Diego F. Torres ∗ , Juan Cortina † and for the MAGIC collaboration ∗∗

Highlights of MAGIC observations of galactic sources

Diego F. Torres ^∗ , Juan Cortina ^† and for the MAGIC collaboration ^∗∗

∗ ICREA and Institut de Cienciès de l’Espai, IEEC-CSIC, E-08193 (Barcelona) Spain

† Institut de Física d’Altes Energies, Edifici Cn., E-08193 (Barcelona), Spain

∗∗ Updated collaborator list at

http://wwwmagic.mppmu.mpg.de/collaboration/members

Abstract. During its first cycle, the MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov) telescope was performing an observational campaign covering a total of about 250 hours on galactic sources. Here we review the results for the very high energy ( > 100 GeV) γ -ray emission from some of those sources. We focus on LS I +61 303 and PSR 1951+32.

THE MAGIC TELESCOPE

The Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescope is a very high energy (VHE) γ -ray telescope, operating in an energy band from 100 GeV to 10 TeV, exploiting the Imaging Air Cherenkov technique. Located on the Canary Island of La Palma, 2250 m above sea level, the telescope has a 17-m diameter tessellated parabolic mirror, and is equipped with a 3.5 ^◦ -3.8 ^◦ field of view camera. In this work we leave aside published results on SNRs ([1, 2]) and the Galactic Center ([3]) to showcase the released highlights on pulsars and X-ray binaries observations.

A HIGHLIGHT FROM PULSARS OBSERVATIONS

Based on results from EGRET, we know that pulsars have relativistic particles in their

magnetospheres, that emit γ -rays up to energies of several GeV. PSR B1951+32 is

a prime candidate for observation by ground based γ -ray detectors with low energy

thresholds: Its spin down age is ∼ 10 ⁵ years, i.e. about 100 times older than the Crab

pulsar. The magnetic field strength of 4 . 9 · 10 ¹¹ G is lower than in most rotation-powered

pulsars. It is therefore expected that the screening of γ -rays due to pair production in

the magnetosphere is reduced and the cutoff of the high energy emission subsequently

shifts to higher energies. For references and more detailed discussion on PSR 1951+32

see [4]. The pulsar is located in the core of the radio nebula CTB 80, thought to be

physically associated with the pulsar. The current tightest constraint on the > 100 GeV

emission from the pulsar and its nebula, obtained by the Whipple collaboration [5], puts

an upper limit of 75 GeV on the cutoff energy of the pulsed emission and an upper limit

of ≤ 1 . 95 × 10 ⁻¹¹ cm ⁻² s ⁻¹ on the > 260 GeV steady emission. In total 30.7 hours of

MAGIC data were processed. The zenith angle range of the observation was restricted

to between 5 ^◦ and 25 ^◦ , guaranteeing the lowest possible energy threshold. We searched

for steady γ -ray emission from the direction of PSR B1951+32 with different analysis thresholds between 140 GeV and 2 . 6 TeV. As no significant signal ( > 5 σ ) of γ -rays was found, we calculated upper limits on the number of excess events with a confidence level of 95%. The limits on the integral flux of γ -rays are shown in Figure 1 (left panel), together with the measurement of [5] and the predictions of [6]. MAGIC has also performed a search for pulsed γ -ray emission from PSR B1951+32 in 5 differential bins of reconstructed energy between 100 GeV and 2 TeV. To test for periodicity we applied the Pearson- χ ² -test, the H-Test and the Bayesian-Test (see [4] for details). No signature of pulsed emission was found in any of the energy intervals. From the result of the H-Test we calculated an upper limit on the number of excess events, from which we derived an upper limit on the cutoff energy of the pulsed emission. The measured spectrum of PSR B1951+32 at lower gamma-ray energies multiplied by an exponential cutoff of 32 GeV is shown as a solid red line in Figure 1 (right panel). The analysis threshold, 75 GeV, is marked with the red arrow in the figure.

