R. De los Reyes for the MAGIC collaboration

(1)

1

Recent results on galactic sources with MAGIC telescope

R. De los Reyes for the MAGIC collaboration

^a

a

Universidad Complutense de Madrid, Madrid, Spain

Located at the Canary island of La Palma, the single-dish MAGIC telescope currently has the lowest energy threshold achieved by any Cherenkov telescope, which can be as low as 25 GeV. In the last two years, the MAGIC telescope has detected a significant amount of galactic sources that emit at very high energies (up to several TeV). Here we present the most recent results that have yielded important scientific highlights in astrophysics, which include the first detection of gamma-ray emission from a pulsar, an X-ray binary system and a stellar-mass black hole. We also make a review of the latest results of the MAGIC observations on galactic sources, which will include also EGRET unidentified sources, the Galactic Centre, pulsars and supernova remnants.

1. Introduction

In the latest years, ground-based Cherenkov telescopes have opened a new window in the ob- servation of our Universe, due to the high sensi- tivity of the new generation of telescopes. This new window observes the very high energy (VHE) γ-rays which are photons with energies from 10 GeV up to 100 TeV. The understanding of the production, acceleration and transport of these high energy photons is the main objective of γ- ray astronomy, which will allow us to definitely understand the physical processes that take place in the astronomical γ-ray emitting objects. One of the main objectives of the physical program of the MAGIC telescope is the study of the origin of the cosmic rays and the physical processes that produce γ-rays in our Galaxy. This article will summarize the results of the MAGIC telescope on galactic sources in the last years (2004-2008).

2. The MAGIC telescope

The MAGIC telescope [11] is the largest single- dish Imaging Air Cherenkov Telescope (IACT) currently operating. It is located at the Roque de los Muchachos observatory (28.8

^◦

N, 17.8

^◦

W, 2.2 km a.s.l) in Canary Island of La Palma. The tele- scope has a 17-m diameter tessellated parabolic mirror, supported by a light-weight carbon fiber frame which allows the telescope to reposition in less than 40 seconds. Its 3.5

^◦

FoV camera

is equipped with 577 enhanced high-quantum- efficiency photomultiplier tubes (PMTs). These modified PMTs allow the observations during moonlight and twilight which increases the tele- scope duty cycle by up to 20% and 12%, respec- tively. The analogue signal from PMTs is trans- ported via optical fibers to the nearby electronic room, which hosts the trigger system and the 2 GHz FADC system [19]. Operating with the standard 2-level trigger the telescope reaches an energy threshold of 50 GeV and a sensitivity of 1.6% the Crab flux in 50 hours. The development of a new trigger system [22], the so called SUM- trigger, designed specifically for the detection of low energy γ-rays [15] has allowed the MAGIC telescope to reach the lowest energy threshold ( ∼ 25 GeV) ever achieved by any Cherenkov tele- scope up to date.

The data analysis uses the standard MAGIC analysis and reconstruction software [14]. In the first stage of the analysis, the FADC signal is cal- ibrated [17]. After this, the calibrated image is cleaned from noise applying image-cleaning tails cuts [18] and parameterized by the so-called Hillas image parameters [20]. The standard method used in MAGIC for the γ/hadron separation, as well as for the estimation of the γ-ray energy, is the Random Forest [12,13]. With this method the energy resolution achieved is around 20-30%.

An Active Mirror Control (AMC) system and the

estimation of γ-ray arrival direction through the

(2)

2 DISP-method [16] allows an angular resolution of

∼ 0.1

^◦

, while the pointing uncertainty has a sys- tematic error of 1’ [6]. The differential spectrum is corrected for the instrumental energy resolu- tion yielding an slope resolution of 35% and a systematic error in the spectral index of 0.2 [6]

for a typical Crab-like spectra.

3. Galactic sources

VHE gamma-rays from our Galaxy are pro- duced by different mechanisms: one is the in- teraction of cosmic rays with the magnetic field (electron acceleration) or matter (proton accel- eration) present in our galaxy or in the source surroundings. Another one is the transfer of the electrons kinetic energy to the background pho- tons (CMB, IR, UV photons). The emission at VHE depends on the energy of the cosmic rays which can be accelerated in shocks like relativis- tic winds (from pulsars or young stars), shell- type SNRs or pulsar magnetospheres. In the last years MAGIC has discovered emission at high en- ergies from IC443, the Crab pulsar, the X-ray bi- nary LSI +61 303 and Cygnus X-1. It has also confirmed the γ-ray emission from 4 SNRs, the Galactic Center and the unidentified high energy source TeV J2032+4130, and gives upper limits to the pulsar and SNR PSR B1951+32/CTB 80, the X-ray binary Cygnus X-3 and a possible new kind of γ-ray source: the WR stars.

