X-ray emission from star-forming galaxies - I. High-mass X-ray binaries

Abstract: Based on a homogeneous set of X-ray, infrared and ultraviolet observations from Chandra, Spitzer, GALEX and 2MASS archives, we study populations of high-mass X-ray binaries (HMXBs) in a sample of 29 nearby star-forming galaxies and their relation with the star formation rate (SFR). In agreement with previous results, we find that HMXBs are a good tracer of the recent star formation activity in the host galaxy and their collective luminosity and number scale with the SFR, in particular, Lx~2.6 10^{39} SFR. However, the scaling relations still bear a rather large dispersion of ~0.4 dex, which we believe is of a physical origin. We present the catalog of 1057 X-ray sources detected within the $D25$ ellipse for galaxies of our sample and construct the average X-ray luminosity function (XLF) of HMXBs with substantially improved statistical accuracy and better control of systematic effects than achieved in previous studies. The XLF follows a power law with slope of 1.6 in the logLx~35-40 luminosity range with a moderately significant evidence for a break or cut-off at Lx~10^{40} erg/s. As before, we did not find any features at the Eddington limit for a neutron star or a stellar mass black hole. We discuss implications of our results for the theory of binary evolution. In particular we estimate the fraction of compact objects that once upon their lifetime experienced an X-ray active phase powered by accretion from a high mass companion and obtain a rather large number, fx~0.2 (0.1 Myr/tau_x) (tau_x is the life time of the X-ray active phase). This is ~4 orders of magnitude more frequent than in LMXBs. We also derive constrains on the mass distribution of the secondary star in HMXBs.

• The paper demonstrates that HMXBs strongly correlate with star formation, with X-ray luminosity scaling as approximately 2.6 × 10^39 erg/s per unit SFR.
• The HMXB luminosity function follows a power law with a slope of 1.6 and shows a break near 10^40 erg/s, indicating a physical cutoff in binary properties.
• The study identifies a 0.4 dex dispersion in the scaling relation, suggesting intrinsic variations in binary evolution and galactic conditions.

Overview of the Paper: X-ray Emission from Star-forming Galaxies - High-mass X-ray Binaries

The paper presents an in-depth study analyzing high-mass X-ray binaries (HMXBs) as the key sources of X-ray emissions in star-forming galaxies. The authors, Mineo, Gilfanov, and Sunyaev, leverage data from several observatories, including Chandra, Spitzer, GALEX, and 2MASS, to explore the relationship between the populations of HMXBs and the star formation rate (SFR) in galaxies.

Key Findings

1. HMXBs as Star Formation Tracers: The study confirms the established notion that HMXBs are effective tracers of recent star formation activity. A strong correlation between the collective X-ray luminosity of HMXBs and the SFR of the host galaxies was found, characterized by the scaling relation $$L_\text{X} \approx 2.6 \times 10^{39} \times \text{SFR}$$ erg/s. This signifies that high-mass stars, which evolve into compact objects, are significant contributors to a galaxy's X-ray background, directly relating their presence to star formation activities.
2. Luminosity Function Analysis: The paper presents a detailed HMXB X-ray luminosity function (XLF) constructed from 1,057 X-ray sources detected within the $D25$ ellipse across 29 galaxies. The XLF follows a power law with a slope of 1.6 within the luminosity range of $\log(L_\text{X})\sim 35-40$. Notably, the study finds a significant break at $L_{\text{X}} \sim 10^{40} \, \text{erg}/\text{s}$, suggesting a potential cutoff or transition related to the physical characteristics of the binary systems.
3. Dispersion in Scaling Relation: While strong scaling relations are established, the study highlights a large dispersion ($r \sim 0.4$ dex) around these relations, indicating physical origins rather than observational uncertainties. Potential contributing factors to this dispersion could include variability in XLF normalization, differing star-formation histories, or intrinsic variance in galaxy properties such as metallicity.
4. Binary Evolution and Population Synthesis: The research presents important implications for the theory of binary evolution. Notably, it estimates a high fraction $(f_\text{X} \sim 0.2)$ of compact objects eventually undergo an X-ray-visible phase powered by accretion from a high-mass companion, with this occurrence being substantially more frequent than in low-mass X-ray binaries (LMXBs).

Theoretical and Practical Implications

This comprehensive study provides critical insights into HMXBs' role as indicators of star formation, reinforcing the necessity of understanding X-ray emissions in mapping stellar and galaxy evolution. Practically, the observations can enhance galaxy surveys and models by accounting for HMXBs in evaluating the star formation metrics of extragalactic systems.

It also elucidates constraints on binary evolution parameters, suggesting high specific frequencies of X-ray luminous phases among compact objects, with direct implications on X-ray luminosity functions used in galaxy formation models.

Future Directives

Further exploration should focus on reducing uncertainty in SFR determinations and additional, indirect methods of discerning contributions between neutron star and black hole binaries. Expanding the sample size and including more refined measurements of galaxy characteristics, especially metallicity, could mitigate current dispersions in XLF scaling relations.

The paper, by establishing robust observational insights, sets a foundational groundwork for future theoretical frameworks and practical studies in galactic and stellar astrophysics. This meticulous approach towards investigating HMXBs within star-forming galaxies significantly contributes to the broader comprehension of their evolutionary trajectories and interrelation with galactic compositions.