MAGIC observes a gravitational lens at very high energies
According to Einstein's General Relativity, light is deflected passing close to a large mass. To a distant observer the mass focuses light like a giant lens. The
result is a much brighter, although distorted, image of the source and a chance to see distant objects that might otherwise be far too faint to detect. And just like a
lens, light can pass through the lens with slightly different path lengths. On cosmic scales, this means photons -- parcels of light -- traveling along different
lines of sight arrive at slightly different times. If, in addition, the source is variable, this is "imprinted" on the light with a time delay relative to a fixed first arrival. And
this should not depend on the energy of the photons, according to the theory. That makes such observations especially important.
QSO B0218+357 harbors a supermassive black hole in a galaxy located halfway across the Universe from Earth. Over 7 billion years ago a gigantic explosion
occurred in this object, which led to the emission of an intense flare of gamma rays, which is the highest-energy form of light. In its long journey toward Earth,
these photons passed in the vicinity of a foreground — still distant — galaxy, B0218+357G, over one billion years later. In passing and being deflected, those
photons traveling along the shorter path finally arrived at Earth on July 14th, 2014 and were observed by the Large Area Telescope on board the orbiting Fermi
satellite, which scans the entire sky every 3 hours. The detection of this gammaray outburst alerted the astronomical community, and many telescopes
worldwide were immediately pointed at QSO B0218+357 to learn more from this distant cosmic explosion. Researchers operating the MAGIC telescopes, located
on La Palma in the Canary Islands, became excited about the possible observation of this object in very-high-energy gamma rays. These could provide
the most extreme perspective of this outburst, but, unfortunately, at that time there was full moon in La Palma, which prevented the operation of the MAGIC
telescopes. The MAGIC telescopes measure very-high-energy gamma rays, which are a thousand times more energetic than those measured by Fermi, and a
hundred billion times more energetic than any light we see from our Sun.
From the earlier measurements of this object in 2012 by Fermi and by radio telescopes the scientists knew that photons arriving along the longer path should
arrive about 11 days later. “In other words, Nature could award us with a replay, a second chance to look at the same interesting phenomenon.” says the MAGIC
Collaboration member Julian Sitarek (University of Łódz, Poland and IFAE former member of the Institut de Fisica d'Altes Energies in Barcelona, Spain, when he
started this project) who led this study, and continues: “When the time came, the MAGIC telescopes were pointed at QSO B0218+357, and, in accordance with
the prediction, a flare of very-high-energy gamma rays was observed, making QSO B0218+357 the most distant object detected in the very-high-energy
gamma-ray domain to date.” These very-high-energy gamma-rays from any distant source have a high chance to interact with the numerous low-energy
photons emitted by galaxies and stars, being lost in the process. With this observation, MAGIC has doubled the previously known visibility range of the
Universe in very-high-energy gamma rays. Observation of the delayed signal from QSO B0218+357 by MAGIC showed for the first time that these very
energetic photons are also deflected in agreement with General Relativity, a result that is both striking and potentially profound. The signal arriving at the
predicted time may rule out some theories of the structure of the vacuum. That awaits further analysis. For the moment, this observation demonstrates a new
capability of the very-high-energy gamma-ray observatories and highlights what awaits the next generation of such telescopes, the Cherenkov Telescope Array
Figure 1: Photons are emitted from a galaxy QSO B0218+357 in the direction of the Earth. Due to the gravitational effect of the intervening galaxy B0218+357G
photons form two paths that reach Earth with a delay of about 11 days. Photons were observed by both the Fermi-LAT instrument and the MAGIC telescopes.
(MAGIC Image credits: Daniel Lopez/IAC; Hubble image of B0218+357G credits: NASA/ESA; AGN image credits: NASA E/PO - Sonoma State University, Aurore Simonnet)
The original MAGIC article can be seen
A message from Crab Pulsar in TeraelectronVolt fonts
The Crab Pulsar, PSR J0534+220, is a young neutron star that was created after the supernova explosion SN1054.
