No periodicity revealed for an "eclipsing" ultraluminous supersoft X-ray source in M81

Abstract

Luminous supersoft X-ray sources found in the Milky Way and Magellanic Clouds are likely white dwarfs that steadily or cyclically burn accreted matter on their surface, which are promising type Ia supernova progenitors. Observations of distant galaxies with Chandra and XMM-Newton have revealed supersoft sources that are generally hotter and more luminous, including some ultraluminous supersoft sources (ULSs) that are possibly intermediate mass black holes of a few thousand solar masses. In this paper we report our X-ray spectral and timing analysis for M81-ULS1, an ultraluminous supersoft source in the nearby spiral galaxy M81. M81-ULS1 has been persistently supersoft in 17 Chandra ACIS observations spanning six years, and its spectrum can be described by either a $k{T}_{bb}\approx 70$ eV blackbody for a $\sim 1.2M_{\odot }$ white dwarf, or a $k{T}_{in}\approx 80$ eV multicolor accretion disk for a $\gtrsim 10^3{M}_{\odot }$ intermediate mass black hole. In two observations, the light curves exhibited dramatic flux drop/rise on time scales of $10^3$ seconds, reminiscent of eclipse ingress/egress in eclipsing X-ray binaries. However, the exhaustive search for periodicity in the reasonable range of 50 ksec to 50 days failed to reveal an orbital period. The failure to reveal any periodicity is consistent with the long period ($\ge 30$ yrs) predicted for this system given the optical identification of the secondary with an asymptotic giant star. Also, the eclipse-like dramatic flux changes in hours are hard to explain under the white dwarf model, but can in principle be explained by disk temperature changes induced by accretion rate variations under the intermediate mass black hole model.

19 pages, 7 figures, 1 table, to appear in ApJS

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• Identification of the X-Ray Thermal Dominant State in an Ultraluminous X-Ray Source in M82
• Nature of the soft ULX in NGC 247: super-Eddington outflow and transition between the supersoft and soft ultraluminous regimes
• Revisiting the ultraluminous supersoft source in M101: an optically thick outflow model
• Relativistic baryonic jets from an ultraluminous supersoft X-ray source
• Discovery of a recurrent spectral evolutionary cycle in the ultra-luminous X-ray sources Holmberg II X-1 and NGC 5204 X-1
• XMM-Newton Observations of NGC 247: X-ray Population and a Supersoft Ultraluminous X-ray Source
• Chandra and HST Observations of the Supersoft ULX in NGC 247: Candidate for Standard Disk Emission
• Discovery of two eclipsing X-ray binaries in M51
• Broadband study and the discovery of pulsations from the Be/X-ray binary eRASSU J052914.9-662446 in the Large Magellanic Cloud
• Spectroscopic Studies of an Ultraluminous Supersoft X-Ray Source in M81
• New outburst from the luminous supersoft source SSS1 in NGC 300 with periodic modulation