X-ray outburst of IGR J06074+2205 faint again

ATel #17152; Roberto Nesci (INAF/IAPS), Mariateresa Fiocchi (INAF/IAPS), Antonio Vagnozzi (MPC589)
on 18 Apr 2025; 15:31 UT
Credential Certification: Roberto Nesci (roberto.nesci@inaf.it)

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The HMXB IGR J06074+2205 in the last years had recurrent X-ray flares, during a few days, with a recurrence 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 historic maximum Equivalent Width of 12 Angstrom. Indeed 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 E.W. was just 5.0 Angstrom at the beginning of November 2024, so that the expected X-ray outburst around November 10 should be very small or undetectable: actually our Swift/XRT observations showed a peak count rate of 0.21 c/s (ATel #16899). Our H-alpha monitoring of the source showed a fast increase of the H-alpha emission since February 2025, reaching 10 Angstrom E. W. in April, so that a large flare was expected at the next periastron passage around April 15. Our Swift XRT daily monitoring started on April 11: the source was at the quiescent level (0.065 c/s) until April 17, when a clear flux increase (0.21 c/s) was detected, but the flux decreased down to 0.12 c/s already on April 18. Our Swift/XRT monitoring is scheduled until April 21 but, if the orbital period is correct, the periastron passage of the pulsar should already be over and no further flux increases should happen. The modest X-ray flux occurred in this periastron passage, similar to that of November when the H-alpha emission was low, poses some doubts about the simple model of the X-ray outburst described above: a possible way out could be that the structure of the circumstellar disk of the Be star is rather patchy. We thank the Swift Time Allocation Committee for the approval of our ToO request.