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 ] \sim 0.7 and [ 3.6 ] - [ 4.5 ] \sim 0.0 , 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 } \sim 1 \times 10 ^ { -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 \pm 0.1 , we suggest that the mid-IR emission from Holmberg IX X-1 originates from a variable jet .