4XMM J181330.1−175110: A new supergiant fast X-ray transient

Context. Supergiant fast X-ray transients (SFXTs) are a subclass of high-mass X-ray binaries (HMXBs) in which a compact object accretes part of the clumpy wind of the blue supergiant companion, triggering series of brief X-ray flares lasting a few kiloseconds. Currently, only about 15 SFXTs are known.Aims. The EXTraS (Exploring the X-ray Transient and variable Sky) catalog provides the timing signatures of every source observed by the EPIC instrument onboard XMM-Newton. Among the most peculiar sources in terms of variability, we identified a new member of the SFXT family: 4XMM J181330.1−175110 (J1813).Methods. We analyzed all publicly available XMM-Newton, Chandra, Swift, and NuSTAR data pointed at the J1813 position to determine the source’s duty cycle and to provide a comprehensive description of its timing and spectral behavior during its active phase. Additionally, we searched for the optical and infrared counterpart of the X-ray source in public databases and fitted its spectral energy distribution (SED).Results. The optical-to-mid infrared SED of J1813 is consistent with a highly absorbed (A_{V ∼ 38) B0 star at ∼10 kpc. During its X-ray active phase, the source is characterized by continuous thousands seconds-long flares with peak luminosities (2–12 keV) ranging from 10^{34 to 4 × 1035 erg s−1. Its X-ray spectrum is consistent with a high-absorbed power-law model with NH ∼ 1.8 × 1023 cm−2 and Γ ∼ 1.66. No spectral variability was observed as a function of time or flux. J1813 is in a quiescent state ∼60% of the time, with an upper-limit luminosity of 8 × 1032 erg s−1 (at 10 kpc), implying an observed long-term X-ray flux variability > 500.Conclusions. The optical counterpart alone indicates J1813 is an HMXB. Its transient nature, duty cycle, the amplitude of observed X-ray variability, the shape and luminosity of the X-ray flares – and the lack of known X-ray outbursts (> 1036 erg s−1) – strongly support the identification of J1813 as an SFXT.}}