From ESA via Manu: “Rho Ophiuchi A confirmed as a cosmic lighthouse”

Manu Garcia, a friend from IAC.

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27 February 2018.

Ignazio Pillitteri
INAF-Osservatorio Astronomico di Palermo
Palermo, Italy
Phone: +39 091 233 420

Lida Oskinova
Institute of Physics and Astronomy, University of Potsdam
Potsdam, Germany
Phone: +49 331-9775910

Norbert Schartel
XMM-Newton Project Scientist
European Space Agency

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XMM-Newton detects the first X-ray flares of a massive stellar beacon.

ESA/XMM Newton

Credit : ESA / XMM-Newton; I. Pillitteri (INAF-Astronomical Observatory of Palermo).
This image of space observatory XMM-Newton of the ESA shows a massive star called Rho Ophiuchi A . The star, visible in the center of the frame, is in the heart of Rho Ophiuchi dark cloud, a nearby region known for actively forming new stars, located about 350 light years away.

A rich collection of colourful astronomical objects is revealed in this picturesque image of the Rho Ophiuchi cloud complex from NASA’s Wide-field Infrared Explorer, or WISE.

NASA/WISE Telescope

The Rho Ophiuchi cloud (pronounced ‘oh-fee-yoo-ki’ and named after a bright star in the region) is found rising above the plane of the Milky Way in the night sky, bordering the constellations Ophiuchus and Scorpius. It’s one of the nearest star-forming regions to Earth, allowing us to resolve much more detail than in more distant similar regions, like the Orion nebula.

In 2014, a team of scientists used X – rays with the X-ray observatory XMM-Newton ESA emanating from the massive star Rho Ophiuchi A. Numerous and subsequent telescope observations showed that these periodically fluctuated as intense flames , ranging over a period of about 1.2 days as the star he turned. The team used the ESO Very Large Telescope to discover that the star has a strong magnetic field, confirming its status as cosmic X – ray lighthouse.

ESO VLT Platform at Cerro Paranal elevation 2,635 m (8,645 ft)

The FORS instruments (Focal Reducer and low dispersion Spectrograph 2) located at ESO’s VLT. The FORS2 is in the foreground while the FORS1 is at the bottom. The two seem similar instruments but perform completely different tasks. Credit: ESO.

This finding was unexpected given what is known about the massive stars and their behavior: it is known that sun – like stars produce flares of X rays strong, but massive stars seem to be very different. In stars more than eight solar masses, the X – ray emission is constant, and has not been observed with certainty that said star SHINES repeatedly in this part of the spectrum before observing Rho Ophiuchi A.

It is known that sun-like stars produce powerful X-ray flares, but massive stars seem to be very different. In stars from eight solar masses, the X-ray emission is continuous and had not reached any star to issue such flares repeatedly in that part of the spectrum … Until recently observed.

“We spent almost 40 hours observing the star with XMM-Newton and discovered something even more unexpected, recognizes Ignazio Pillitteri, INAF-Osservatorio Astronomico the di Palermo (Italy) and head of the research team. Instead of a homogeneous and continuous emission periodically pulsed X-rays toward the outside of Rho Ophiuchi A, with a variation of about 1.2 days as rotating star-just like a lighthouse X-ray! This is a new phenomenon in greater than the Sun “stars.

Rho Ophiuchi A is much hotter and more massive than our parent star. It is not yet known how X – rays are generated in this type of stars; One possibility is a strong intrinsic magnetism, which would be observable by signs of surface magnetism. However, it remains unclear how the magnetic field would originate and how to be associated with X-ray emissions

“We suspect that there may be a giant active magnetic point on the surface of Rho Ophiuchi A, something like a sunspot, only much larger and more stable, added Pillitteri. As the star rotates, this stain would be hidden or visible repeatedly, causing the observed X-ray pulses. However, this hypothesis is not very likely: stains stars form when a magnetic field inside comes to the surface, and we know that only one in ten massive stars have a measurable magnetic field. ”

Furthermore, the ‘lighthouse effect’ pulse also may be due to low mass companion orbital, which would add their own and abundant X rays to light attributed to Rho Ophiuchi A . This X – ray emission power would vary due to the passage of this hypothetical smaller star ahead or behind Rho Ophiuchi A during orbit of 1.2 days. The team also considered the possibility that Rho Ophiuchi A could have an inconspicuous, small, low – mass companion in a close orbit.

“To confirm what was the case, we hasten to obtain measurements of Rho Ophiuchi A using one of the largest ground-based observatories: the ESO VLT says Lida Oskinova, University of Potsdam (Germany) and member of the international team that carried the study. Fortunately, one of our measurements confirmed predictions by showing that X-rays were probably due to magnetic structures on the surface of the star. ”

The measurements were made in visible light with a technique known as spectropolarimetry, which involves studying various wavelengths of polarized light emitted by a star. The data showed that Rho Ophiuchi A has a strong magnetic field, about 500 times stronger than the Sun ‘s .

“Such a strong field may easily produce the type of detected flares, Pillitteri points. This confirms that what we found with XMM-Newton were really X-ray emissions from Rho Ophiuchi A massive stars can be magnetically active (as shown by optical observations) and that this activity can be seen in X-rays “.

The combined data indicate that Rho Ophiuchi A is the only star of this type in which confirmed an active magnetic region on the surface that emits X – ray search for similar behavior in stars like Rho Ophiuchi A will help scientists understand the prevalence of this phenomenon and learn more about the magnetic properties of these stars.

“This study is important for the exploration of massive stars, as there is much we do not know about these objects emphasizes Norbert Schartel, XMM-Newton scientist ESA project. By combining the extraordinary capabilities of XMM-Newton and the VLT we have managed to fit another piece of the puzzle. ”

“In addition, it illustrates perfectly the scientific process: find something interesting or unusual, investigate and launched several hypotheses, then keep watching to find out which one is correct. It is a fantastic example of international collaboration between telescopes, both orbiting and ground, coming together to explore and explain the phenomena we see in the cosmos. ”

X-ray flares Rho Ophiuchi A. The flickering light of the massive star Rho Ophiuchi A is observed by the XMM-Newton space observatory of the ESA in 2016.

These and earlier observations XMM-Newton showed that this star periodically throws flares X-ray of its surface as it rotates, a behavior something like a cosmic lighthouse. Follow-up observations made by the research team using the Very Large Telescope of ESO confirmed that this star has a strong magnetic field and the X-ray flares are connected to an active magnetic region on the surface of the star turns in and out of sight.

This sequence consists of 40 frames obtained between 22 and 23 February 2016 each taken approximately one hour apart. It shows the emission of the star on the X – ray part of the spectrum; The clearer it is blue tone, stronger is the issue, and the white represents the maximum intensity. It can be seen that the intensity of X – ray emission of Rho Ophiuchi A rises sharply at the beginning and end of this sequence; This is because the data cover more than one cycle period X – ray burning star, which lasts 1.2 days.

These findings are described in three papers published in Astronomy & Astrophysics:
Smooth X-ray variability from ρ Ophiuchi A + B: A strongly magnetized primary B2 star? By Pillitteri et al. (2014), doi: 10.1051 / 0004-6361 / 201424243;
The early B-type star Rho Ophiuchi A is an X-ray lighthouse of Pillitteri et al. (2017), doi: 10.1051 / 0004-6361 / 201630070; Y
Detection of magnetic field in the B2 star ρ Oph A with ESO FORS2 of Pillitteri et al. (2018), doi: 10.1051 / 0004-6361 / 201732078.

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