[Paper Review] Peculiar X-ray transient SRGA J043520.9+552226/AT2019wey discovered with SRG/ART-XC

I. A. Mereminskiy, А. В. Додин+25 more|arXiv (Cornell University)|Jul 12, 2021

Astrophysical Phenomena and Observations57 references13 citations

TL;DR

This paper identifies SRGA J043520.9+552226, a newly discovered X-ray transient detected by the SRG/ART-XC telescope, as a black hole in a low-mass X-ray binary (LMXB) system. Multiwavelength follow-up using X-ray (NuSTAR, INTEGRAL, NICER), optical (2.5-m and RC600 telescopes), and radio observations reveals a prolonged hard-state outburst with low-hard state characteristics, including low-frequency quasi-periodic oscillations (LF QPOs), unusual LX–Lopt correlation, and X-ray irradiated accretion disk features, marking the first such source with a bright optical counterpart for detailed study.

ABSTRACT

Context: During the ongoing all-sky survey, the Mikhail Pavlinsky ART-XC telescope on board the SRG observatory should discover new X-ray sources, many of which can be transient. Here we report on the discovery and multiwavelength follow-up of a peculiar X-ray source SRGA J043520.9+552226=SRGe J043523.3+552234 - the high-energy counterpart of the optical transient AT2019wey. Aims: Thanks to its sensitivity and the survey strategy, the Mikhail Pavlinsky ART-XC telescope uncovers poorly studied weak transient populations. Using a synergy with current public optical surveys, we are aiming at revealing the nature of these transients to study its parent populations. The SRGA J043520.9+552226 is the first transient detected by ART-XC which has a bright optical counterpart suitable for further studies. Methods: We have used available public X-ray and optical data and observations with SRG, INTEGRAL, NuSTAR, NICER and ground-based telescopes to investigate the source spectral energy distributions at different phases of the outburst. Results: Based on X-ray spectral and timing properties derived from space observations, optical spectroscopy and photometry obtained with the 2.5-m and RC600 CMO SAI MSU telescopes, we propose the source to be a black hole in a low-mass close X-ray binary system.

Motivation & Objective

• To identify the nature of SRGA J043520.9+552226, a newly discovered X-ray transient with a bright optical counterpart, using multiwavelength follow-up.
• To investigate the X-ray spectral and timing properties of the transient during its outburst phase.
• To determine the system's distance, binary parameters, and accretion physics through optical spectroscopy and broadband SED modeling.
• To explore the role of X-ray irradiation in shaping the accretion disk spectrum and optical emission.

Proposed method

• Utilized near-real-time X-ray detection from the SRG/ART-XC telescope and eROSITA for source localization and initial characterization.
• Conducted follow-up optical spectroscopy and photometry using the 2.5-m and RC600 CMO SAI MSU telescopes to identify absorption lines and constrain the system's distance and binary parameters.
• Analyzed X-ray data from NuSTAR, INTEGRAL, and NICER to model the broadband X-ray spectrum, including hard X-ray emission and strong reflection features.
• Performed broadband spectral energy distribution (SED) modeling from NIR to UV, fitting the continuum with power laws (fν ∼να) and accounting for interstellar extinction (EB−V = 0.8–1.2).
• Identified LF QPOs in the X-ray light curve, indicating the source was in a low-hard state typical of black hole X-ray binaries.
• Modeled the accretion disk as an X-ray-irradiated, modified black-body with electron scattering and viscosity effects to explain the shallow absorption lines and flat continuum.

Experimental results

Research questions

• RQ1 What is the nature of the X-ray transient SRGA J043520.9+552226, and how does it relate to known X-ray binary systems?
• RQ2 Why does the source exhibit a prolonged X-ray outburst with a slow rise time, unlike typical black hole LMXBs?
• RQ3 How does the broadband SED from NIR to UV evolve over time, and what does it reveal about the accretion disk and irradiation effects?
• RQ4 What is the origin of the shallow Balmer absorption lines and the Bowen blend in the optical spectrum, and can they be used to measure orbital parameters?
• RQ5 To what extent does X-ray irradiation modify the accretion disk spectrum, and how does this affect the observed fν ∼να power-law index?

Key findings

• SRGA J043520.9+552226 is identified as a black hole in a low-mass X-ray binary (LMXB), with a likely distance between 1 and 10 kpc and a secondary star mass ≤0.8 M☉.
• The source exhibited a prolonged X-ray outburst with a slow rise time, characteristic of a 'hard-only' outburst, and displayed low-frequency quasi-periodic oscillations (LF QPOs), confirming its classification as a black hole LMXB in the low-hard state.
• The optical spectrum shows Balmer lines in absorption and a Bowen blend, indicating irradiation of the companion star’s atmosphere, which enables future orbital parameter determination.
• The broadband SED from NIR to UV is well-fit by a power law fν ∼να with α ≈0.7–1.7, depending on interstellar extinction (EB−V = 0.8–1.2), consistent with X-ray irradiated accretion disk models.
• The shallow absorption lines and flat continuum suggest significant X-ray irradiation of the outer disk, which may explain the inverted or shallow lines and the observed spectral flattening.
• The source remains in outburst over a year after discovery, with evolving high-energy emission, indicating sustained accretion and making it a prime target for long-term multiwavelength monitoring.