Energy [MeV]

105 10⁶ 10⁷

]-1 s-2Integral Flux [ cm

10-15 10-14 10-13 10-12 10-11 10-10 10-9 10-8 10-7

MAGIC 95% C.L. Upper Limits Upper Limits (Srinivasan et al. (1997)) 3σ

Predicted PWN Emission (Bednarek & Bartosik (2003)) Crab

10% Crab 1% Crab

PSR B1951+32 / CTB 80

Energy [MeV]

102 10³ 10⁴ 10⁵ 10⁶ 10⁷

]-1 s-2 dN/dE [ MeV cm2E

10-6 10-5 10-4

C.L. Upper Limits (H-Test) MAGIC 2σ

MAGIC U.L. on cutoff energy (32 GeV) Whipple (Srinivasan 1997) EGRET (Fierro 1996) EGRET Spectrum + 75 GeV CutOff Polar Cap (Harding 2001) Outer Gap (Hirotani 2006b)

FIGURE 1. Left: Integral upper limits (95 % Confidence Level) on the steady γ -ray emission from the direction of PSR B1951+32. For comparison, the γ -ray flux of the Crab nebula. Right: Results of the analysis in the search for pulsed emission from PSR B1951+32. Upper limits are given with a 95 % confidence level. The upper limit on the cutoff energy from Whipple is shown as the dot dashed curve.

The upper limit on the cutoff of 32 GeV by MAGIC is shown as the solid red curve. The analysis threshold (75 GeV) is marked by the arrow on the X-Axis.

THE γ -RAY BINARY LS I +61 303

LS I +61 303 belongs, together with LS 5039 and PSR B1259-63, to a new class of

objects, the so-called γ -ray binary systems. LS I +61 303 is composed of a B0 main

sequence star with a circumstellar disc, i.e. a Be star, located at a distance of ∼ 2 kpc. A

compact object of unknown nature is orbiting around it, in a highly eccentric (e = 0 . 72 ±

0 . 15) orbit. LS I +61 303 was considered as one of the two microquasar candidates

positionally coincident with EGRET γ -ray sources although the large uncertainty of

the position of the EGRET source did not allow an unambiguous association. MAGIC

observations lasted 54 hours (after standard quality selection, discarding bad weather

data) between October 2005 and March 2006 [7]. The reconstructed γ -ray map is shown

in Figure 2. The data were first divided into two different samples, around periastron

passage (0.2-0.3) and at higher (0.4-0.7) orbital phases. No significant excess in the

number of γ -ray events is detected around periastron passage, whereas there is a clear detection (9.4 σ statistical significance) at later orbital phases. The distribution of γ -ray excess is consistent with a point-like source located at (J2000): α = 2 ^h 40 ^m 34 ^s , δ = 61 ^◦ 15 ^' 25 ^'' , with statistical and systematic uncertainties of ± 0 . 4 ^' and ± 2 ^' , respectively.

This position is in agreement with the position of LS I +61 303. In the natural case in which the VHE emission is produced by the same object detected at EGRET energies, this result identifies a γ -ray source that resisted classification during the last three decades. MAGIC measurements show that the VHE γ -ray emission from LS I +61 303 is

FIGURE 2. Smoothed maps of γ -ray excess events above 400 GeV around LS I +61 303. (A) Observa- tions over 15.5 hours corresponding to data around periastron (phase 0.2-0.3). (B) Observations over 10.7 hours at orbital phase 0.4-0.7. The position of the optical source LSI +61 303 (yellow cross) and the 95%

confidence level contours for two EGRET sources are shown. From Albert et al. [7].

variable. The γ -ray flux above 400 GeV coming from the direction of LS I +61 303 has a maximum corresponding to about 16% of the Crab nebula flux, and is detected around phase 0.6. The combined statistical significance of the 3 highest flux measurements is 8.7 σ , for an integrated observation time of 4.2 hours. The probability for the distribution of measured fluxes to be a statistical fluctuation of a constant flux (obtained from a χ ² fit of a constant function to the entire data sample) is 3 × 10 ⁻⁵ .

Acknowledgements.. We thank the IAC for the excellent working conditions at the ORM in La Palma. The support of the German BMBF and MPG, the Italian INFN, the Spanish CICYT is gratefully acknowledged. This work was also supported by ETH research grant TH-34/04-3, and the Polish MNiI grant 1P03D01028.

REFERENCES

1. Albert J. et al. [MAGIC Collaboration] 2006, ApJ 637, L41 2. Albert J. et al. [MAGIC Collaboration] 2006, ApJ 643, L53 3. Albert J. et al. [MAGIC Collaboration] 2006, ApJ 638, L101

4. Albert J. et al. [MAGIC Collaboration] 2007, astro-ph 0702077, submitted to the ApJ 5. Srinivasan R., et al. 1997, ApJ 489, 170

6. Bednarek W. & M. Bartosik M. 2003, A&A 405, 689

7. Albert J. et al. [MAGIC Collaboration] 2006, Science 312, 1771