Its mass is about 1.5 times the mass of the Sun, concentrated in an object of about 10 km diameter. It rotates 33 times per second, and it is surrounded by a
region of intense magnetic field ten thousand billion times stronger than that of the Sun.
This field is strong enough to dominate the motion of charges and forces charged particles to rotate and spiral around the neutron star.
This region is called the magnetosphere.
The rotation of the magnetic field generates intense electric fields that literally tear charged particles from the surface.
As accelerated charged particles stream outward, they produce beams of radiation that we receive every time the beam crosses our line of sight, like it happens with a lighthouse.
The Crab is the most powerful pulsar in our Galaxy. It is one of the few pulsars that has been detected
across the electromagnetic spectrum from radio up to gamma rays, and is one of the brightest at high energies.
Its pulsar wind nebula, the Crab nebula, is also the standard candle for
many experiments detecting gamma-rays, as MAGIC: in 2008 for the first time pulsed emission was discovered
in the Very High Energy range (E. Aliu et al., Science, 322, 1221(2008)), revolutionizing the existing models of pulsars as the Polar Cap model.
Following that discovery, in 2011 and 2012 the energy range of the emission observed from Crab Pulsar expanded up to 400 Giga Electron Volts (GeV), and also bridge emission between
the pulsed peaks was detected: now after an extensive work collecting ~320 hours of very good quality data observing Crab, MAGIC reported in the paper published
in A&A vol. 585, A133 (2016)
the discovery of the highest pulsed emission ever detected in our Universe, 1.5 Tera Electron Volts(TeV).
The mechanism behind such an unexpected highly energetic gamma-ray emission is not easy to be understood. The existing models fail to describe the extension of
the emitted energy up to 1.5 TeV, that could only be ascribed to an Inverse Compton process at work in the Crab pulsar, dominating the emission of gamma-rays
above 50 GeV.
The new results probe the Crab Pulsar as the most compact TeV accelerator known to date, and require a revision of the state-of-the-art models proposed to explain
how and where gamma-ray pulsed emission up to 1.5 TeV are produced.
Due to the big amount of data used for this research it was also possible to perform a detailed study of how the pulsar emission (phaseogram) changes with energy.
These results are key for the understanding of pulsars and will be an hard test for new theories to come.
Image Credits: Patricia Carcelén Marco
Figure: The neutron star (red sphere) with its strong magnetic field (white lines) spins
around itself nearly 30 times per second injecting energetic electrons in the space region
around it. The green and blue shaded regions depict different particle acceleration zones
from where the detected photons could originate. The green zone lies in the vicinity of the
pulsar's magnetosphere, whereas the blue zone could be as far as 100.000 km away from the pulsar
Very-high-energy gamma rays from the Universe's middle age: the first
detection of the distant galaxy PKS1441+25
PKS 1441+25 is one of the two most distant active galaxy detected up to such extreme energies. The very high-energy emission is attenuated on its way to Earth due to the interaction with the diffuse light filling the Universe, the extragalactic background light. This diffuse light retains the history of the stars and galaxies evolution and, hence, the story of the evolution of the Universe. PKS 1441+25 was used as a lighthouse to derive information on the evolution of the Universe from half its age up to the present day. Beside its distance, PKS 1441+25 is one of the very few Flat Spectrum Radio Quasars detected at such extreme gamma-ray energies, allowing to study its intrinsic characteristics.
On April 2015, the Large Area Telescope detected an increase of the activity of PKS 1441+25 and MAGIC followed up the alert, discovering VHE gamma rays from the object.
Some of the galaxies are called active, as they produce in their central parts much more light that can be explained by the stellar and dust emission. Active galaxies, hosting at their centers a supermassive black hole with a mass of million up to few billion times the mass of the Sun are amongst the most powerful objects of the Universe and dominate the gamma-ray sky: they are able to accelerate charged particles up to very high energies. Following a classification based on their luminosity and other astrophysical important features, PKS 1441+25 belongs to the Flat Spectrum Radio Quasars family and emits very energetic gamma rays from the vicinity of its central black hole. Flat Spectrum Radio Quasars are enigmatic sources and scientists struggle to explain the emission of VHE gamma rays from them, observations such as those performed by MAGIC give crucial inside into those sources. Gamma rays depict the extreme Universe, but such highly energetic emission suffers absorption in the journey to the Earth due to the interaction with diffuse emission, the extragalactic background light. The extragalactic background light acts as a sort of ‘haze’ which dims the gamma-ray brightness of distant galaxies. This ‘haze’ was generated by stars and dust throughout the history of the Universe. It traces the evolution of the Universe after the appearance of the first stars. In this context, powerful active galaxies, such as PKS 1441+25, can be used as distant lighthouses to infer the characteristics of the extragalactic background light between the Earth and the position of the active galaxy.
