Kavli Affiliate: Robert Simcoe
| First 5 Authors: Kishalay De, Deepto Chakrabarty, Roberto Soria, Michael C. B. Ashley, Charlie Conroy
| Summary:
The symbiotic X-ray binary Sct X-1 was suggested as the first known neutron
star accreting from a red supergiant companion. Although known for nearly 50
years, detailed characterization of the donor remains lacking, particularly due
to the extremely high reddening towards the source ($A_Vgtrsim25$ mag). Here,
we present i) improved localization of the counterpart using Gaia and Chandra
observations, ii) the first broadband infrared spectrum ($approx1-5,mu$m;
$Rapprox 2000$) obtained with SpeX on the NASA Infrared Telescope Facility and
iii) $J$-band light curve from the Palomar Gattini-IR survey. The infrared
spectrum is characterized by i) deep water absorption features (H$_2$O index
$approx 40$%), ii) strong TiO, VO and CO features, and iii) weak/absent CN
lines. We show that these features are inconsistent with known red supergiants,
but suggest a M8-9 III type O-rich Mira donor star. We report the discovery of
large amplitude ($Delta Japprox3.5$ mag) periodic photometric variability
suggesting a pulsation period of
$621pm36,{rm(systematic)}pm8,{rm(statistical)}$ days, which we use to
constrain the donor to be a relatively luminous Mira ($M_K=-8.6pm0.3$ mag) at
a distance of $3.6^{+0.8}_{-0.7}$ kpc. Comparing these characteristics to
recent models, we find the donor to be consistent with a $approx 3-5$
M$_odot$ star at an age of $approx 0.1-0.3$ Gyr. Together, we show that Sct
X-1 was previously mis-classified as an evolved High Mass X-ray Binary; instead
it is an intermediate mass system with the first confirmed Mira donor in an
X-ray binary. We discuss the implications of Mira donors in symbiotic X-ray
binaries, and highlight the potential of wide-field infrared time domain
surveys and broadband infrared spectroscopy to unveil their demographics.
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