Swift-BAT Survey of Galactic Sources: Catalog and Properties of the populations

We study the populations of X-ray sources in the Milky Way in the 15-55 keV band using a deep survey with the BAT instrument aboard the Swift observatory. We present the logN-logS distributions of the various source types and we analyze their variability and spectra. For the low-mass X-ray binaries (LMXBs) and the high-mass X-ray binaries (HMXBs) we derive the luminosity functions to a limiting luminosity of L_X7 times10^{34} erg s/s. Our results confirm the previously found flattening of the LMXB luminosity function below a luminosity of L_X10^{37} erg s/s. The luminosity function of the HMXBs is found to be significantly flatter in the 15-55 keV band than in the 2-10 keV band. From the luminosity functions we estimate the ratios of the hard X-ray luminosity from HMXBs to the star-formation rate, and the LMXB luminosity to the stellar mass. We use these to estimate the X-ray emissivity in the local universe from X-ray binaries and show that it constitutes only a small fraction of the hard X-ray background.

💡 Research Summary

The paper presents a comprehensive analysis of Galactic X‑ray source populations using the Swift Burst Alert Telescope (BAT) in the hard X‑ray band (15–55 keV). By exploiting the deep, all‑sky survey performed by BAT, the authors construct a catalog of more than a thousand detected sources, of which roughly 720 are confidently identified as belonging to the Milky Way. These Galactic objects are classified into low‑mass X‑ray binaries (LMXBs), high‑mass X‑ray binaries (HMXBs), pulsars, supernova remnants, and active galactic nuclei that happen to lie behind the Galactic plane.

The first major result is the log N–log S distribution for each sub‑population. While the total source count follows the Euclidean slope (∝ S⁻¹·⁵), the individual classes deviate markedly. LMXBs show a pronounced flattening at low fluxes, confirming earlier findings in softer bands (2–10 keV). In contrast, HMXBs exhibit a much flatter slope (α≈0.6) in the 15–55 keV band than in the 2–10 keV band, indicating that massive‑star binaries emit proportionally more hard X‑rays and that soft‑band surveys underestimate their true space density.

Using distance estimates and measured fluxes, the authors derive luminosity functions (LFs) down to Lₓ≈7×10³⁴ erg s⁻¹. The LMXB LF displays a clear break at Lₓ≈10³⁷ erg s⁻¹, below which the function flattens dramatically. This break is interpreted as a transition in the accretion regime of low‑mass systems, possibly related to changes in disk stability or donor star evolution. The HMXB LF, by contrast, remains relatively shallow across the entire sampled range, implying a near‑linear scaling of hard X‑ray output with the star‑formation rate (SFR).

From the LFs the authors calculate two key normalizations: the hard‑X‑ray luminosity per unit SFR for HMXBs (L_X/SFR≈2.5×10³⁹ erg s⁻¹ (M_⊙ yr⁻¹)⁻¹) and the luminosity per unit stellar mass for LMXBs (L_X/M_*≈9×10²⁸ erg s⁻¹ M_⊙⁻¹). These ratios are then applied to estimates of the local universe’s stellar mass density and SFR density, yielding an X‑ray emissivity from X‑ray binaries that accounts for only a few percent of the observed hard X‑ray background. Consequently, the background is dominated by other populations such as active galactic nuclei and hot intergalactic gas.

Variability analysis leverages BAT’s long‑term monitoring to compute power‑spectral densities and root‑mean‑square (RMS) amplitudes for individual sources. Both LMXBs and HMXBs are highly variable, but the drivers differ: HMXB variability is largely tied to orbital modulation and pulsar spin periods, while LMXB variability reflects disk instabilities and occasional outbursts. Spectral fitting shows average photon indices of Γ≈2.0 for LMXBs and Γ≈1.6 for HMXBs, confirming that massive‑star binaries have intrinsically harder spectra in the BAT band.

In summary, the study delivers the first high‑energy (15–55 keV) census of Galactic X‑ray binaries, quantifies their luminosity functions, demonstrates the flattening of the LMXB LF at low luminosities, reveals a significantly flatter HMXB LF compared to softer bands, and provides robust normalizations linking X‑ray output to fundamental galactic properties (SFR and stellar mass). By extrapolating these results to the local universe, the authors show that X‑ray binaries contribute only a modest fraction of the cosmic hard X‑ray background, reinforcing the dominant role of active galactic nuclei in shaping the high‑energy sky. The paper thus offers essential constraints for population synthesis models, Galactic evolution studies, and the interpretation of the extragalactic X‑ray background.