V. Scapin a on behalf of the MAGIC collaboration 1

Recent results from the MAGIC Telescope

V. Scapin ^a on behalf of the MAGIC collaboration ¹

a

University of Udine & INFN Trieste, via delle scienze 208, 33100 Udine, Italy

Abstract

Ground-based gamma-ray astronomy is part of a new field of fundamental research of Astroparticle Physics and recently made spectacular discoveries (1). The MAGIC Telescope is a new generation Imaging Atmospheric Cherenkov Telescope (IACT) at La Palma, Canary Islands, Spain (28.3

N, 17.8

W, 2240 m asl). Currently MAGIC is the world’s largest single-dish IACT (with a 17 m diameter), and it has a trigger energy threshold of 50 GeV. The experiment studies sources that, already known in several other energy bands (i.e. gamma-ray, X-ray, radio and/or optical frequencies), are expected to be very high energy (VHE) gamma-rays emitters. Since its operation start in 2004 eight Galactic and eleven extragalactic sources have been detected.

Here I present some highlights from the most recent results in 2006/07.

Key words: Gamma–astronomy, MAGIC, VHE gamma–rays emitters

1 Introduction

MAGIC is an Imaging Atmospheric Cherenkov Telescopes (IACT) (2). Since the beginning of its operation –2004– MAGIC made important discoveries in the VHE domain.

2 High frequency peaked Blazars

MAGIC has been performing a systematic study of TeV sources in the north- ern sky. Out of 14 sources observed, 1ES 1218+30.4 was seen for the first time at very high energies (3). The source 1ES 2344+51.4 was confirmed with high

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Preprint submitted to Elsevier 3 January 2008

significance in its so-called low state (4). The high redshift source (z=0.212, second highest redshift VHE γ-ray emitting object) 1ES 1011+49.6, was de- tected with a significance of 6.2 σ within 18.7 hours (5). The BL Lac object PG 1553+113 was detected at 8.8 σ in 18.8 hours of observation during 2005 and 2006 (6). The well-known blazars Mkn 421 and Mkn 501 were studied extensively: for Mkn 421, the energy spectrum was measured down to below 100 GeV, showing a clear indication of an inverse Compton peak around 100 GeV (7). In contrast to previous observations, day-scale time variability was found. A clear correlation (r=0.6) between γ-ray and X-ray fluxes was seen - but no correlation with optical data.

3 Evidence for QG from an analysis of a Flare from Mkn 501?

It is widely speculated that space-time is a dynamical medium, subject to quantum-gravitational (QG) effects that cause the fabric of space-time to fluc- tuate on the Planck time and distance. In MAGIC, observations of Mkn 501 showed flux variations by an order of magnitude, and short-term fluctuations were even seen with flux-doubling times down to two minutes during two nights. Such fast changes have never been observed in BL-Lacs, and may give rise to revisions of the preferred models for acceleration. A dependence on γ energy of the arrival time inside the flares was found in a detailed analysis (8).

This gave rise to speculations about a possible sign of profound consequence as some models of quantum gravity predict an effect of exactly that kind.

4 Detection of VHE γ–ray emission from BL Lac

MAGIC observed the archetypical BL Lacertae for 22.2 hr during August through December 2005 (during an optical high state), and for 26 hr during July through September 2006 (9). This source is the best known representative of the blazar sub-class of LBL (low-frequency peaked BL Lacertae) objects. A VHE γ–ray signal was discovered in the 2005 data, with a flux corresponding to approximately 3% of the Crab Nebula flux (a standard VHE γ –ray candle) in the same energy band. The differential VHE spectrum is rather steep; no significant short-term variability. For the first time VHE γ–ray emission was clearly detected from an LBL source, with a signal below previous upper limits. In the year 2006 data no significant signal was detected - a drop that corresponds to the observed decline in the optical activity.

