Optical photometry and Halpha Spectroscopy of SWIFT J1910.2-0546/MAXI J1910-057

ATel #4347; Jorge Casares, Pablo Rodriguez-Gil, Cristina Zurita, Ignacio Gonzalez Martinez-Pais. Jesus M. Corral-Santana, Romano Corradi and Remon Cornelisse (IAC) and Phil A. Charles (Southampton/UCT)
on 29 Aug 2012; 21:38 UT
Credential Certification: Jorge Casares (jcv@iac.es)

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We report on optical photometry and spectroscopy of the black hole X-ray transient Swift J1910.2-0546/MAXI J1910-057 (Krimm at al. ATel #4139, Usui et al. ATel #4140). R-band photometry (time resolution 27s) over an interval of 3.1 hr was obtained using ALFOSC on the 2.5m Nordic Optical Telescope (NOT) on the night of 21 July 2012. The light curve is dominated by flickering superimposed on a smooth, approximately sinusoidal variability of amplitude ~0.02 mags and ~4 hr time scale. We see no evidence for short-time scale variations repeating with a ~2.2 hr period, as were observed early on in the outburst (Lloyd et al. ATel #4246). We measure a mean magnitude R=16.20, indicating that the transient has only faded by ~0.5 mags since its discovery (Rau et al. ATel #4144). A further image was acquired on 15 Aug 2012 22:20UT using ACAM on the 4.2m William Herschel Telescope (WHT). This new image was obtained after the transient was reported to enter the hard state (Nakahira et al. ATel #4273, Bodaghee et al. ATel #4328) and we find that the source is still optically bright at r'=16.38 Time-resolved spectroscopy was obtained on the nights of 28 July and 16 Aug 2012 using ACAM on the WHT. We obtained thirty five 300s spectra on 28 July and ten 600s spectra on 16 Aug, covering a useful spectral range 4300-9200A at 3.4 A/pix. Given the excellent seeing conditions (~0.5"-0.7") on both nights we employed a 0.5" slit which resulted in a spectral resolution of 280 km/s, as measured from the FWHM of the OI 5577 sky line. These spectra show significant changes with respect to previous spectroscopy obtained during the soft state (Charles et al. ATel #4210). The nightly averages display a weak (~2A EW) HeII 4686 emission line, broad (FWHM ~ 2000-3000 km/s) Hbeta and Hgamma absorptions and a wide Halpha absorption trough with a weak (~2A EW) embedded emission component. The Halpha emission is very narrow (FWHM=550 +/ 20 km/s) and exhibits clear velocity motions, with a smooth drift of ~220 km/s over 3.1 hr on the night of 28 July. With an intrinsic broadening ~470 km/s, the Halpha emission is likely to arise from a turbulent region in the accretion disc, such as the hot spot. If the velocities are caused by the binary motion, our observations would constrain the orbital period to > 6.2 hr. The WHT is operated on the island of La Palma by the Isaac Newton Group (ING). The NOT is operated jointly by Denmark, Finland, Iceland, Norway and Sweden. Both telescopes are installed in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias (IAC). These observations have made use of IAC Service Time and IAC Director's Discretionary Time.