A missed X-ray outburst of IGR J06074+2205 ?
ATel #16899; Roberto Nesci (INAF/IAPS), Mariateresa Fiocchi (INAF/IAPS), Antonio Vagnozzi (MPC589)
on 10 Nov 2024; 13:37 UT
Credential Certification: Roberto Nesci (roberto.nesci@inaf.it)
Subjects: Optical, X-ray, Binary, Neutron Star, Pulsar
The HMXB IGR J06074+2205 in the last years had recurrent X-ray flares during a few days with a period of 80 +/-2 days (Mihara et al. 2023 ATel #16351). These outbursts are believed to be produced by the crossing of the circumstellar disc of the Be star by the companion pulsar. The optical indicator of the disc is a strong H-alpha emission, with maximum Equivalent Width of 12 Angstrom. Long term spectroscopic monitoring (Reid et al. 2016 A\&A 590, 122) shows that the H-alpha line has a variable intensity, so that no, or very small, outburst should happen at the time of crossing if the line is faint. A large outburst (1.9 c/s) happened on March 2024-03-14 (Nesci et al. ATel #16525) when the E.W. was 10 Angstrom. Our spectroscopic monitoring with the 50 cm telescope of the S. Lucia di Stroncone Observatory (MPC 589) showed that the EW was only 4 Angstrom on August 30 and remained low, raising just to 5.0 Angstrom at the beginning of November, so that the expected X-ray outburst around November 10 should be very small or undetectable if our understanding of the phenomenon is correct. Our ToO with Swift-XRT started on 2024-11-07 and will go on at 2 days steps until 2024-11-15, to encompass the expected outburst date. Quick look measures show a count rate of 0.21 (+/-0.02) c/s on Nov 07 and 0.19 (+/-0.02) c/s on Nov 09, which is 3-4 times higher than the quiescent level. We believe therefore that the outburst is actually happening at the expected time but is very faint due to the small quantity of material in the circumstellar disc of the Be star.
