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Strong Optical Activity of HDE 245770/A0535+26 System

ATEL # 2497; Giovannelli Franco (INAF-IASF, Roma), Gualandi Roberto (INAF-OABO, Bologna) and Sabau-Graziati Lola (INTA-ACUI, Madrid)
on 20 Mar 2010; 21:33 UT
Distributed as an Instant Email Notice (Request for Observations)
Password Certification: Franco Giovannelli (

Subjects: Optical, Request for Observations, Binaries, Pulsars, Transients, Stars
Referred to by ATEL #: 2541

The O9.7IIIe star HDE 245770, nicknamed Flavia’ star, is the optical counterpart of the X-ray pulsar A0535+26. On March 19, 2010 from UT 18hh 45mm 37ss to 19hh 28mm 07ss at the Loiano 1.52 m telescope we performed optical spectroscopy of this star 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 the spectra shows a strong optical activity of the star. The equivalent widths of the Balmer lines are -1.6 +/- 0.1, -0.24 +/- 0.01, -0.03 +/- 0.02 nm for H-alpha, H-beta and H-gamma, respectively. H-delta and H-epsilon appear in absorption, but the determination of their equivalent widths is extremely difficult because of the low signal-to noise ratio. The equivalent widths of He I lines are -0.04 +/- 0.01, -0.14 +/- 0.01, -0.06 +/- 0.01, -0.20 +/- 0.01 nm for the lines at 501.5, 587.5, 667.8 and 706.5 nm, respectively. Such a strong activity pointed out by the Balmer and He I lines strongly in emission and especially by H-gamma in emission, which is usually in absorption, is a powerful indicator of an incoming X-ray activity. We urge X-ray community to perform measurements of A0535+26 in order to detect the very probable incoming X-ray outburst of the system which is just one orbital cycle later the last one, started on November 30, 2009 (ATel # 2324).

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