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MAXI J1807+132: GTC spectroscopy reveals a LMXB-like optical spectrum.

ATel #10221; T. Munoz-Darias, F. Jimenez-Ibarra, D. Mata Sanchez, M. Armas Padilla, J. Casares (IAC-Tenerife), P. A. Charles (Southampton)
on 28 Mar 2017; 23:11 UT
Credential Certification: Phil Charles (pac2soton@gmail.com)

Subjects: Optical, X-ray, Binary, Black Hole, Neutron Star, Transient

Referred to by ATel #: 10222, 10223, 10224

Following the recent detection of the new X-ray transient MAXI J1807+132 and its optical counterpart (ATEL #10208, #10215, #10216, #10217) we performed spectroscopic observations with the OSIRIS spectrograph attached to the Gran Telescopio Canarias (GTC) 10.4m telescope located in La Palma, Canary Islands, Spain. 2x300s observations were taken on 2017 March 28 (05:30 UT) using the R1000B grism (2.12 Angs pix-1) ), which covers the spectral range 3800-7800 Angs. We clearly detect several single-peaked emission lines that are typically observed in Low Mass X-ray Binaries (LMXBs) in outburst, such as H-alpha, H-beta, HeI-6678 and HeII-4686. For the strongest emission line (H-alpha) we measure an equivalent width of ~ 2.1 Angs. This, combined with the detection of previous source activity (ATEL #10217), strongly suggest that MAXI J1807+132 is a new transient LMXB.

The spectrum also shows interstellar absorption features that can be used to constrain the reddening. Using the equivalent width of the Na I doublet (5889.9 - 5895.9 Angs) and the relation presented in Barbon et al. 1990 (A&A 237, 79) we obtain E(B-V)~0.28 which translates into NH~1.6x1021 cm-2 (Predehl & Schmitt 1995, A&A, 293, 889). This (relatively) low value is similar to that derived from the Swift X-ray analysis (NH = (2.3 +/- 0.5)x1021 cm-2; ATEL #10216) and makes it a valuable target for study over a wide range of wavelengths.

We are grateful to the GTC team for their fast and efficient response to our ToO trigger.