Optical Spectroscopy of a Candidate O-Star X-ray Binary in M33
Abstract
We present new observations of a candidate O-type donor star in a high mass X-ray binary (HMXB) in M33. The candidate donor star is spatially coincident with a hard X-ray point source. The star's optical magnitude and colors are consistent with what is expected for a massive star. The four band optical/UV spectral energy distribution (SED) of the donor star is well fit by an O-giant model with an effective temperature of ∼43 kK, mass of 75+9−0.3 M⊙, and surface gravity of log(g) of ∼3.8. We present four epochs of optical spectroscopy of the donor star taken with the Keck/DEIMOS spectrograph over the course of approximately one year. Features in the star's spectrum are consistent with an O-type star. The star's spectrum exhibits a strong Hα emission line during all epochs of observation with a variable line profile. We measure the star's radial velocity for each epoch, which we use to fit for a systemic velocity. We find that the systemic velocity is offset from the local gas velocity from HI measurements, suggesting that the system has a peculiar velocity relative to the local gas. We observe a radial velocity shift in the He I absorption lines that is consistent with motion within a binary system.
Versions
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BibTeX
@misc{lazzarini2026optical,
title = {{Optical Spectroscopy of a Candidate O-Star X-ray Binary in M33}},
author = {Margaret Lazzarini and Lara Cullinane and Karoline Gilbert and Puragra GuhaThakurta and Kyros Hinton and Daniel Stern and O. Grace Telford and Tobin M. Wainer and Benjamin F. Williams and Robert Alexander and Ernesto Ramirez Jr},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.16174},
eprint = {2609.16174},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.16174},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Optical Spectroscopy of a Candidate O-Star X-ray Binary in M33
AU - Margaret Lazzarini
AU - Lara Cullinane
AU - Karoline Gilbert
AU - Puragra GuhaThakurta
AU - Kyros Hinton
AU - Daniel Stern
AU - O. Grace Telford
AU - Tobin M. Wainer
AU - Benjamin F. Williams
AU - Robert Alexander
AU - Ernesto Ramirez Jr
PY - 2026
DA - 2026/09/14
PB - arXiv
DO - 10.48550/arxiv.2609.16174
UR - https://arxiv.org/abs/2609.16174
AB - We present new observations of a candidate O-type donor star in a high mass X-ray binary (HMXB) in M33. The candidate donor star is spatially coincident with a hard X-ray point source. The star's optical magnitude and colors are consistent with what is expected for a massive star. The four band optical/UV spectral energy distribution (SED) of the donor star is well fit by an O-giant model with an effective temperature of ∼43 kK, mass of 75_+9^−0.3 M_⊙, and surface gravity of log(g) of ∼3.8. We present four epochs of optical spectroscopy of the donor star taken with the Keck/DEIMOS spectrograph over the course of approximately one year. Features in the star's spectrum are consistent with an O-type star. The star's spectrum exhibits a strong Hα emission line during all epochs of observation with a variable line profile. We measure the star's radial velocity for each epoch, which we use to fit for a systemic velocity. We find that the systemic velocity is offset from the local gas velocity from HI measurements, suggesting that the system has a peculiar velocity relative to the local gas. We observe a radial velocity shift in the He I absorption lines that is consistent with motion within a binary system.
ER -