First Detection of Mid-Infrared Variability from an Ultraluminous X-Ray Source Holmberg II X-1

Abstract

We present mid-infrared (IR) light curves of the Ultraluminous X-ray Source (ULX) Holmberg II X-1 from observations taken between 2014 January 13 and 2017 January 5 with the \textit{Spitzer Space Telescope} at 3.6 and 4.5 $\mu$m in the \textit{Spitzer} Infrared Intensive Transients Survey (SPIRITS). The mid-IR light curves, which reveal the first detection of mid-IR variability from a ULX, is determined to arise primarily from dust emission rather than from a jet or an accretion disk outflow. We derived the evolution of the dust temperature ($T_{\mathrm{d}}\sim 600-800$ K), IR luminosity ($L_{\mathrm{IR}}\sim 3\times 10^4$ ${\mathrm{L}}_{\odot }$), mass ($M_{\mathrm{d}}\sim 1-3\times 10^{-6}$ ${\mathrm{M}}_{\odot }$), and equilibrium temperature radius ($R_{\mathrm{eq}}\sim 10-20$ AU). A comparison of X-1 with a sample spectroscopically identified massive stars in the Large Magellanic Cloud on a mid-IR color-magnitude diagram suggests that the mass donor in X-1 is a supergiant (sg) B[e]-star. The sgB[e]-interpretation is consistent with the derived dust properties and the presence of the [Fe II] ($\lambda =1.644$ $\mu$m) emission line revealed from previous near-IR studies of X-1. We attribute the mid-IR variability of X-1 to increased heating of dust located in a circumbinary torus. It is unclear what physical processes are responsible for the increased dust heating; however, it does not appear to be associated with the X-ray flux from the ULX given the constant X-ray luminosities provided by serendipitous, near-contemporaneous X-ray observations around the first mid-IR variability event in 2014. Our results highlight the importance of mid-IR observations of luminous X-ray sources traditionally studied at X-ray and radio wavelengths.

9 page, 4 figures, 1 table, Accepted to ApJ Letters

References in corpus (14)

• Infrared Emission from Interstellar Dust. IV. The Silicate-Graphite-PAH Model in the Post-Spitzer Era
• An Ultraluminous X-ray Source Powered by An Accreting Neutron Star
• An accreting pulsar with extreme properties drives an ultraluminous x-ray source in NGC 5907
• Discovery of coherent pulsations from the Ultraluminous X-ray Source NGC 7793 P13
• Spitzer SAGE Infrared Photometry of Massive Stars in the Large Magellanic Cloud
• Multi-wavelength observations of Galactic hard X-ray sources discovered by INTEGRAL. II. The environment of the companion star
• NuSTAR, XMM-Newton and Suzaku Observations of the Ultraluminous X-ray Source Holmberg II X-1
• Near-infrared counterparts of ultraluminous X-ray sources
• Luminous and Variable Stars in M31 and M33. IV. Luminous Blue Variables, Candidate LBVs, and the B[e] Supergiants; How to Tell Them Apart
• X-ray spectral state is not correlated with luminosity in Holmberg II X-1
• Dusty Blue Supergiants: News from High-Angular Resolution Observations
• The Rich Mid-Infrared Environments of Two Highly-Obscured X-ray Binaries: Spitzer Observations of IGR J16318-4848 and GX 301-2
• Herschel observations of dust around the high-mass X-ray binary GX 301-2
• Spitzer IRAC observations of IR excess in Holmberg IX X-1: A circumbinary disk or a variable jet?

Cited by in corpus (9)

• On rapid binary mass transfer -- I. Physical model
• Uncovering Red and Dusty Ultraluminous X-ray Sources with Spitzer
• Outbursts of the intermediate-mass black hole HLX-1: a wind instability scenario
• CI Camelopardalis: The first sgB[e]-High Mass X-ray Binary Twenty Years on, a Supernova Imposter in our own Galaxy?
• Infrared outbursts as potential tracers of common envelope events in high-mass X-ray binary formation
• Multi-wavelength spectroscopic analysis of the ULX Holmberg II X-1 and its nebula suggests the presence of a heavy black hole accreting from a B-type donor
• New Transient ULX Candidate in NGC 4254: Evidence of Circumbinary Disk?
• Simultaneous X-ray/UV observations of ultraluminous X-ray source Holmberg II X-1 with Indian space mission AstroSat
• Identification of three new ultraluminous X-ray sources in NGC 4631 and NGC 1097: Evidence for stellar-mass black holes