Recent results from the observation of extragalactic gamma sources with the MAGIC telescope
V. Scalzotto for the MAGIC collaborationa
aUniversit`a di Padova and INFN, Via Marzolo 8, 35131 Padova, Italy
The MAGIC telescope is the largest Cherenkov telescope worldwide, operating since late 2004. One of its main research topics is the discovery of extragalactic gamma-ray emitters at very high energies and high redshift, as active galactic nuclei. From the observations of such distant sources, it is also possible to study the nature of the extragalactic background light, and thanks to the time resolution achievable during high flaring emissions, to investigate on possible light propagation effects. The talk will present an overview of the most recent results concerning extragalactic sources.
1. INTRODUCTION
MAGIC is a new generation of Imaging Atmo- spheric Cherenkov Telescopes (IACTs), studying the gamma-ray emission from many sources in the cosmos. It is situated on the Canary island of La Palma (Spain, 28.3◦N,17.8◦W, 2240 m asl). The collaboration counts 22 institutions in nine coun- tries, enrolling more than 130 physicists. Thanks mainly to the 17 m (diameter) tessellated mir- ror of parabolic shape, MAGIC can reach a low energy threshold (which depends on the zenith angle of observation and on the kind of analysis [1]), of about 50 GeV.
The light is collected on a hexagonally shaped camera of 576 hemispherical photo-multiplier tubes (PMT), with a field of view of 3.5 de- grees of diameter. The fast PMT analog signals are routed via optical fibers to the DAQ-sytem electronics where the signals are digitized by a 2GSample/s FADC system and saved on disk.
The energy resolution from the analysis is of 30%
at 100 GeV and 20% at 1 TeV. The angular reso- lution on a point-like source is of 0.1 degrees, and the flux sensitivity (5σin 50 h) is of 1.6% of Crab flux.
The most difficult task for the Gamma Astron- omy experiments is the identification of the sig- nal. These gammas, with respect to the cosmic radiation background, can be considered as ex-
tremely rare events: at these energy they are typ- ically less than 1 over 104 hadrons. Their flux is also very low: about 5·10−7ph s−1m−2for gam- mas of 1 TeV, in the case of the Crab Nebula, considered the brightest steady gamma-ray emit- ting source. In this context, ground based as- troparticle experiments have been built in order to enhance the collection area by using the at- mosphere as a calorimeter. The principle is that the telescopes do not observe directly the primary gammas, but the atmospheric showers induced by them.
In particular, being an Image Atmospheric Cherenkov Telescope (IACT), MAGIC collects the Cherenkov radiation emitted by the ultra- relativistic particles of the atmospheric show- ers, produced by their interaction with the at- mospheric molecules. The physical and phe- nomenological characterization of these showers, and the morphological study of the shape of the Cherenkov image collected in the camera plane, gives the possibility to select the showers induced by the gamma events among the huge amount of the cosmics [2].
Until now, a last observational energy gap from around 30 to 200 GeV is still about to be covered, and its investigation represents the main task of IACTs.
In particular, looking to the extragalactic sky, one
of the main subject for MAGIC is the class of Ac- tive Galactic Nuclei (AGNi) up toz∼0.5.
These objects are among the most energetic and complex sources of the Universe, and are char- acterized by a variable spectrum. For example, AGNi with one jet aligned with the line of sight, called blazars, are spanning from radio to TeV energies. The relationship between the variabil- ity at different wave bands appears rather compli- cated. The nature of the Very High Energy com- ponent of these spectra is still under debate, so the role played by IACTs is fundamental. More- over, while EGRET identified dozens of AGNi at different redshifts [3], only two of them were de- tected by ground experiments at about 250 GeV at a very close distance from the Earth (z∼0.03).
Lowering the threshold of the IACTs is the way to study more AGNi and, related to this, to in- vestigate the extragalactic background light in a very crucial portion of its spectrum [4].
We report here a brief summary of four extra- galactic sources recently observed by MAGIC, which can give an idea of the wide scientific sce- nario this experiment can deal with.
2. SELECTED HIGHLIGHTS
2.1. A gamma-flare emission from M87, a non blazar radio galaxy
M87 is a giant elliptical radio galaxy M87, of Fanaroff Riley-I-type. It lies in the Virgo Clus- ter at a distance of about 16 Mpc from earth.
The angle between the jet and the line of sight is around 30 degrees. It hosts in its core a su- per massive black hole with a mass of 109 solar masses [5].
This source is widely studied in all the wavelength spectrum, revealing an extended jet from radio to X-rays, with a rich structure [6], which can be resolved, thanks to its proximity. M87 has also been established as TeV gamma-ray emitter by the HEGRA collaboration [7] and was later con- firmed by HESS [8] and VERITAS [9].