The emission detected from the active galaxy PKS 1441+25 has been traveling for half of the age of the Universe. PKS1441+25 and QSO B0218+357, another source recently detected by MAGIC, are the oldest VHE gamma rays ever detected from ground observatories.
In the paper published the 15th of dec 2015 in the AstroPhysics Journal Letters, vol. 815 L23,
the current models of the extragalactic background light which constrain the evolution of the Universe from its middle age until today were tested for the first time up to energies never reached before for such distant galaxies.
MAGIC Telescopes observe the birth of a jet
close to the event horizon.
Over 250 millions years ago a powerful burst of very high energy
gamma-ray radiation left the vicinity of a supermassive black hole
in the nucleus of the galaxy named by astronomers as IC 310.
The flickering of the gamma-ray flare on time scales of less than
five minutes shows that it originated from a region smaller than
the event horizon of the black hole. This finding supports the idea
that such an extraordinary emission was due to particles accelerated
in an extremely narrow region located near the event horizon of the
black hole, and permeated by strong electric fields. Such structures
are expected to form near rapidly-spinning black holes that power
radio jets by their rotational energy loss.
IC 310 is a galaxy belonging to the Perseus cluster of galaxies at a
distance of about 260 million light-years from Earth. It is famous for hosting a
supermassive black hole of over 300 million solar masses.
Supermassive black holes with masses ranging from a million up to
few billion times the mass of the Sun are thought to reside in the center
of all galaxies. In some fraction of these, accretion of matter onto these
objects produces unusually intense emission visible as a star-like core called
an active galactic nucleus, AGN. This accreting matter does not fall directly
into the central black hole, but circulates around it in a dense disk.
Supermassive black holes are expected to expel plasma jets at the
expense of their rotational energy. They induce currents in the
surrounding accretion flow associated with large-scale magnetic fields,
collimating and accelerating charged particles. Jets of plasma are thus
produced and ejected into the interstellar medium at tremendous velocities
approaching the speed of light. The core of an active galaxy, powered by
the central black hole and composed by the disk and the jets, is thus emitting
light across the entire electromagnetic spectrum.
IC 310 is famous for an extended radio jet emerging from its center.
For decades, the origin of this jet has been a puzzle. It emits a power that
corresponds to the radiation output from ten billion stars and emerges
from an extremely compact central part of the galaxy. Employing the
EVN network of radio antennas, high-resolution images of the jet in
IC 310 revealed a very straight structure emerging from a compact
core smaller than a light year.
Fermi/LAT and MAGIC discovered very high energy emission from this
galaxy in 2009. After these findings, IC 310 has been considered
a very peculiar source and has been carefully monitored at all wavelengths.
Left panel: artist view of
the core on an active galactic nucleus. The supermassive black hole is at the
center and, both, the accretion disk and the jet are shown.
Right panel: Significance
map (false color scale) of the Perseus cluster sky region in gamma rays
observed in the night of November 12th, 2012, with the MAGIC telescopes.
The bright source shown in the gamma ray image is IC 310. The insert
shows the radio jet image of IC 310 at 5.0GHz obtained with the European
VLBI Network (EVN) on October 29th, 2012.
