Very little magnetism in this odd couple
Heads are turning in our universe. There are stellar couples composed of a very bright blue supergiant around which a compact object orbits such as a white dwarf or even something as extreme as a neutron star or a black hole. These objects can strip part of the outer layers of the supergiant. While falling, spiralling in tighter and tighter orbits, this material heats up so much it emits huge amounts of X-ray radiation. Among these systems, called Supergiant X-Ray Binaries (SGXRB), even odder couples can be found, that suddenly, and within a few hours, brighten up in their X-ray emission by up to a thousand times, to later return to a dimmer, quieter fluxes. These are called Supergiant Fast X-ray Transients.
The best example of all peculiar SFXT properties is the source IGR J17544-2619, discovered by the ESA satellite INTEGRAL in 2003. Since then, it was studied in depth with all satellites the high-energy astrophysicists had access to, including XMM-Newton, Chandra, and Swift. The latter, has been meticulously observing for the past seven years not only IGR J17544-2619 but also the rest of the dozen or so sources in the SFXT class, both in their bright flares and in their fainter emission when not flaring.
The reasons behind their odd behaviour are far from being understood, and different theoretical, sometimes incompatible models are still incomplete or occasionally in contrast with observational evidence. Some propose that the accretion onto the compact object is a process regulated by the gas structure (a discontinuous flow), others that the magnetic field of the compact object and its spin velocity may regulate the accretion flow, hence the X-ray luminosity.
The new observations of IGR J17544-2619 obtained by the NASA satellite NuSTAR have shed light onto some of the most obscure properties and behaviours of SFXTs. The unprecedented accuracy of these measurements has allowed the team led by Varun Bhalerao at the Inter University Center for Astronomy and Astrophysics (India), with the participation of INAF researchers Patrizia Romano and Lorenzo Natalucci, to see a cyclotron line in the X-ray spectrum of this source, the unambiguous signature of the interaction between the stellar matter and the magnetic field of the compact object that is attracting it.
“The NuSTAR observation allowed us for the very first time to reveal and hence measure the energy of a cyclotron line in a SFXT. Consequently, we could obtain the first direct measurement of the magnetic field of the compact object onto which matter from the companion is falling. This is a key ingredient in our understanding of the mechanisms causing the different behaviour of SFXTs when compared to the other normal X-ray binaries,” says Patrizia Romano, researcher at INAF-IASF Palermo, and lead of the investigation on SFXT with Swift, second author of the paper describing this measurement, to appear on Monthly Notices of the Royal Astronomical Society.
“This is another clear demonstration of NuSTAR’s ability to detect cyclotron lines in binary systems. After all, it’s one of the strategic goals of the mission,” adds Lorenzo Natalucci, researcher at INAF-IAPS in Rome and member of the NuSTAR scientific team, “This result follows on the footsteps of important studies performed with INTEGRAL and Swift, results obtained with a substantial contribution from Italian researchers.”
The detection of the cyclotron line in IGR J17544-2619 is, therefore, a turning point. SFXTs share with high mass binaries both the companion (a massive OB supergiant), and the compact object, probably a neutron star. Under similar conditions, for instance the same binary orbit, we would expect the same phenomenology, the same behaviour. On the contrary, the former emit peculiar bright flares, the latter are basically persistent.
As an example: if we give two chefs working in the same kitchen the same recipe, the same ingredients in the same quantities so that they can make a pie, then we expect that they will make very similar, if not indistinguishable pies. Unless they mix the ingredients differently, for example.
“This is what happens with SFXTs, and the most accepted models tend to demonstrate that the cooks, in keeping with the kitchen analogy, are pouring the flour (that is the gas accreting onto the neutron star) in a different way, one smoothly, the other by the spoonful,” adds Patrizia Romano.
And the actual reason for pouring one way or the other depends on the cook, that is, the compact object. Some current models, in fact, predict that the compact object is a magnetar, a neutron star endowed with a very high magnetic field (1014 Gauss). NuSTAR’s measurement is 100 times lower than the typical magnetar fields, thus rejecting a magnetar nature for the compact object in the SFXT class prototype and demonstrating that, at least in this object, the magnetic field has the same strength as those in the other high mass X-ray binaries.
The paper NuSTAR Detection Of A Cyclotron Line In The Supergiant Fast X-ray Transient IGR J17544-2619 (Varun Bhalerao et al.) accepted for publication on MNRAS available on arxiv website repository
The Swift satellite is a NASA mission with an international participation (Italy and UK). INAF OA Brera produced the XRT optics, while ASI Science Data Center (ASDC) wrote the software for the scientific analysis of the XRT data. The Italian team, led by Gianpiero Tagliaferri, contributes to the scientific operations of the satellite. The Italian participation is possible thanks to the support of ASI, which also provides the Malindi ground station.
The NASA NuSTAR satellite is a collaboration of several US institutions and led by Principal Investigator Fiona Harrison at California Institute of Technology. The Italian contribution includes the Malindi ground station (ASI) and the science analysis software developed by ASDC, led by Paolo Giommi. Further contributions include the calibration and scientific data interpretation in conjunction with a team of scientists belonging to several Italian scientific institutions.
