Absence of radial velocity variations in MWC 148 during the recent activity of HESS J0632+057

ATel #3209; Jorge Casares (Instituto de Astrofisica de Canarias), Marc Ribo (Universitat de Barcelona), Josep M. Paredes (Universitat de Barcelona), Artemio Herrero (Instituto de Astrofisica de Canarias), Ignacio Negueruela (Universidad de Alicante), Francesc Vilardell (Universidad de Alicante)
on 7 Mar 2011; 12:46 UT
Credential Certification: Marc Ribo (mribo@am.ub.es)

Tweet

MWC 148 is a B0pe-type star suspected to be the counterpart of the variable TeV gamma-ray source HESS J0632+057, which also shows variable radio and X-ray emission (Aharonian et al. 2007, A&A, 469, L1; Acciari et al. 2009, ApJ, 698, L94; Hinton et al. 2009, ApJ, 690, L101; Skilton et al. 2009, MNRAS, 399, 317; Falcone et al. 2009, ApJ, 708, L52). MWC 148 is thought to be a gamma-ray binary, although evidence of binarity from radial velocity measurements is still lacking (Aragona et al. 2010, ApJ, 724, 306).

Falcone et al. 2011 (ATEL #3152) have reported increased X-ray activity detected by Swift-XRT between January 23 and at least February 6, 2011 (MJD 55584-55598). These authors propose a binary orbital period of ~320 days based on three X-ray activity periods. Increased activity has also been reported in very high energy gamma rays by VERITAS and MAGIC from February 7 to 9, 2011 (MJD 55599-55601) (Ong et al. 2011, ATEL #3153; Mariotti et al. 2011, ATEL #3161). The radio counterpart has also been detected at milliarcsecond scales with the e-EVN on February 15, 2011 (MJD 55507) (Moldon et al. 2011, ATEL #3180).

We are obtaining intermediate resolution spectroscopy of MWC 148 as part of our long-term project on radial velocity studies of high-mass X-ray binaries. Observations conducted with the 2.0-m Liverpool Telescope between January 5 and February 24 (MJD 55566-55616) with a resolution of 55 km/s reveal no change in the radial velocities of the B0pe star larger than +/- 12 km/s. The equivalent width of Halpha, observed to vary by a factor of 2 in the past (Casares et al. 2010, arXiv:1012.4351), only shows a marginal 4 percent increase from 44.5 to 46.3 Angstrom.

These results suggest that the current activity period is not related to the periastron passage of a compact object in an elliptical orbit around MWC 148. Therefore the activity period seems to have a different origin.

We will continue our program on radial velocity measurements approximately on a weekly basis from now on.

The Liverpool Telescope is a fully robotic telescope located at the Observatorio del Roque de Los Muchachos of the Instituto de Astrofisica de Canarias, on the Canary island of La Palma, Spain. The Liverpool Telescope is owned and operated by Liverpool John Moores University, with financial support from STFC.