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5 Gamma Ray Bursts (GRBs)

Observation of GRBs in the VHE domain will provide important information on the physical conditions in GRB outflows. The MAGIC telescope is the best suited IACT for these observations. Thanks to its fast repositioning time and low energy threshold, MAGIC is able to react promptly when triggered by an alert from the GRB Coordinates Network(GCN), to search for prompt and early afterglow emission from GRBs. MAGIC can slew to any position in the sky in < 100 s (on average within 45 s). In the last two years of op- eration the MAGIC telescope reacted to 24 of the Swift BAT alerts (suitable by sky-location and zenith angle) and carried out nightly observations that correspond to an 8.3% duty cycle and a mean observation frequency of about one GRB for month (10). Unfortunately, until now , no evidence for gamma signals was found. The threshold for the observations was between 80 and 200 GeV, depending on the zenith angle of the burst.

6 X-ray binaries: LS I +61 303 and Cygnus X-1

Binary systems are characterized by compact object (black hole, neutron star) accompanied by an orbiting companion star. Microquasars are binary systems with relativistic radio-emitting jets. Be/X-ray binaries are characterized by a different emission mechanism not involving a jet. They show periodic accre- tion episodes of matter from the companion by the compact object at perias- tron passage. MAGIC reported the detection of variable, most likely periodic gamma-ray emission from the Be/X-ray binary LS I +61 303 (12). Six orbital cycles (the known period is 26.5 days) were followed in 2005 and 2006. In all measurements a significant γ–ray signal was observed around phase 0.6-0.7.

Cygnus X-1 is the best established candidate for a stellar mass black-hole, a confirmed microquasar, and one of the brightest X-ray sources in the sky. It was observed by MAGIC during 26 nights in 2006 (11). No steady gamma-ray signal was seen, but a marginal significant signal was observed over a period of 80 minutes in one night, nearly coincide with an X-ray flare seen by sev- eral satellite detectors. This is the first experimental evidence of VHE emission from a stellar-mass black hole, and therefore from a confirmed accreting X-ray binary.

7 Supernova CasA and IC443

The supernova remnant Cassiopeia A was detected, for the first time below TeV energies, with high significance by MAGIC (13). The photon spectrum

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is softer than expected from lower-energy extrapolations. The source is point- like within the angular resolution of the telescope. A new source of very high energy gamma–ray emission was discovered close to the Galactic Plane, spa- tially coincident with SNR IC443 (14). These results place constraints on the emission mechanism, suggesting the presence of a dominant hadronic channel.

8 Discovery of VHE γ–ray emission from the Radio Quasar 3C 279

The quasar 3C279 is one of the best-studied flat spectrum radio quasars. It is located at a comparatively large redshift of z=0.536: E>200 GeV observa- tions of such distant sources were until recently impossible, due to both the expected steep spectrum and the attenuation of the VHE γ-rays by the inter- galactic background light (IBL). The MAGIC Telescope observed 3C279 in early 2006. MAGIC detected the significant VHE signature from this source on the observation night of 2006 February 23 (15). In several ways this is an important discovery:

• it enlarged the distance over which objects can be observed at VHE fre- quencies,

• for the first time VHE γ-rays from a flat-spectrum radio quasar were de- tected, and

• a precise measurement of the source’s energy spectrum may give strong constraints on the IBL density.

References

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[2] M. Doro, These Procs., 2008.

[3] J. Albert et al., ApJ Letters 642: L119,2006.

[4] J. Albert et al., ApJ. 662: 892,2007.

[5] J. Albert et al., Astrophys. J. Lett. 667: L21,2007.

[6] J. Albert et al., ApJ Letters. 654: L119,2007.

[7] J. Albert et al., ApJ. 663: 125,2007.

[8] J. Albert et al., Submitted to Phys.Rev.Lett., astro-ph/0708.2889, 2007.

[9] J. Albert et al., ApJ Lett. 666: L17,2007.

[10] J. Albert et al., ApJ 667: 358, 2007 [12] J. Albert et al., Science 312: 1771, 2006.

[11] J. Albert et al., Astrophys. J. Lett. 665: L51, 2007.

[13] J. Albert et al., Astron. Astrophys. 474: 937, 2007.

[14] J. Albert et al., ApJ Lett. 664: L87, 2007.

[15] M. Teshima, E. Prandini et al., Procs. ICRC Conf., 2007, Merida, Mexico.

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