miércoles, 10 de agosto de 2011

Cygnus X-1 y su agujero negro/Cygnus X-1 and its black hole

Cygnus X-1 lleva ese nombre porque fue la primera fuente de Rayos X que se encontró en la constelación de Cygnus (Cisne). En 1971, los astrónomos descubrieron Rayos X procedentes de una brillante estrella azul que orbitaba alrededor de un misterioso objeto oscuro. Supusieron que los Rayos-X se originaban a partir de la material arrancada de la estrella y que caía en el objeto oscuro, que era quizás un agujero negro. A comienzos de este año, se ha encontrado que Cygnus X-1 está a 6.000 años-luz de la Tierra. A partir de la distancia se determinaron las masas de la estrella y del objeto oscuro, y es tan masivo que solo puede ser un agujero negro.
Ahora, Krzysztof Belczynski y Tomasz Bulik de la Universidad de Varsovia y Charles Bailyn de la Universidad de Yale, han utilizado estos datos tan precisos para predecir el futuro de Cygnus X-1. Según la publicación enviada a la revista Astrophysical Journal Letters, el equipo calcula que el agujero negro, cuya masa actual equivale a 14,8 soles, arrancará otros tres soles a su compañera, mientras que esta última, que ahora tiene una masa de 19,2 soles disminuirá hasta solo 4 soles, perdiendo materia hacia su compañera y hacia la galaxia.
Según se transfiere masa de la estrella al gujero negro, ambos se alejan lentamente entre sí reajustando sus órbitas a la pérdida de masa. Hoy día, sus centros están a una distancia igual a la mitad de la existente entre Mercurio y el Sol, pero el equipo de Belczynski opina que la distancia aumentará hasta ser ligeramente mayor que la que hay entre la Tierra y el Sol.
Cuando la estrella explote, dentro de 2,6 millones de años, estará tan lejos del agujero negro, que hay un 70% de probabilidades de que se aleje de éste, dejándole orbitar en solitario por la galaxia. La estrella será entonces una estrella de neutrones, una variedad de estrella muerta menos extrema que un agujero negro e incluso si el agujero negro y la estrella de neutrones permanecen juntos, lo más probable es que sigan así y no se fusionen.
Esta mezcla o fusión es un evento que podría generar ondas gravitacionales, es decir rizos en el espacio tiempo que han sido predichos por la Teoría General de la relatividad de Albert Eisntein, y que se están buscando en la actualidad. Por tanto, el equipo de Belczynski concluye que si Cygnus X-1 es representativo de futuros sistemas binario agujero negro-estrella de neutrones, los observadores que buscan ondas gravitacionales no deberían esperar verlas como resultado de la fusión de tales sistemas. De hecho, todavía no se conocen sistemas binarios agujero negro-estrella de neutrones.
A modo de curiosidad, la banda de rock canadiense Rush anticipó la existence de un agujero negro en Cygnus X-1, donde los viajeros se aventuran “a través del vacío para ser destrozados”



Cygnus X-1 bears its name because it was the first source of x-rays found in the constellation Cygnus. In 1971, astronomers discovered that the x-rays came from the direction of a bright blue star whirling around a mysterious dark object. They speculated that the x-rays were resulting from material being torn away from the bright star and falling onto the dark object, perhaps a black hole. Earlier this year, astronomers measured the distance to Cygnus X-1, finding it to be about 6000 light-years from Earth. This distance then yielded the mass of the bright star as well as the dark object; the latter is so massive it can only be a black hole.
Now Krzysztof Belczynski and Tomasz Bulik of the University of Warsaw and Charles Bailyn of Yale University have used these precise parameters to predict the fate of Cygnus X-1. According to the paper submitted to The Astrophysical Journal Letters, the team calculates that the black hole, now weighing 14.8 suns, will gobble an additional three suns' worth of material from its companion, whereas this last, whose mass is now 19.2 suns, will slim down to just four solar masses by losing material both to its partner and the galaxy.
As the bright star transfers mass to the black hole, the two will slowly move away from each other readjusting their orbits to the change in mass. Today, their centers are about half as far apart as Mercury is from the sun, but Belczynski's team says this distance will grow until it's slightly greater than that between the sun and Earth.
In the future, about 2.6 million years from now, when the star will explode it will be so far from the black hole that there's about a 70% chance it will shoot away from the black hole, leaving its dark companion to sail through the galaxy alone. The star will have become a neutron star, a dead star less extreme than a black hole. Even if the black hole and neutron star remain in orbit, they will probably stay far apart and they won't merge into one.
This merging is an event that would trigger gravitational waves, i.e. ripples in spacetime predicted by Albert Einstein's general theory of relativity that observers are currently hoping to detect. Thus, Belczynski's team concludes that if Cygnus X-1 is representative of future black hole-neutron star binaries, observers seeking to detect gravitational waves should not expect to see them from mergers of such systems. In fact, no black hole-neutron star binaries are yet known.
As a curiosity, the Canadian rock band Rush did anticipate the existence of a black hole in Cygnus X-1, where voyagers venture "through the void to be destroyed”.

Tomado de/Taken from Science

Resumen de la publicación/Abstract of the paper
The fate of Cyg X-1: an empirical limit on BH-NS merger rate
Krzysztof Belczynski, Tomasz Bulik, Charles Bailyn
arXiv:1107.4106v1 [astro-ph.GA]
The recent distance determination allowed precise estimation of the orbital parameters of Cyg X-1, which contains a massive 14.8 Msun BH with a 19.2 Msun O star companion. This system appears to be the clearest example of a potential progenitor of a BH-NS system. We follow the future evolution of Cyg X-1, and show that it will soon encounter a Roche lobe overflow episode, followed shortly by a Type Ib/c supernova and the formation of a NS. It is demonstrated that in majority of cases (>70%) the supernova and associated natal kick disrupts the binary due to the fact that the orbit expanded significantly in the Roche lobe overflow episode. In the reminder of cases (<30%) the newly formed BH-NS system is too wide to coalesce in the Hubble time. Only sporadically (1%) a Cyg X-1 like binary may form a coalescing BH-NS system given a favorable direction and magnitude of the natal kick. If Cyg X-1 like channel (X-ray active phase shorter than 10 Myr) is the only or dominant way to form BH-NS binaries in the Galaxy we can estimate the empirical BH-NS merger rate in the Galaxy at the level of 0.001 per Myr. This rate is so low that the detection of BH-NS systems in gravitational radiation is highly unlikely, generating Advanced LIGO/VIRGO detection rates at the level of only 1 per century. If BH-NS inspirals are in fact detected, it will indicate that the formation of these systems proceeds via some alternative and yet unobserved channels.