The Extremes of Black Hole Accretion

Some of the most energetic processes seen in the Universe arise close to a super-massive black hole such as relativistic jets and winds. These are now known to play a key role in determining the growth of galaxies across cosmic time, but the mechanisms by which they are launched remain unclear.

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The Evolving Blazar Paradigm

The international meeting “The Evolving Blazar Paradigm” is a follow up of our previous Krakow meetings focused on the physics of astrophysical jets in AGN and in the Galactic sources “Challenges of Relativistic Jets” (2006)” and “Understanding Relativistic Jets” (2011).

The innermost regions of relativistic jets and their magnetic fields

The study of relativistic jets in active galactic nuclei and other sources involving the accretion onto compact objects, like gamma-ray bursts and microquasars, has gained added interest thanks to the recent simultaneous multi spectral-range observations and the advances in the theoretical and numerical modeling. During June 10-14th, 2013, the Instituto de Astrofísica de Andalucía-CSIC in Granada, Spain, will host a meeting (poster) aimed to discuss the recent results in the study of relativistic jets. The meeting will focus on the study of the innermost regions of AGN jets to obtain a better understanding of the jet formation mechanisms and determine the origin and location of the high energy emission, as well as the role played by the magnetic field.

Topics to be discussed include:

• Jet formation
• Black hole, accretion disk, jet connection
• Multi-spectral-range emission
• Magnetic fields and polarization
• Jet dynamics and stability
• Unification models, microphysics, particle acceleration
• Relativistic stellar jets

All the light in the Universe since the Big Bang

Vi siete mai chiesti quanta luce è stata emessa da tutte le galassie da quando è nato l’Universo? Pensate un attimo a ciascun fotone di qualsiasi lunghezza d’onda, dall’ultravioletto all’infrarosso, che sta viaggiano ancora nello spazio fino a raggiungere i nostri rivelatori. Se riuscissimo a misurare in maniera accurata il numero e l’energia di tutti i fotoni, non solo quelli dei nostri giorni ma anche quelli più antichi, potremmo ricavare indizi fondamentali sulla natura e l’evoluzione dell’Universo e comprendere come le galassie più antiche siano differenti rispetto a quelle che vediamo oggi.

That bath of ancient and young photons suffusing the Universe today is called the extragalactic background light (EBL). An accurate measurement of the EBL is as fundamental to cosmology as measuring the heat radiation left over from the Big Bang, the cosmic microwave background, at radio wavelengths. A new paper, called “Detection of the Cosmic γ-Ray Horizon from Multiwavelength Observations of Blazars”, by Alberto Dominguez at University of California at Riverside and six coauthors, based on observations spanning wavelengths from radio waves to very energetic gamma rays, obtained from several NASA spacecraft and several ground-based telescopes, describes the best measurement yet of the evolution of the EBL over the past 5 billion years. Directly measuring the EBL by collecting its photons with a telescope, however, poses towering technical challenges, harder than trying to see the dim band of the Milky Way spanning the heavens at night from midtown Manhattan. Earth is inside a very bright galaxy with billions of stars and glowing gas. Indeed, Earth is inside a very bright Solar System: sunlight scattered by all the dust in the plane of Earth’s orbit creates the zodiacal light radiating across the optical spectrum down to long-wavelength infrared. Therefore ground-based and space-based telescopes have not succeeded in reliably measuring the EBL directly. So, astrophysicists developed an ingenious work-around method: measuring the EBL indirectly through measuring the attenuation of, that is, the absorption of, very high energy gamma rays from distant blazars. Blazars are supermassive black holes in the centers of galaxies with brilliant jets directly pointed at us like a flashlight beam. Not all the high-energy gamma rays emitted by a blazar, however, make it all the way across billions of light-years to Earth; some strike a hapless EBL photon along the way. When a high-energy gamma ray photon from a blazar hits a much lower energy EBL photon, both are annihilated and produce two different particles: an electron and its antiparticle, a positron, which fly off into space and are never heard from again. Different energies of the highest-energy gamma rays are waylaid by different energies of EBL photons. Thus, measuring how much gamma rays of different energies are attenuated or weakened from blazars at different distances from Earth indirectly gives a measurement of how many EBL photons of different wavelengths exist along the line of sight from blazar to Earth over those different distances. Observations of blazars by NASA’s Fermi Gamma Ray Telescope spacecraft for the first time detected that gamma rays from distant blazars are indeed attenuated more than gamma rays from nearby blazars, a result announced on November 30, 2012, in a paper published in Science, as theoretically predicted. Now, the big news is that the evolution of the EBL over the past 5 billion years has been measured for the first time. That’s because looking farther out into the Universe corresponds to looking back in time. Thus, the gamma ray attenuation spectrum from farther distant blazars reveals how the EBL looked at earlier eras. This was a multistep process. First, the coauthors compared the Fermi findings to intensity of X-rays from the same blazars measured by X-ray satellites Chandra, Swift, Rossi X-ray Timing Explorer, and XMM/Newton and lower-energy radiation measured by other spacecraft and ground-based observatories. From these measurements, Dominguez and collaborators were able to calculate the blazars’ original emitted, unattenuated gamma-ray brightnesses at different energies. The coauthors then compared those calculations of unattenuated gamma-ray flux at different energies with direct measurements from special ground-based telescopes of the actual gamma-ray flux received at Earth from those same blazars. When a high-energy gamma ray from a blazar strikes air molecules in the upper regions of Earth’s atmosphere, it produces a cascade of charged subatomic particles. This cascade of particles travels faster than the speed of light in air, which is slower than the speed of light in a vacuum. This causes a visual analogue to a “sonic boom”: bursts of a special light called Čerenkov radiation. This Čerenkov radiation was detected by imaging atmospheric Čerenkov telescopes (IACTs), such as HESS (High Energy Stereoscopic System) in Namibia, MAGIC (Major Atmospheric Gamma Imaging Čerenkov) in the Canary Islands, and VERITAS (Very Energetic Radiation Imaging Telescope Array Systems) in Arizona. Comparing the calculations of the unattenuated gamma rays to actual measurements of the attenuation of gamma rays and X-rays from blazars at different distances allowed Dominquez and colleagues to quantify the evolution of the EBL, that is, to measure how the EBL changed over time as the Universe aged, out to about 5 billion years ago, corresponding to a redshift of about z = 0.5. “Five billion years ago is the maximum distance we are able to probe with our current technology”, Domínguez said. “Sure, there are blazars farther away, but we are not able to detect them because the high-energy gamma rays they are emitting are too attenuated by EBL when they get to us, so weakened that our instruments are not sensitive enough to detect them”. This measurement is the first statistically significant detection of the so-called “Cosmic Gamma Ray Horizon” as a function of gamma-ray energy. The Cosmic Gamma Ray Horizon is defined as the distance at which roughly one-third or, more precisely, 1/e, that is, 1/2.718 where “e” is the base of the natural logarithms, of the gamma rays of a particular energy have been attenuated. This latest result confirms that the kinds of galaxies observed today are responsible for most of the EBL over all time. Moreover, it sets limits on possible contributions from many galaxies too faint to have been included in the galaxy surveys, or on possible contributions from hypothetical additional sources, such as the decay of hypothetical unknown elementary particles.

