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Optical Activity of HDE 245770/A0535+26 System

ATel #3176; Franco Giovannelli - INAF-IASF, Roma; Roberto Nesci - Physics Dpt. Un. La Sapienza, Roma; Corinne Rossi - Physics Dpt. Un. La Sapienza, Roma
on 18 Feb 2011; 17:32 UT
Credential Certification: Franco Giovannelli (franco.giovannelli@iasf-roma.inaf.it)

Subjects: Optical, Binary, Neutron Star

Referred to by ATel #: 3198, 3204

The O9.7IIIe star HDE 245770, nicknamed Flavia, is the optical counterpart of the X-ray pulsar A0535+26. On February 5, 2011 at the Loiano 1.52 m telescope from UT 18hh 40mm 40ss we performed optical B, V spectrophotometry of HDE 245770 and starting from UT 18hh 51mm 11ss to 19hh 15mm 37ss we performed optical spectroscopy in the range 330 - 785 nm [Delta(lambda) from about 0.4 to 0.3 nm] by using the BFOSC (Bologna Faint Object Spectrograph & Camera). The first analysis of data shows B = 9.67 +/- 0.04 mag and V = 9.25 +/- 0.04 mag, and the spectra shows a moderate optical activity of the star. The equivalent widths of the H-alpha and H-beta emission lines are -1.01 +/- 0.05, -0.13 +/- 0.05, respectively. H-gamma, H-delta and H-epsilon appear in absorption with equivalent widths, 0.13 +/- 0.02 nm, 0.25 +/- 0.02 and 0.14 +/- 0.02, respectively. The He I line at 667.8 nm is in emission and shows a clear doubling whose peaks are separated by 0.85 nm. This doubling could be an indicator of an accretion disk temporary formed around the neutron star (e.g. Giovannelli et al., 2007, A&A 475, 651). On Febr 10, 2011 (MJD 55602) Fermi/GBM detected pulsations from A0535+26 and three days later an X-ray emission of ~ 157 mCrab (ATel #3166). Such emission could be easily explained. Indeed, we suggest that the periastron passage was on MJD 55586 (Jan 25, 2011) following the ephemerides JD(opt-outb)=JD(0)(2,444,944) +/- n(110.856 +/- 0.02) days derived by optical measurements by Giovannelli & Sabau-Graziati (2010, IBWS in Karlovy Vary, Czech Republic). Optical measurements of HDE 245770 from La Palma and Sierra Nevada observatories gave the same equivalent width of -1.00 nm for H-alpha on Jan 19, 2011 (MJD 55580) (ATel #3166), a few days earlier of the suggested periastron passage. Since the life-time of the temporary accretion disk is of order of 30-40 days, we suggest optical measurements in order to detect the disappearing of the doubling in the He I lines. This could definitively prove the existence of the temporary accretion disk around the neutron star near the periastron passage (e.g. Giovannelli & Ziolkowski, 1990, AcA 40, 95).