Gold, Platinum and neutron stars

Gravitational waves are revolutionizing the world of physics and providing first level headlines. Just two years ago the first direct detection of gravitational waves (GW150914, September 14, 2015), produced by the coalescence (merger) of two black holes, for part of the LIGO collaboration. The official announcement of this detection, the 11 February 2016, marked a before and an after in the history of physics, providing a new tool for observing the universe, complementary to the usual electromagnetic window (and others, such as neutrinos). Until that date, the existence of gravitational waves had only been able to infer in an indirect way, by means of measures of accuracy of the variation of the orbital period of double star systems, such as binary pulsars PSR B1913 + 16. The celebrated GW150914 detection was followed by 3 more star systems, all of them composed of black holes (the latest, announced on September 27, marked the first simultaneous detection of observatories LIGO and Virgo).

Without time to dampen the echoes of these discoveries, revitalitzats for the award of the Nobel Prize in Physics in 2017 Riner Weiss, Barry c. Barish and Kip s. Thorne (the last, awarded last May with an honorary doctorate by the UPC) for "decisive contributions to the development of LIGO detector and the detection of gravitational waves ", a new object, GW170817, destined to occupy a prominent place in the annals of Astrophysics, and of science in general, has just been featured in society. GW170817 is a unique object. This is the first detection of the coalescence of two neutron stars. And the first to be detected via gravitational waves and electromagnetic radiation in multiple bands, ushering the so-called astronomy multi Messenger. 

On 17 August 2017, gravitational waves coming from a double star system, located at 130 million light years away, were detected by observatories LIGO and Virgo. 1.7 seconds after, a gamma-ray flare – a gamma-ray burst of short duration – was located by the FERMI space telescope (and later, for INTEGRAL), near the area of emission of gravitational waves. These detections were allowed to refine the position and distance to the source radio station with relative accuracy and undertake their possible optical detection. So, 10 hours and 52 minutes later, identified the optical counterpart of the event from the southern hemisphere, to a point on the NGC4993 Galaxy, in the constellation Hydra. The optical observation was followed up by a number of detections in other bands (infrared, 11 hr 36 min; ultraviolet, 15 hr; radio waves, 16 days). In total, some 70 observatories have observed the phenomenon in different bands of the electromagnetic spectrum, from radio waves to gamma rays. The magazine "The Astrophysical Journal", for example, has just published a monographic volume dedicated to GW170817 (the main results will be reviewed in Abbott et al., ApJ 848, L12).

What does this discovery in scientific terms? First of all, the first detection of a coalescence of neutron stars, in the form of kilonova (explosion a thousand times more energy than a new classic), and the possible confirmation of this stage as head of the gamma-ray bursts of short duration. The simultaneous observation of gravitational waves and electromagnetic radiation have allowed also to verify that the first move at the speed of light.

 At the same time, provide a new tool for delving into the structure and properties of neutron stars, as well as for the study of the rate of expansion of the universe (Abbot et al., Nature, in press).  Finally, but no less relevant, studies a photometric system of the light curves (evolution of the energy emitted as a function of time) in the infrared and the ultraviolet, combined with the corresponding spectral analysis, have allowed to infer the presence of lanthanides ( chemical elements between the lanthanum and ytterbium) in the material ejected during the kilonova, particularly in a layer of a mass 0.04 solares, with a concentration of lanthanides in the order of 1% by mass (Chornock et al., ApJ 848, L19; Tanvir et al., ApJ 848, L27; Pian et al., Nature, in press).