Optical Spectroscopy of the Candidate Be Star Counterpart to 4U1901+03
ATel #12554; J. Strader, L. Chomiuk, S. Swihart, E. Aydi (MSU)
on 7 Mar 2019; 02:51 UT
Credential Certification: Jay Strader (strader@pa.msu.edu)
Subjects: Optical, Binary, Pulsar
We obtained an optical spectrum of the candidate Be star counterpart to the X-ray pulsar 4U1901+03 (ATel #12514, #12515, #12519) with SOAR/Goodman on 2019 Mar 5. The Gaia DR2 J2000 coordinates of this source are (RA,Dec) = (19:03:39.392, +03:12:15.74), which is about 0.42" from the Chandra X-ray position of 4U1901+03 reported by ATel #12519.
The spectrum shows clear H-alpha emission at a barycentric velocity of -59 km/s, with a FWHM of about 450 km/s and an equivalent width of 9 A. There are strong DIB features in absorption, but besides H-alpha, no other clear emission or absorption features associated with the star. Consistent with the high foreground N_H, the spectrum is highly reddened, with minimal flux below ~ 5000 Ang. Overall, the spectrum is consistent with that of a reddened Be star and hence are suggestive of the identification of 4U 1901+03 as a Be/X-ray binary.
We note that the parallax of this star in Gaia DR2 is not significant and hence the distance is not well-constrained. In fact, the high N_H and optical reddening are consistent with an E(B-V) ~ 2.5-3.5, which via the PS1 reddening maps of this region (Green et al, 2018, MNRAS, 478, 651) would imply a large distance of > 12 kpc. This distance constraint is not definitive, but a near distance of a few kpc is not favored.
