[Paper Review] Discovery of a giant and luminous X-ray outburst from the optically inactive galaxy pair RXJ1242.6-1119

S. Komossa, J. Greiner|ArXiv.org|Aug 19, 1999

Astrophysical Phenomena and Observations7 citations

TL;DR

This paper reports the discovery of a giant, luminous X-ray outburst from the otherwise optically inactive galaxy pair RXJ1242.6-1119, exhibiting >20-fold variability over 1.5 years and an extremely soft X-ray spectrum (Γₓ ≈ -5). The source's high intrinsic luminosity (Lₓ > 9×10⁴³ erg/s at z=0.05) and lack of optical activity suggest a rare tidal disruption event or accretion onto a dormant supermassive black hole.

ABSTRACT

We report the discovery of large-amplitude X-ray variability from the direction of the previously unknown, optically inactive galaxy pair RXJ1242.6-1119. The X-ray source shows variability by a factor > 20 between the ROSAT all-sky survey and a later pointed observation separated by ~1.5 yr. Its spectrum is extremely soft with photon index Gamma_x ~ -5, among the steepest ever observed among galaxies. Based on the redshift derived from the optical spectra, z=0.05, the source's intrinsic luminosity is large, L_x > 9x10^{43} erg/s. Surprisingly, the optical spectra of both galaxies are characterized by absorption lines and do not show signs of (Seyfert) activity. This makes RXJ1242-11 the third candidate for giant-amplitude variability in an otherwise non-active galaxy, the first two being NGC 4552 (in the UV; Renzini et al. 1995) and NGC 5905 (in X-rays; Bade et al. 1996, Komossa & Bade 1999). Several mechanisms to explain this unexpected and peculiar behavior are investigated. The most likely one seems to be an accretion event onto an otherwise dormant supermassive black hole (SMBH), e.g., by a tidal disruption event.

Motivation & Objective

• To investigate the origin of a previously unknown, extremely luminous X-ray outburst detected from the direction of the optically inactive galaxy pair RXJ1242.6-1119.
• To determine whether the X-ray variability and spectral properties can be explained by known active galactic nucleus (AGN) phenomena or require a novel mechanism.
• To assess the nature of the central engine in the absence of optical signs of activity, such as emission lines or Seyfert characteristics.
• To evaluate the possibility that the outburst results from a transient event like a tidal disruption of a star by a supermassive black hole in a quiescent system.

Proposed method

• Utilized archival data from the ROSAT All-Sky Survey and a subsequent pointed X-ray observation to detect and characterize the X-ray variability.
• Measured the X-ray spectrum of the source, finding an extremely soft energy distribution with a photon index Γₓ ≈ -5, among the steepest ever observed in galaxies.
• Determined the redshift (z = 0.05) from optical spectroscopy of both galaxies to estimate the intrinsic X-ray luminosity.
• Compared the source's properties with known cases of giant-amplitude variability in non-active galaxies, such as NGC 4552 and NGC 5905.
• Evaluated competing physical models, including accretion events and tidal disruption, to explain the observed behavior.
• Used luminosity and spectral energy distribution analysis to constrain the nature of the X-ray emission mechanism.

Experimental results

Research questions

• RQ1 What causes the extreme X-ray variability (factor >20) in an otherwise optically inactive galaxy pair?
• RQ2 Why does the X-ray spectrum exhibit such an unusually soft shape (Γₓ ≈ -5) in a quiescent galactic system?
• RQ3 Can the high intrinsic X-ray luminosity (Lₓ > 9×10⁴³ erg/s) be explained by a dormant supermassive black hole undergoing a transient accretion event?
• RQ4 Why is there no optical evidence of active galactic nucleus activity, despite the extreme X-ray output?
• RQ5 Is this outburst consistent with known mechanisms such as tidal disruption of a star or other transient phenomena?

Key findings

• The X-ray source in RXJ1242.6-1119 exhibited a flux increase by a factor greater than 20 between the ROSAT All-Sky Survey and a follow-up pointed observation.
• The X-ray spectrum is extremely soft, with a photon index Γₓ ≈ -5, one of the steepest ever measured in a galactic X-ray source.
• The source's intrinsic X-ray luminosity exceeds 9×10⁴³ erg/s at a redshift of z = 0.05, indicating a highly energetic emission process.
• Optical spectra of both galaxies show only absorption lines, with no signs of emission-line activity or Seyfert-like features, confirming the system's optical quiescence.
• This is the third known case of giant-amplitude X-ray variability in a non-active galaxy, following NGC 4552 (UV) and NGC 5905 (X-ray).
• The most plausible explanation is a transient accretion event, such as a tidal disruption of a star by a dormant supermassive black hole.