Multiwavelength Analysis of X-ray Binary Candidate 1eRASS J085039.9-421151 Suggests a Neutron Star Primary

Core Concepts

A multiwavelength analysis of 1eRASS J085039.9-421151, utilizing data from eROSITA, NuSTAR, and X-shooter, suggests the source is likely an X-ray binary system hosting a neutron star accreting from a late-type M2-3 supergiant companion.

Abstract

• Bibliographic Information: Zainab, A., Avakyan, A., Doroshenko, V., et al. Multiwavelength study of 1eRASS J085039.9-421151 with eROSITA, NuSTAR and X-shooter. Astronomy & Astrophysics manuscript no. main ©ESO 2024.

• Research Objective: This research paper aims to characterize the nature of the X-ray source 1eRASS J085039.9-421151 and its companion star, utilizing multiwavelength observations.

• Methodology: The authors analyzed X-ray data from eROSITA and NuSTAR, optical and near-infrared spectra from X-shooter, and archival data from Swift/BAT, Swift/XRT, INTEGRAL, and ART-XC. They investigated the X-ray timing and spectral properties of the source and characterized the spectral type and variability of the optical counterpart.

• Key Findings:

  • The X-ray spectrum of J085039 is consistent with an absorbed power law model, typical of accreting high-mass X-ray binaries.
  • The optical counterpart, UCAC2 13726137, exhibits emission lines and TiO absorption bands, leading to a reclassification as an M2-3 supergiant.
  • The presence of emission lines in the optical spectra, combined with the lack of significant X-ray variability, suggests a complex interplay between intrinsic stellar variability and photoionization from the compact object.
  • The low X-ray luminosity, spectral parameters, and the rarity of RSG-black hole systems point towards a neutron star as the compact object.

• Main Conclusions: The authors conclude that 1eRASS J085039.9-421151 is likely a neutron star accreting material from an M2-3 supergiant companion. The emission lines observed in the optical spectra are attributed to a combination of intrinsic stellar variability and photoionization from the X-ray source.

• Significance: This study contributes to the growing body of knowledge on X-ray binaries and provides valuable insights into the properties and behavior of these systems. The identification of a late-type supergiant companion is particularly interesting, as such systems are relatively rare.

• Limitations and Future Research: Further investigation is needed to fully disentangle the origin of the emission lines observed in the optical spectra. Longer X-ray observations are required to search for pulsations or other timing signatures that could confirm the nature of the compact object and constrain the binary parameters.

Stats

eROSITA detected the source four times, with exposures of a few hundred seconds each.
NuSTAR observed the source with an effective exposure time of 55.6 ks.
The Gaia parallax for the optical counterpart corresponds to a distance of 7.45+0.75−0.71 kpc.
The X-ray spectrum is well-described by an absorbed power law with a high energy cutoff.
A strong fluorescent neutral iron line with an equivalent width of ∼700 eV is detected.
The X-ray spectrum also exhibits an absorption edge, suggesting a partial covering absorber.
No significant pulsations were detected in the NuSTAR lightcurves.
The optical counterpart shows semi-periodic photometric variability with a periodicity of ∼39.24 days and an amplitude of about ±0.1 mag.

Quotes

Key Insights Distilled From

Multiwavelength study of 1eRASS J085039.9-421151 with eROSITA NuSTAR and X-shooter

by Aafia Zainab... at arxiv.org 11-06-2024

https://arxiv.org/pdf/2411.02655.pdf

Deeper Inquiries

How might the variability of the M2-3 supergiant companion affect the accretion processes and X-ray emission in this system?

