This paper reports the first detection of the Crab pulsar above 100 GeV, thus placing stringent constraints on the gamma-ray radiation mechanisms and on the locations of the emission regions in the magnetosphere.
We report the detection of pulsed gamma-rays from the Crab pulsar at energies above 100 Gigaelectronvolts (GeV) with the VERITAS array of atmospheric Cherenkov telescopes. The detection cannot be explained on the basis of present pulsar models. The photon spectrum of pulsed emission between 100 Megaelectronvolts (MeV) and 400 GeV is described by a broken power law that is statistically preferred over a power law with an exponential cut-off. The observation is unlikely to be explained by curvature radiation as the primary origin of gamma rays above 100 GeV. Our findings require that these gamma rays be produced beyond 10 stellar radii from the neutron star.
Figures from paper (click to get full size image):
Figure 1: Pulse profile of the Crab pulsar. The shaded histograms show the VERITAS data. The pulse profile in panel A is shown twice for clarity. The dashed horizontal line in panel A shows the background level estimated from data in the phase region between 0.43 and 0.94. Panels B and C show expanded views of the pulse profile with a finer binning than in panel A and are centered at P1 and P2, which are the two dominant features in the pulse profile of the Crab pulsar. The data above 100 MeV from the Fermi-LAT (13) are shown beneath the VERITAS profile. The vertical dashed lines in the panels mark the best-fit peak positions of P1 and P2 in the VERITAS data. The solid black line in B and C shows the result of an unbinned maximum-likelihood fit of Gaussians to the VERITAS pulse profile (described in text). Note that from the figure no definite conclusion can be made about the alignment of the peak positions between the Fermi-LAT and VERITAS data from this figure because different ephemerides were used in the two analyses. Flaring, low-state and daily upper limits are shown superimposed on the Swift XRT light curve (Evans et al. 2007). The extent of the daily upper limit horizontal bars represents the approximate time interval during which the VERITAS observations were taken.
Figure 2: Spectral energy distribution (SED) of the Crab pulsar in gamma rays. VERITAS flux measurements are shown by the solid red circles, Fermi-LAT data (13) by green squares, and the MAGIC flux point (16) by the solid triangle. The empty symbols are upper limits from CELESTE (25), MAGIC (17), and Whipple (26). The bowtie and the enclosed dotted line give the statistical uncertainties and the best-fit power-law spectrum for the VERITAS data using a forward-folding method. The result of a fit of the VERITAS and Fermi-LAT data with a broken power law is given by the solid line and the result of a fit with a power-law spectrum multiplied with an exponential cut-off is given by the dashed line. Below the SED we plot χ2 values to visualize the deviations of the best-fit parametrization from the Fermi-LAT and VERITAS flux measurements.