During a further observation campaign of IC 310, in the night of
12 November 2012 a very powerful gamma ray emission, coming from
this source, was observed by MAGIC: this emission varied on time
scales of less than five minutes. Information about the structure of the
source is encoded in its variability. The shorter the variability time scale,
the smaller the size scales that can be probed. A source region must be
smaller than the distance that light can travel during the variability time
scale. Since the shortest variability time scales arise at the highest photon
energies, gamma-ray observations hold the clue to uncover the size of
the jet formation region. Moreover, the event horizon of the supermassive
black hole in IC 310 is known to be about three times as large as the
Sun-Earth distance, and finding variable gamma-ray emission at only one
fifth of this distance was a complete surprise. According to the current
picture, the observed gamma rays arise at shocks traveling down the jet.
However, the minimum time scale for the emission originating at shocks
is given by the light crossing time across the event horizon which limits
the radius of the jet from below. Now, the MAGIC observations tell a
different story. If the plasma around the supermassive black hole is
very rarified, the jets should develop regions near the rotational pole
where the density is too low for charges to shortcut the electric fields
arising due to the rotation of the magnetic field. These intensely
electrified regions resemble the vacuum gaps in pulsar magnetospheres,
which are known to accelerate particles to very high energies, exceeding
more than ten thousand times the rest mass equivalent energy of the
elementary constituents of matter, protons and neutrons, and tapping
a significant fraction of the total outflow energy.
Thus, the observations of IC 310 may be the first hint providing
direct clues on the enigmatic jet formation process near black holes.
This exceptional result obtained by using the MAGIC
and EVN telescopes is now published on the issue of November 6th
2014 of Science Express.
Discovery of 3C 58 (MAGIC J0205+6451)
When a massive star has consumed almost the
totality of its nuclear fuel, it undergoes a supernova explosion and its fate depends on its mass: if it
is heavy enough, it becomes a black hole where not even the light can escape, but if its mass does not
reach this limit, the remnant is a rapidly spinning neutron star called pulsar as a leftover. Pulsar wind
nebulae (PWNe) are nebulae powered by the magnetized wind of a pulsar. 3C 58 is a young PWN powered by a
powerful pulsar (PSR J0205+6449). The source's age is very controversial, since it is spatially coincident
with a Supernova recorded by Chinese and Japanese astronomers between August 4th and 6th in 1181 AD, although
measurements do not support this hypothesis and derive several ages, usually larger than 2500 years. The
distance to the source is also uncertain, since it was always considered to be located at 3.2 kpc but recently
a new distance of 2 kpc was proposed (Kothes, R., 2013, A&A, 560, A18 ). Due to morphology similarities
(see Figure 1 from Chandra X-Ray Observatory), 3C 58 has been compared to the best studied source in the sky,
the Crab Nebula.
Figure 1: (left panel) Crab pulsar and Nebula and (right panel) 3C58 as seen
in the X-ray (courtesy CXO). In both cases the Chandra image shows the center of the nebula, which contains a
rapidly spinning neutron star surrounded by a thick ring, or torus, of X-ray emission.
Nevertheless, it was observed and undetected by
several ground-based VHE gamma-ray telescopes. 3C 58 was first detected at gamma rays by the LAT instrument
on-board Fermi satellite and the observed spectrum is extending beyond energies of 100 GeV. MAGIC finally
succeeded in discovering VHE gamma rays emitted by 3C 58 (MAGIC J0205+6451) (Figure 2) and in extending its
spectrum up to a few TeV energies (as shown in Figure 3).
Figure 2: Skymap obtained by MAGIC. The gamma ray emission is shown
together the optical position of the pulsar and the radio contours.
Figure 3: The Very High Energy spectrum of 3C58 including Fermi and
MAGIC points and theoretical models.
MAGIC observations established that 3C 58
(MAGIC J0206+6451) is the lowest luminosity and the one with the lowest flux ever detected at VHE gamma rays
as shown in Figure 4).
The assumed age that best fits the data is 2500 years,
while a distance to the source of 2 kpc or a high local infrared density for a distance of 3.2 kpc might explain the
multiwavelngth emission. Since the high local infrared density is unexpected, we favor the recently proposed distance
of 2 kpc to the source. The magnetic field derived by the different models is very low (below 35 microGauss) and far
from equipartition, which is ~ 80 microGauss.
The original MAGIC article can be seen