The variability of the M2-3 supergiant companion, potentially exhibiting characteristics of Mira variables or semi-regular variables, can significantly influence the accretion processes and X-ray emission in the system through several mechanisms:
Variable Stellar Wind: M-type supergiants are known for their dense and clumpy stellar winds. The variability, potentially driven by pulsations, can lead to changes in the wind's density, velocity, and clumpiness. These fluctuations directly impact the amount of material available for accretion onto the compact object.
Enhanced Accretion during Wind Enhancement: Periods of increased wind strength can channel more material towards the compact object, leading to enhanced accretion. This could manifest as increased X-ray luminosity, potentially observable as flares or outbursts in the X-ray light curve.
Orbital Modulation of Accretion: If the orbital plane of the binary is inclined, the compact object's passage through different regions of the companion's variable wind can lead to periodic changes in the accretion rate. This could result in orbital modulation of the X-ray emission, with the X-ray flux varying systematically over the binary orbit.
Influence of Clumpiness: The clumpy nature of the wind can create variations in the density of the accreting material. As denser clumps accrete onto the compact object, they can release bursts of energy, leading to short-timescale X-ray variability.
Shielding and Absorption: Changes in the wind density and structure can also affect the absorption of X-rays from the compact object. Denser regions of the wind can scatter or absorb more X-rays, leading to variations in the observed X-ray flux and spectrum.
Further investigation into the correlation between the optical variability of the M2-3 supergiant and the X-ray emission properties of J085039 is crucial to disentangle the specific mechanisms at play. Long-term monitoring campaigns in both optical and X-ray bands would be invaluable in this regard.

Could the lack of observed pulsations be due to a very short or very long neutron star spin period, rather than the presence of a black hole?

Yes, the lack of observed pulsations in J085039 does not necessarily rule out the presence of a neutron star. Both very short and very long neutron star spin periods can pose challenges in detecting pulsations:
Very Short Spin Periods (Milliseconds): If the neutron star is spinning rapidly, with a period on the order of milliseconds, the time resolution of the observations might be insufficient to resolve individual pulses. The pulsations would be smeared out in time, appearing as a constant or nearly constant emission.
Very Long Spin Periods (Hours to Days): Conversely, if the neutron star has a very long spin period, spanning hours to days, the observation time might not be long enough to cover a significant fraction of a single rotation. In such cases, the pulsations would be too slow to be detected within the limited observation window.
Other Factors:

Large Absorption: As mentioned in the abstract, significant absorption by a partial covering component can also mask pulsations. The absorber can scatter and absorb X-rays from different parts of the neutron star's surface differently, smearing out the pulsations.
Geometric Effects: The orientation of the neutron star's magnetic axis and rotation axis with respect to our line of sight can also affect the detectability of pulsations. If the magnetic axis is aligned close to the rotation axis, the pulsations might be intrinsically weak.
Therefore, while the absence of pulsations in the current data does not provide conclusive evidence for the nature of the compact object, it does not exclude the possibility of a neutron star with a very short or very long spin period. Deeper and longer X-ray observations with high time resolution are needed to search for rapid pulsations, while continuous monitoring over extended periods is required to probe for long-period pulsations.

What are the implications of this discovery for our understanding of binary evolution and the formation of X-ray binaries with late-type supergiant companions?

The discovery of J085039, a potential X-ray binary with a late-type M2-3 supergiant companion, presents intriguing implications for our understanding of binary evolution and the formation of such rare systems:
Constraints on Binary Evolution Models: The presence of an M2-3 supergiant as the mass donor in an X-ray binary is uncommon. Most known HMXBs host earlier-type (O/B) supergiants. This discovery suggests that binary evolution pathways can lead to the formation of X-ray binaries with late-type supergiant companions, challenging existing models and demanding further refinement.
Formation Mechanisms: The formation of such systems likely involves significant mass transfer from the progenitor of the M2-3 supergiant to the compact object. Understanding the details of this mass transfer process, including its duration, efficiency, and impact on the binary orbit, is crucial to unraveling the evolutionary history of J085039.
Rarity and Implications: The rarity of X-ray binaries with late-type supergiant companions suggests that their formation might require specific and relatively uncommon conditions during the evolution of the binary system. Studying J085039 in detail can provide valuable insights into these conditions and shed light on the broader population of such systems.
Supergiant Winds and Accretion: The properties of the M2-3 supergiant's wind, particularly its density, clumpiness, and variability, directly influence the accretion processes and X-ray emission from the compact object. J085039 offers a unique laboratory to study the interaction of compact objects with the winds of late-type supergiants.
Future Studies: Further investigations of J085039, including:

Precise determination of the compact object's nature (neutron star or black hole)
Detailed characterization of the orbital parameters
Long-term monitoring of the X-ray and optical variability
will be essential to refine our understanding of binary evolution, the formation of X-ray binaries with late-type supergiant companions, and the physics of accretion in these extreme environments.