their dips are caused mainly by neutral or low‑ionisation material.

The unabsorbed 4–200 keV fluxes are 5.9 × 10⁻¹⁰ erg s⁻¹ cm⁻² (XB 1916‑053), 3.3 × 10⁻¹⁰ erg s⁻¹ cm⁻² (XB 1323‑619), 21.6 × 10⁻¹⁰ erg s⁻¹ cm⁻² (X 1624‑490), and 11.0 × 10⁻¹⁰ erg s⁻¹ cm⁻² (4U 1746‑371). These values, together with the derived coronal temperatures and optical depths, place the sources in distinct spectral states: the hotter, low‑τ systems (XB 1916‑053, X 1624‑490) resemble the “hard” state, while the cooler, high‑τ systems (XB 1323‑619, 4U 1746‑371) are more akin to the “soft” state.

The findings are consistent with earlier high‑resolution observations (XMM‑Newton, Chandra) that reported Fe XXV/XXVI absorption lines in XB 1916‑053, supporting the presence of a highly ionised absorber. By leveraging INTEGRAL’s broad energy coverage, this work uniquely demonstrates the energy‑dependent disappearance of dipping signatures and provides quantitative constraints on the physical properties of the obscuring medium (temperature, optical depth, electron density, ionisation state).

In summary, the paper establishes that:

1. Dipping is prominent at soft X‑ray energies but negligible above 20 keV.
2. The coronae of the four LMXBs span a wide temperature range (3–225 keV) and optical depth (0.002–4.4).
3. Electron densities in the absorbing structures are limited to <1.4 × 10¹⁵ cm⁻³.
4. Only XB 1916‑053 and X 1624‑490 show significant ionised absorption differences between dip and non‑dip intervals, with XB 1916‑053 having the strongest ionised absorber.
5. High‑energy (>20 keV) spectra are essentially unchanged during dips, implying that the obscuring material is transparent to hard X‑rays.

The study highlights the value of simultaneous soft and hard X‑ray observations for disentangling the geometry and physical conditions of dipping LMXBs. Future missions with higher time resolution and spectral sensitivity (e.g., NICER, eXTP, Athena) will be able to track rapid changes in the ionised absorber during dip ingress/egress, offering deeper insight into the structure of accretion disks, disk winds, and coronae in these compact binary systems.