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X-ray flashes detected from the low-mass X-ray binary system IGR J17407−2808

Astronomers have recently observed a low-mass X-ray binary known as IGR J17407−2808 with NuSTAR and XMM-Newton space telescopes. In result, they detected several fast X-ray flares from this source. The finding was reported in a paper published April 18 on the arXiv pre-print repository.

X-ray binaries (XRBs) are composed of a normal star or a white dwarf transferring mass onto a compact neutron star or a black hole. Based on the mass of the companion star, astronomers divide them into low-mass X-ray binaries (LMXBs) and high-mass X-ray binaries (HMXBs).

Discovered on October 9, 2004, by the International Gamma-Ray Astronomy Laboratory (INTEGRAL), IGR J17407−2808 (or J17407 for short) is an LMXB at a distance of some 12,400 light years away from the Earth. Although J17407 exhibited several peculiarly quick and strong flares in the past, it remained a poorly studied source.

A team of astronomers led by Lorenzo Ducci of the University of Tuebingen in Germany decided to change this by conducting a comprehensive investigation of J17407's enigmatic nature using NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) and ESA's XMM-Newton space observatories.

"In an attempt to clarify the nature of this object, in this work we report on the remarkable flaring activity detected for the first time from J17407 by NuSTAR during an observation performed in 2022. We also analyze the first source broadband spectrum (∼0.2-60 keV) obtained by combining the NuSTAR data with a quasi-simultaneous observation carried out with XMM-Newton during the persistent low luminosity state of the source," the researchers wrote.

NuSTAR observations conducted by Ducci's team detected J17407 in a flaring state characterized by a variability as large as three orders of magnitude on time scales of a few tens of seconds. During this state, the source exhibited several fast X-ray flares, lasting between 1 to 100 seconds and profiles with either single or multiple peaks.

The astronomers noted that the fast and strong variability of J17407 is similar to that observed in some other LMXB systems like Swift J1858.6−0814, V404 Cygni, and V4641 Sgr or in a HMXB designated A0538−66. However, they underlined that given that some fundamental characteristics of the stellar components hosted in J17407 are still unknown, it is difficult to conclude what accretion mechanisms are triggering its X-ray variability.

According to the authors of the paper, analysis of the collected data suggests that the donor star in J17407 can be either a rare K or M-type sub-subgiant or a K-type main sequence (MS) star, or sub-giant star. Therefore, the researchers propose spectroscopic observations of J17407 in optical near-infrared in order to disentangle the true nature of the donor star, noting that their recent results further support the classification of this system as an LMXB.