3.1. LSI +61

^◦

303 LSI +61

^◦

303 is a X-ray binary system com- posed by a Be star orbiting an unknown object with an orbital period of 26.4950 days. MAGIC observed this source for a total of ∼ 166 hours during different periods. The result was the de- tection of point-like VHE emission in the orbital phase φ=0.65 coincident with the EGRET γ-ray source 3EG J0241+6103 and confirmed by VERI- TAS. No significant excess where detected around the periastron passage (Figure 1). The emission detected is consistent with a power law flux with slope -2.6 ± 0.2 between 200 GeV and 4 TeV with a maximum of 16% of the Crab flux [1] and con- stant within 30-75 min. An analysis of the peri- odicity of the VHE emission during several orbital

cycles results on an estimation of a 26.8 ± 0.2 days period above 400 GeV with a post-trial probabil- ity of 4.4x10

⁻⁷

[7], compatible with the orbital period observed at other wavelengths. Trying to understand the nature of this source, some of the observations were part of multi-wavelength cam- paigns [8]. The results show a clear X/γ-ray cor- relation while the radio emission is uncorrelated with the emission at higher energies. This points to a different particle population producing the high-energy emission and the radio emission, this lat one confined into a region smaller than 6 mas.

Figure 1. Smoothed sky map of LSI +61

^◦

303 above 400 GeV around the periastron passage (left) and at orbital phase 0.4-0-.7 (right) when the maximum emission is observed [1]. The green contours correspond to the 95% c.l. of 3 EG J0229+6151 and 3EG J0241+6103.

3.2. Cygnus X-1

Cygnus X-1 is the first evidence of VHE emis-

sion from a well established black hole X-ray bi-

nary (BHXB). This binary system consists on a

stellar black hole of 21M

_#

orbiting each 5.6 days a

40M

_#

O9.7 star. For the whole observation time

(154 min.), MAGIC detected a marginal 3.2σ

point-like excess consistent with the position of

Cygnus X-1. A search of variability yielded a flare

detection at significance of 4.1σ after post-trial

correction for 79 minutes of observation time, cor-

responding to an orbital phase of 0.91 [3]. The

flux during the flare was consistent with a power

(3)

3 law of slope -3.2 ± 0.6 above 150 GeV. This flare is correlated with an X-ray flare seen by IN- TEGRAL, Swift and RXTE 1-2 hours after the MAGIC maximum. These results lead to the hy- pothesis of an emission produced at different re- gions of an aligned mas jet: the X-rays at the jet base while the γ-rays could be produced in a jet interaction with the stellar wind.

3.3. IC 443

IC 443 is a 45’ diameter asymmetric shell- type SNR detected in radio, γ-rays and X-rays.

However, the emission from the X-ray pulsar CXOU J061705.3+222127 is not consistent with the high-energy emission detected by EGRET and MAGIC. MAGIC observations ( ∼ 29 hours) resulted in a point-like excess of 5.7σ signifi- cant (MAGIC J0616+225) centred at (α,δ) = (6

^h

16

^m

3

^s

,+22

^◦

31’48”) [5], coincident with the position of the SNR IC 443 and a CO emission region. This suggests a possible origin for the VHE γ-rays due to π

⁰

decays from interactions between cosmic rays accelerated in IC 443 and the dense molecular cloud.

3.4. CrabNebula

The SNR Crab Nebula is one of the best stud- ied non-thermal objects and the standard can- dle at high-energies. The MAGIC telescope has measured for the first time its spectrum above 60 GeV [6]. The steady flux resulting from these ob- servations fits to a power law above 500 GeV, con- sistent with previous VHE measurements, while it seems to be a change on the pure lower law spectrum for low energies fitted to a slope -2.31- 0.26log(E/300GeV). This softening in the spec- trum has yielded an estimation of the SED peak, due to the inverse Compton process, at E=77 ± 47 GeV. The point-like excess measured by MAGIC is consistent with the pulsar (PSR B0531+21) position and within the optical synchrotron neb- ula. Although the observations support the syn- chrotron self-Compton model, other models in- volving bremsstrahlung and π

⁰

decay processes can not be excluded.

3.5. PSR B0531+21 - Crab pulsar

The central engine of the Crab Nebula is the Crab pulsar (PSR B0531+21). It is a compact

neutron star with a rotational period of 33 ms which emits beamed radiation detected from ra- dio to gamma-rays below 10 GeV. The analysis of the pulsed emission of previous MAGIC ob- servations of the Crab Nebula [6] resulted in a no detection of γ-rays above 180 GeV coming from the pulsar magnetosphere. However, re- cent improvements in the trigger system of the MAGIC telescope [22] have decreased MAGIC’s energy threshold down to 25 GeV. This has made possible the first detection of the pulsar emission at high-energies with Cherenkov telescopes [10].

