Uncovering Red and Dusty Ultraluminous X-ray Sources with Spitzer
  We present a mid-infrared (IR) sample study of nearby ultraluminous X-ray
sources (ULXs) using multi-epoch observations with the Infrared Array Camera
(IRAC) on the Spitzer Space Telescope. Spitzer/IRAC observations taken after
2014 were obtained as part of the Spitzer Infrared Intensive Transients Survey
(SPIRITS). Our sample includes 96 ULXs located within 10 Mpc. Of the 96~ULXs,
12 have candidate counterparts consistent with absolute mid-IR magnitudes of
supergiants, and 16 counterparts exceeded the mid-IR brightness of single
supergiants and are thus more consistent with star clusters or non-ULX
background active galactic nuclei (AGN). The supergiant candidate counterparts
exhibit a bi-modal color distribution in a Spitzer/IRAC color-magnitude
diagram, where "red" and "blue" ULXs fall in IRAC colors $[3.6] - [4.5]\sim0.7$
and $[3.6] - [4.5]\sim0.0$, respectively. The mid-IR colors and absolute
magnitudes of 4 "red" and 5 "blue" ULXs are consistent with that of supergiant
B[e] (sgB[e]) and red supergiant (RSG) stars, respectively. While "blue",
RSG-like mid-IR ULX counterparts likely host RSG mass donors, we propose the
"red" counterparts are ULXs exhibiting the "B[e] phenomenon'' rather than hosts
of sgB[e] mass donors. We show that the mid-IR excess from the "red" ULXs is
likely due to thermal emission from circumstellar or circumbinary dust. Using
dust as a probe for total mass, we estimate mass-loss rates of
$\dot{M}\sim1\times10^{-4}$ M$_\odot$ yr$^{-1}$ in dust-forming outflows of red
ULXs. Based on the transient mid-IR behavior and its relatively flat spectral
index, $\alpha=-0.19\pm0.1$, we suggest that the mid-IR emission from Holmberg
IX X-1 originates from a variable jet.
