Pulsar observations with the MAGIC Telescope
M. López ∗ , N. Otte † , E. Aliu ∗∗ , W. Bednarek ‡ , J.L Contreras ∗ , K. Hirotani § , M.
Rissi ¶ and for the MAGIC Collaboration "
∗ Univ. Complutense de Madrid, Spain
† MPI Munich, Germany
∗∗ IFAE, Spain
‡ Lodz University, Poland
§ ASIAA, Taiwan
¶ ETH Zurich, Switzerland
" http://magic.mppmu.mpg.de/collaboration/members
Abstract. Pulsars were detected by EGRET up to energies below 20 GeV. Observations at higher energies with ground based experiments so far failed to detect pulsars, indicating a sharp cutoff of the pulsed emission. Here we present the results of the search for very high γ-ray emission from the pulsar PSR B1951+32.
INTRODUCTION
MAGIC [1] is the largest air Cherenkov telescope worldwide with a reflector of 17 m. It is located at the Roque de los Muchachos observatory (La Palma, Spain) at 2250 m above sea level. The current trigger threshold is about 50 GeV, well suited to perform pulsar studies. In addition, a special pixel located at the camera center has been developed to detect optical emission from pulsars to perform correlation studies [2]. Several pulsars have been already observed by MAGIC for few hours during the first observation campaigns. Some of them, as PSR J0218+4232 or PSR J1856+0113, are candidates to γ -ray pulsars, and others like Crab, Geminga and PSR B1951+32, have already been detected in γ - rays. In this work we focus on the latest results obtained with MAGIC from the observations of PSR B1951+32 [3].
OBSERVATIONS OF PSR B1951+32
PSR B1951+32 (rotation period 39 ms) is one of the 7 pulsars firmly detected in γ -rays, and the the only one detected by EGRET up to energies of ∼ 20 GeV, with an spectrum which does not show evidence of a turnover. The pulsar is surrounded by the radio nebula CTB 80, which could be also a source of high-energy γ -rays. Assuming that high energy leptons can accumulate for long periods of time in the nebula, in [4] it is predicted a flux above 200 GeV from the nebula at a level of ∼ 4 . 4% of the Crab flux, a factor 2 bellow previous upper limits reported by Whipple.
The observations of PSR B1951+32 with MAGIC were performed between July and September 2006, at low zenith angles. A total 31 hours of data were selected for the present analysis.
Search for steady emission
We search for steady γ -ray emission from the association PSR B1951+32/CTB 80. As no significant signal ( > 5 σ ) of γ -rays was found, we calculated upper limits at different energies. The limits on the integral flux of γ -rays for a 95% confidence level are shown in Fig. 1 (left panel) together with results by other experiments. The upper limits are below the flux predicted by the time dependent model of [4].
We explored also the possibility of an extended and/or dislocated emission region of γ -rays around the position of
the pulsar (the latter scenario due to the pulsar proper motion). Sky maps with the reconstructed direction of γ -rays
were obtained. The maps in Fig. 1 (right) show the significance calculated in bins of (0.1x0.1) degrees 2 for events with
energies above 200 GeV, and the derived upper limits on the integral γ -ray emission. From this study we can exclude
steady γ -ray emission on the level predicted by the model of [4], which we would have detected if a) the emission
originates from within a circle of radius 0 . 4 ◦ centered at the pulsar position, and b) the apparent emission region is restricted to lest than 0 . 3 ◦ in diameter. Nevertheless, that model depends on free parameters. For instance, a value for the magnetization parameter much larger than the one assumed in it would suppress the inverse-Compton γ -ray flux.
DEC [deg]
32.2 32.4 32.6 32.8 33 33.2 33.4 33.6
-3 -2 -1 0 1 2 3
PSR B1951
+
PSF
RA [h]
19.82 19.84 19.86 19.88 19.9 19.92 19.94
Significance DEC [deg]
32.2 32.4 32.6 32.8 33 33.2 33.4 33.6
2 3 4 5 6 7
PSR B1951
+
RA [h]
19.82 19.84 19.86 19.88 19.9 19.92 19.94
Upper Limit on the Flux > 200 GeV [ x10 photons sec cm ]-1-2-12
FIGURE 1. Left: Integral upper limits on the steady γ -ray emission from the direction of PSR B1951+32. Right: Significances in bins of (0.1x0.1) deg 2 around the pulsar position and derived upper limits (95% confidence level) on the integral γ -ray emission.
Search for pulsed emission
To search for the presence of a pulsed signal at the known pulsar frequency, the event arrival times were transformed to the solar system barycentre, and folded to the pulsar frequency (using the ephemeris provided by [5]). The data set was divided in different bins of reconstructed energy, and for each of them, the resulting light curve was tested against uniformity. For instance, Fig. 2 (left) shows the light curve for energies ≤ 180 GeV. No signature of pulsed emission was found in any of the energy intervals. The corresponding 95% confidence level upper limits are shown in Fig. 2 (right). The limits were calculated using the H-test, and assuming a 36% duty cycle for the pulsar light curve.
To constraint the energy cutoff of the pulsar, all the events with energies above 75 GeV are analyzed together, obtaining an upper limit of 4 . 3 · 10 −11 cm −2 s −1 . The power law spectrum measured by EGRET was multiplied with a simple exponential cutoff and convoluted with the effective collection area of the telescope. The cutoff values were changed interactively until matching the obtained upper limit. The highest cutoff compatible with our upper limits is E < 32 GeV, constraining the pulsar cutoff to a narrow energy region. Polar cap models predict a cutoff within the allowed region derived from our results. Conversely, the upper limits obtained are already below the upper boundary of possible inverse-Compton emission at Tev energies expected from current outer gap models.
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)