New MAGIC observations ( ∼ 22 hours) with this new system have yielded the detection of the Crab pulsar at 6.4σ of significance (Figure 2) in the same phase bins as the EGRET detected photons and simultaneously with MAGIC-optical data [21]. The resulting spectrum shows a pref- erence for an exponential cut-off energy at ∼ 16 GeV ( E

0

= 21 GeV in case of a super-exponential behaviour). These results contradict the basic features of the polar-cap model [10].

Figure 2. Light curve of Crab pulsar for E > 35 GeV corresponding to a χ

²

-Test for null hypothe- sis equivalent to a 6.4σ rejection. The grey bands correspond to the EGRET signal region P1 (φ=(- 0.06,0.04)) and P2 (φ=(0.32,0.43)) [10].

3.6. Cassiopeia A

This shell-type SNR (of ∼ 4’ diameter) is a

bright source of synchrotron radiation detected

from radio to γ-rays. 47 hours of MAGIC

(4)

4 observations have confirmed HEGRA detection above 1 TeV within 0.13

^◦

of its position with- out flux variability within the 8 years gap be- tween MAGIC and HEGRA observations. This source was detected above 250 GeV an excess, with a significance of 5.2σ yielding a flux estima- tion consistent with a pure power law with a slope of -2.4 ± 0.2 at a level of 3% the Crab flux [4]. Al- though the detection of VHE γ-rays provide evi- dence of the particle acceleration of TeV particles in SNR shocks, the current observations do not discriminate between leptonic and hadronic sce- narios in multi-zone models as the responsible of the high-energy emission.

3.7. TeV J2032+4130

This unidentified VHE source was first detected by HEGRA without any counterpart at other wavelengths. Likely Galactic, it is coincident with the Cygnus OB2 core. MAGIC observed this source, centred at HEGRA position, for ∼ 94 hours yielding a total significance of 5.6σ for a source extended up to ∼ 5’ centred at (α,δ) = (20

^h

32

^m

20

^s

,41

^◦

30’36.0”) [9]. The total flux mea- sured above 500 GeV fits to a power law of slope -2.0 ± 0.3 at a level 1.6% of the Crab flux. This result is consistent with a steady flux due to the no significant variability observed during the 3 years gap between HEGRA and MAGIC observa- tions. The TeV emission detected is compatible with both leptonic and hadronic scenarios.

3.8. Galactic center

The Galactic Center region contains different objects which could emit γ-rays: regions of star formation with more than 100 OB stars and gas, young SNRs, non-thermal radio arcs and, moreover, the dynamical centre of the Galaxy, the compact radio source SgrA

^∗

which is be- lieved to be a massive black hole. Recent ob- servations (EGRET, HESS, VERITAS, CANGA- ROO...) have confirmed the Galactic Center as an important region for high-energy processes.

MAGIC observed the Galactic Center for a to- tal of 24 hours, detecting a point-like excess of 7.3σ significance above 1 TeV centered at (α,δ) = (17

^h

45

^m

20

^s

,-29

^◦

2’), which is spatially consistent with SgrA

^∗

, SgrA East (a supernova remnant)

and the candidate PWN G359.95-0.04 [2]. The γ-ray flux above 1 TeV is fitted to a power law of slope -2.2 ± 0.2 at a level of 13% of the Crab flux.

This is consistent with all the observations car- ried on by different experiments during the last 2 years, which seems to indicate an steady high- energy source, possibly an SNR or PWN and not the central back hole. However, the nature of the VHE γ-rays has not been yet identified.

REFERENCES

1. Albert, J. et al, 2006, Science, 321, 1771.

2. Albert, J. et al, 2006, ApJ, 638, L101.

3. Albert, J. et al, 2007, ApJ, 665, L51.

4. Albert, J. et al, 2007, A&A, 474, 937.

5. Albert, J. et al, 2007, ApJ, 664, L87.

6. Albert, J. et al, 2008, ApJ, 674, 1037.

7. Albert, J. et al, 2008, arXiv:0806.1865.

8. Albert, J. et al, 2008, ApJ, 684, 1351.

9. Albert, J. et al, 2008, ApJ, 675, L25.

10. Aliu, E. et al, 2008, Science, 322, 121.

11. Baixeras, C. et al, 2004, NIM A, 518, 188.

12. Bock, R. K. et al, 2004, NIM A, 516, 511.

13. Breiman, L., 2001, Machine Learning, 45, 5.

14. Bretz, T. & Wagner, R., 2003, Proc. 28th ICRC, Tsukuba, 2947.

15. De los Reyes, R. & de Ona-Wilhelmi, E., 2004, Low energy γ-ray detection (E < 30 GeV): effect of Earth’s magnetic field and a novel trigger technique, Proc. 19th ECRS, Florence.