Indeed, the combination of its proximity and the small viewing angle of its jet, leads M87 to be considered as a unique laboratory for the study of the connections between the physics of the jet and the measured VHE gamma-ray emission.
MAGIC Telescope has been monitoring M87, from January until April 2008, under a small zenith angle of observation (from 16 to 35 de- grees). In total, the amount of analyzed time was of 23 hours.
During this period a rapid flare has been detected, reaching, on its maximum, a value for the flux of 15% of the Crab Nebula at those energies. In addition, during the 13 days of observation, the flux was found to be variable above 350 GeV on a timescale as short as 1 day, at a significance level of 5.6σ.
For the lower energies (150-350 GeV), instead, the flux was compatible with a constant value (figure 1).
Figure 1. Differential energy spectra of M87 divided into high (filled circles) and low (open circles) states. The best-fit functions, assum- ing power laws, are given by the black solid and red dashed-dotted curves, respectively. The flare gives an hardening of the spectrum, pass- ing from an alpha = −2.6±0.3 (low state) to alpha=−2.2±0.2 (high state). The low energy range looks steady.
For the first time, MAGIC obtained an energy spectrum below 250 GeV, where it can be de- scribed by a power law with a photon index of 2.30±0.11 stat ±0.20 syst, without any hint of flattening.
The observed day-scale flux variability at VHE prefers the M87 core as source of this emission and implies that either the emission region is very compact (just few Schwarzschild radii) or the Doppler factor of the emitting blob is rather large in case of a non-expanding emission region.
Finally, we want to remark here that M87 has been observed in a joint campaign by H.E.S.S., MAGIC and VERITAS. Thus, MAGIC played a fundamental role in this agreement, triggering these other IACTs as well as Swift satellite.
2.2. S5 0716+714: the most recent opti- cally triggered detection
MAGIC experiment is also intended to react to Target of Opportunity triggers, given by high flux states in optical or X-ray bands.
In this context, we mention KVA, an optical tele- scope of 35 cm of diameter, situated in La Palma observatory, as MAGIC. This telescope performs joint observations with MAGIC and regular mon- itor campaigns on several AGNi together with the Tuorla 1m optical telescope.
In the past three years, MAGIC received 5 optical alerts for blazars triggered by their high optical states. Three of them led to the discovery of three new VHE emitters. The most recent has been S5- 0716+714.
S5 0716+714 is a highly variable BL Lac object, with rapid variations observed from radio to X- ray bands [10]. Its synchrotron peak is at the optical band, therefore we can classify it as an LBL [11] or an IBL [12].
It is the third LBL object observed at VHE range.
Its redshift is not yet really known, but from re- cent studies [13][14][15], z=0.26 looks like a rather reliable estimation. Nevertheless other studies are needed.
The observation was performed from April 22nd to 24th, 2008, triggered by the KVA telescope, which registered a doubling of the flux level be- tween the 14th to 17th of April. The source regis- tered then its historical maximum. It was also in a high X-ray state as reported by Swift (Giommi et al., ATel #1495).
MAGIC started to take data as soon as the moon conditions allowed it. A preliminary analysis of MAGIC data (3 nights, 2.6 hours in total, at a
zenith angle higher than 45 degrees) revealed a signal of 6.8 sigma corresponding to a flux of F (above 400GeV) = 10−11ph cm−2s−1.
MAGIC announced this discovery by an As- tronomer Telegram on 30th of April (Masahiro, Atel#1500). This result demonstrates the im- portance of this agreement between optical tele- scopes and IACTs, especially in order to further study the connection between optical and VHE gamma-ray high states.
2.3. Mkn501 in 2005 and the new oppor- tunity to study the evolution in time and energy of a VHE flare
Mkn 501 is a strong and variable TeV blazar.
In 1997, Whipple revealed a variation on the time scale of half a day, and a flux up to 10 Crab Units, above 1 TeV [16]. It results that the higher the flux states, the bigger the spectral index. Mkn 501 was also observed by MAGIC at small zenith angles (lower than 30 degrees) with a typical en- ergy threshold of 150 GeV [17].
In 2005 the average flux level above 200 GeV was 30-50% of the Crab Nebula flux. But on five nights, the source was found in a flaring state with the flux reaching up to 4 Crab Units.
On the night of June 30th and July 9th, MAGIC detected rapid flares with a doubling time shorter than two minutes, thanks also to the perfor- mances of the telescope and in particular to its high sensitivity.
Such fast changes have never been observed be- fore in BL-Lacs, and may give rise to revisions of the preferred models for acceleration.
The rapid increase in the flux level was accompa- nied by a hardening of the differential spectrum on time scales of some 10 min.
