New record set at flash emission

The observations of NASA’s Swift satellite have seized a great flash of emission from the head of the class of Supergiant Fast X-ray Transients, during which this source has exceeded 10 times their own personal record of brightness, and even the theoretical maximum limit brightness expected for these sources. This result, obtained by a team largely made up of Italian researchers at INAF and led by Patrizia Romano (INAF researcher at the Institute of Space Astrophysics and Cosmic Physics of Palermo), which raises doubts about some fundamental aspects concerning these unpredictable sources.
“For us, the source IGR J17544-2619 is not just a symbol, but it is ‘The King’ because the object is more extreme and perhaps capricious class of Supergiant Fast X-ray Transients. Every time we think we have caught a clue, sensed a physical mechanism that induces it to behave like this, with these its characteristic sudden and brilliant flashes of X-ray emission, which occurs here in something new, something absolutely amazing. ”

IGR J17544-2619


Credit: University of Salento and INFN section of Lecce

So Patrizia Romano, said the unexpected ‘mood swings’ of IGR J17544-2619, a pair of interacting stars composed of a supergiant whose wind increases the companion of compact type, a neutron star. Patrizia Romano and his team, mostly composed of Italian researchers at INAF, studying for years, this and other similar sources, in particular thanks to the observations of the Swift satellite (NASA mission with participation of the Italian and United Kingdom) trying to figure out what are caused by the sudden increase in the flow of X-rays from these celestial objects. Here is their story ‘choral’ last, powerful, energy blast from IGR J17544-2619 on 10 October and the results of the investigation associated with it, in an article published in the journal Astronomy & Astrophysics. But from the beginning, from the fateful moment when the team Swift receives the notice ‘live’ from the satellite that something, in that source, had begun to change. And much, “When I saw it, together with American colleagues, the first data indicating a flow comparable with that of the Crab Nebula, one of the most intense high energy sources in the sky, I could not believe my eyes,” says Vanessa Mangano, astrophysics at Penn State University, USA.
And after the surprise, came the first calculations that provide estimates of the amount of energy released in the event. “IGR J17544-2619 has not only beaten its own record of 10 times, thus extending its dynamic range, ie the ratio between the maximum and minimum brightness measured, a million – adds Sergio Campana, the Astronomical Observatory of INAF Brera – but we also left with a puzzle to be solved, a big unexpected problem. ”
In recent years, research on SFXTs, binary systems consisting of a neutron star which falls on the matter from the wind by the partner, a blue supergiant, it is in fact focused on their most striking: bright flashes during which these objects reach streams X-ray ‘soft’ even 100,000 times more than the quiet periods. In particular it is known that SFXT are less bright in general of binary ‘normal’, ie, that do not show these flare.
“We were mostly trying to explain the observations by inhibiting accretion onto the compact object, but now we are faced with a new challenge, or understand how to justify the very high brightness achieved, exceeding the brightness typical in systems that increase from stellar wind, “says Enrico Bozzo, INTEGRAL Science Data Centre (ISDC) in Geneva. “The peak brightness of IGR J17544-2619 can be achieved or a stellar wind extremely slow (speed about 100 times lower than those expected for a normal supergiant star) or very high (and improbable) growth rates.”
The winds from the companion supergiant can be considerably slowed down by the ionization due to the X radiation produced from much on the neutron star, especially in systems which, like that of IGR J17544-2619 are characterized by relatively short orbital periods and with a certain eccentricity. In these conditions it is difficult to prevent the formation of an accretion disk transitional, whose dissipation can considerably increase the growth rate on the neutron star, and then explain in a natural way the flare up to 10 times brighter than the previous observed by the team.
According to Paul Esposito INAF-IASF Milano, currently a Fulbright fellow at the Harvard-Smithsonian Center for Astrophysics, “definitive evidence in favor of the formation of an accretion disk (even if temporary) would measure a speed boost rotation of the neutron star during these intense periods of issue, but unfortunately ‘the King’ has so far been reluctant to show pulsations in its X-ray emission, and even in this spectacular event we could only highlight a weak pulsed signal with a period of about 12 seconds. ”
“There are of course alternative explanations” adds Gianluca Israel INAF Astronomical Observatory of Rome, “such as the growth of large lumps of matter from the companion, as observed in the past for another SFXT, but analysis of spectroscopic this event did not reveal any change in spectral their support. ” In short, to explain the behavior of ‘The King’ is certainly not a simple task, but scientists are working team to study more thoroughly this enigmatic source, and finally get to discover its true nature.

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