UC-HiPACC: DETECTION OF THE COSMIC GAMMA RAY HORIZON MEASURES ALL THE LIGHT IN THE UNIVERSE SINCE THE BIG BANG

UCR: Astronomers Measure the Elusive Extragalactic Background Light

arXiv: Detection of the cosmic γ-ray horizon from multiwavelength observations of blazars

Discovered a new class of gamma-ray burst

A team led by the University of Warwick has pinpointed a new type of exceptionally powerful and long-lived cosmic explosion, prompting a theory that they arise in the violent death throes of a supergiant star. These explosions create powerful blasts of high energy gamma-rays, known as gamma-ray bursts, but while most bursts are over in about a minute, this new type can last for several hours.

The first example was found by astronomers on Christmas Day 2010, but it lacked a measurement of distance and so remained shrouded in mystery with two competing theories put forward for its origin. The first model suggested it was down to an asteroid, shredded by the gravity of a dense neutron star in our own galaxy, the second that it was a supernova in a galaxy 3.5 billion light years away, or in the more common language of astronomers at a redshift of 0.33. A new study by a team of scientists led by Andrew Levan at the University of Warwick finds several more examples of these unusual cosmic explosions and shows that the Christmas Day burst took place in a galaxy much further away than the two theories suggested. This research has been presented at the GRB 2013 Symposium in Nashville, Tennessee on Tuesday 16 April. Using data from the Gemini Telescope in Hawaii, the scientists calculated that this ultra-long gamma-ray burst had a redshift of 0.847. This gives it a location of approximately half-way to the edge of the observable Universe, or 7 billion light years away. Armed with its location, Levan’s team, which included scientists from an international collaboration, has developed a new theory to explain how it occurred. They suggest this kind of burst is caused by a supergiant, a star 20 times more massive than the sun, which evolves to become among the biggest and brightest stars in the universe with a radius of up to 1 billion miles – up to 1,000 times that of the sun. They believe the ultra-long durations of the Christmas gamma-ray burst and two other similar bursts are simply down to the sheer size of the supergiants exploding in a supernova.

Most stars that create gamma-ray bursts are thought to be relatively small and dense, and the explosion that destroys them punches through the star in a matter of seconds. In the case of these new ultra-long bursts the explosion takes much longer to propagate through the star, and so the gamma-ray burst lasts for a much longer time. Levan said: “These events are amongst the biggest explosions in nature, yet we’re only just beginning to find them. It really shows us that the Universe is a much more violent and varied place than we’d imagined. Previously we’ve found lots of gamma-ray events with short durations, but in the past couple of years we’ve started to see the full picture.” Nial Tanvir, a professor at the University of Leicester, and second author of the study added: “We believe that powering the explosion is a newly formed black hole in the heart of the star. Predicting the detailed behaviour of matter falling into a black hole in these circumstances turns out to be very difficult, and from a theoretical point of view we didn’t initially expect explosions at all. “The amazing thing is that nature seems to have found ways of blowing up a wide range of stars in the most dramatic and violent way.” The more common type of gamma-ray burst is thought to be caused when a Wolf-Rayet star in the final phase of its evolution collapses into a black hole at its own core. Matter is drawn into the black hole, but some of its energy escapes and is focussed into a jet of material which blasts out in two directions forming copious gamma-rays in the process. These jets are ejected extremely quickly (close to the speed of light), otherwise the material would fall into the black hole from which it can’t escape. For this reason they last only a few seconds. However, a gamma-ray burst in a bigger star the size of a supergiant needs to power through a larger reservoir of material, hence its longer duration.

University of Warwick: Strange new bursts of gamma-rays point to a new way to destroy a star

arXiv: A new population of ultra-long duration gamma-ray bursts