Abstract
Context. Because of the relatively broad angular resolution of current gamma-ray instruments in the MeV–GeV energy range, the photons of a given source are mixed with those coming from nearby sources or diffuse background. This source confusion seriously hampers the search for pulsation from faint sources. Aims. Statistical tests for pulsation can be made significantly more sensitive when the probability that a photon comes from the pulsar is used as a weight. However, computing this probability requires knowledge of the spectral model of all sources in the region of interest, including the pulsar itself. This is not possible for very faint pulsars that are not detected as gamma-ray sources or whose spectrum is not measured precisely enough. Extending the event-weighted pulsation search to such very faint gamma-ray sources would allow improving our knowledge of the gamma-ray pulsar population. Methods. We present two methods that overcome this limitation by scanning the spectral parameter space, while minimizing the number of trials. The first one approximates the source to background ratio yielding a simple estimate of the weight while the second one makes use of the full spatial and spectral information of the region of interest around the pulsar. Results. We tested these new methods on a sample of 144 gamma-ray pulsars already detected by the Fermi Large Area Telescope data. Both methods detect pulsation from all pulsars of the sample, including the ones for which no significant phase-averaged gamma-ray emission is detected.
Highlights
One of the important contributions of the Fermi Large Area Telescope (LAT; Atwood et al 2009) to gamma-ray astronomy is the detection of many pulsars
There are two possible approaches: (1) use the spatial and spectral description of all sources in the region of interest (RoI); (2) take into account the full point spread function (PSF) information. While the former requires precise RoI modelling including spectral fits, the latter can be achieved by a minimal change of the weight definition, as it is a consequence of a refinement of the PSF definition
Test μw = μg, the position of the maximum of the Gaussian passing through the three points μ1 − 0.5, μ1, μ1 + 0.5; The resulting pulsation significance of the simple method, corrected for the 6 trials, is: One of the new features of Pass 8 (Atwood et al 2013), the latest version of the reconstruction and selection of LAT data, is the introduction of the PSF event types
Summary
One of the important contributions of the Fermi Large Area Telescope (LAT; Atwood et al 2009) to gamma-ray astronomy is the detection of many pulsars. Prior to Fermi’s launch, a pulsar timing consortium (PTC) was organized to support LAT observations of pulsars by providing accurate pulsar ephemerides (Smith et al 2008) Another way of finding gamma-ray pulsars is to look at LAT catalog unassociated sources with a pulsar-like spectrum Current event-weighted methods to search for pulsation cannot be applied to very faint gamma-ray sources. The challenge of extending the event-weighted methods to very faint pulsars consists of minimizing the number of trials when scanning the spectral parameter space, while maximizing the pulsation sensitivity, that is, getting as close as possible to the spectral parameters yielding the optimal weights. 4, we describe a more complex method that takes advantage of the complete knowledge of the RoI and the full capabilities of the instrument Both methods are tested on a sample of 144 known LAT pulsars.
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