In addition, having the information on the arrival time of each gamma from the flare, it is interest- ing to investigate on possible Quantum Gravity effects, as well as to obtain new constraints by this new kind of observations.
Some models of quantum gravity predict, in fact, an effect of energy-dependent arrival time, pro- ducing a delay between gammas, due to fluctu- ations of the space-time medium at the Planck time and distance scales.
It is therefore crucial to handle this data under
the assumption that the flare emission can be con- sidered synchronous at all the energies, with re- spect to the QG expected delays.
Through a detailed analysis of Mkn 501 flare, us- ing advanced statistical methods [18] and all the individual gamma-rays information, we obtain a dependence on energy of the arrival time, with the more energetic photons arriving later. This result can establish a lower limit on the Quan- tum Gravity mass scale: MQG>0.21·1018GeV, in case of a linear dependence of the delay from the energy, orMQG>0.26·1011GeV, in case this dependence would be quadratic. The limits are given at 95% of confidence level.
As already stressed, we cannot in principle ex- clude that this effect is due to an intrinsic delay in the emission from the source (thermal plasma effects are negligible, but we cannot exclude some other effect). In any case this is a confirm that MAGIC is sensitive enough to investigate such theorical propagation effects, by this kind of mea- surements.
2.4. 3C279 and the Extragalactic Back- ground Light
There is a background of low energy photons in the Universe, called extragalactic background light (EBL) [19], given by the light produced by stars and dust coming from all the galaxies.
This can give us fundamental informations on the cosmological structure formation, and is also responsible of the attenuation of gamma rays coming from the different gamma-emitters to the Earth, since they interact with them, by means of a pair production.
In the VHE range the deficit increases with the energy, and this is why only few AGNi have been detected by the previous generation of ground- based telescope (with a higher energy threshold).
Moreover, the absorption increases also with the distance of the source, since the probability to in- teract with an EBL photon gets higher. The new Cherenkov telescopes, built to decrease the en- ergy threshold, would instead operate in a range where this pair production is less severe, and would allow to detect blazars at higher redshift.
In this concern, MAGIC detected the gamma sig- nal coming from the active nucleus of the 3C279
quasar [20], at a redshift z = 0.536, more than twice the distance of objects previously observed with this kind of radiation. It was one of the brightest quasars seen by the EGRET instrument on board NASA’s Compton Gamma-Ray Obser- vatory.
The observation was done during an optical high state of activity (figure 2). The signal was strong only for a short period, during one night (with a weaker signal the night before), and extended over the energy range from 80 to more than 300 GeV. Its strength (at 100 GeV) was of the same intensity level as the Crab.
Figure 2. Light curves. MAGIC (top) and optical R-band data (bottom) obtained for 3C 279 from February to March 2006. The long-term baseline for the optical flux is at 3 mJy.
This was the first VHE gamma-ray emission from a flat spectrum radio quasar ever detected, since the emission above 100 GeV is in general expected to be quite faint. In addition, thanks
to that, the distance over which astrophysical objects can be observed at these energy range has now enlarged. In relation to this, it has been possible to give a more stringent constraint on the EBL density. Its spectrum now appears to be lower than what was expected by many models, leading to a Universe more transparent at cos- mological distances than believed.
3. Conclusions and the future
MAGIC observations of extragalactic TeV gamma-ray sources contributed to many physics insights, confirming the rich potential of gamma- astronomy. Most of these new sources could be actually discovered because of the low energy threshold of the telescope and its good sensitiv- ity.
The importance of a multiwavelength monitoring and observation of cosmological sources has been highlighted by the success of the M87 campaing, and by the detection of signals after an alert of other experiments.
The high MAGIC sensitivity allowed also to col- lect data from the Mkn 501 flare, with a time resolution of the order of 2 minutes. This gave the opportunity of a study of the arrival time of each event, giving a Quantum Gravity interpretation of the observed delay for events at high energy. If confirmed in the future, with other sources at dif- ferent redshift, this effect may be interpreted as a stringent limit on the acceleration models, and potentially, assuming a synchronous production, as a limit on the mass scale of quantum gravity.
Finally, the nature of EBL has been further inves- tigated thanks to the detection of a very distant source, 3C279.
Concerning the future of the experiment, a second MAGIC telescope has recently been constructed close to the first, and is entering its commission- ing phase. Among the other implementations, tipically driven by experience and some recent advances in technology, it presents: larger and lighter mirrors, a huge improvement of the DAQ system and a better engineering of the camera design.
By the stereo observation, we expect an increase in the sensitivity by at least a factor two or three (depending on the energy), and other improve- ments in the energy and direction reconstruction.
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