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The detection of neutron stars through gamma-ray pulsations is a key science goal for CTA. Gamma-ray pulsar observations at high energies (over a few tens of GeV) could help to understand the region where pulsed emission takes place by

comparing the measured spectra with predictions by theoretical models.

The Fermi mission has revolutionized the study of gamma-ray pulsars detecting more than 117 sources in the MeV-GeV energy range [157], whose spectra are reasonably fitted with exponential cutoff values between 0.7 to 7.7 GeV.

Nonetheless, the detection of the Crab pulsar above 25 GeV with IACTs [25, 26] has reframed the exponential cutoff observed by Fermi in favor of a broken power- law shape that extends the pulsed emission up to 400 GeV. This recent discovery motivates the need for further pulsar studies in the VHE regime.

To place this situation in context, Fig. 4.7 shows the spectral fits (power-law with exponential cutoff) for 46 Fermi pulsars taken from the 1FGL, in comparison with the standard CTA differential sensitivity curve for the Prod-1 configuration “B” (5 LSTs) in 50h. The fits of Vela, Crab and Geminga pulsars are indicated explicitly, while the shaded area contains the fits of the remaining 43 pulsars.

Figure 4.7: Fermi -LAT pulsars general profile (grey area) with Prod-1 CTA sensitivity curve for configuration “B” in 50h of observation. Vela, Geminga and Crab show ex- trapolated SEDs (dashed lines). Note the CTA sensitivity curve is shown for reference, as it does not account for the background reduction applied within pulsars analysis. Also note that a curve below the differential sensitivity curve can still be detected if the integral flux is high enough.

Initially, a 50 h simulated observation of the Crab pulsar is generated using CTAmacros, modified by the author of this work to properly estimate the sensi-

tivity for pulsed emission (with a duty cycle of 10%). Total emission (P1 + P2) and both P1 and P2 peaks were simulated using the MAGIC power-law fits given in [27]. CTA candidate layouts “B”, “E” and “C” were tested. Fig. 4.8 shows the estimated Crab pulsar spectrum using Prod-1 configuration “B”, were both total signal and resolved peaks are well characterized.

Figure 4.8: Simulated Crab pulsar SED within 50 h of observation with the CTA Prod-1 configuration B. Each of the two Crab phase peaks and the total spectrum are shown, using MAGIC power-law fits given in [27]. Generated using a modified version of CTAmacros.

Results show that the CTA potential for pulsar detection seems encouraging. It will be able to reveal the extent of the Crab pulsed emission up to at least 1 TeV. In fact, the bare detection of the pulsations would take less than one hour.

To explore the detectability of Fermi pulsars in the power-law scenario, their spectra are extended above the cutoff energy with a power-law tail that assumes the same spectral index (β) as the one found for the Crab, when a broken power- law is applied to fit both Fermi-LAT and VERITAS detections, i.e β = 3.52 [26]. The final extrapolated spectral shapes for 3 out of the 46 pulsars considered are shown in Fig. 4.7.

As described in Sec. 4.1.3, a 90% background reduction is considered assuming a pulsed duty cycle of 10%, systematic errors of 5% and standard detectability conditions (S > 5σ in 50 hours of observation time). No gamma-ray emission from a pulsar wind nebula was considered.

Figure 4.9: Pulsars detectable by the future CTA in 50 hours of observation time assuming their spectra are extrapolated with the Crab pulsar power-law index (G=3.57, from [27]). Prod-1 layout “B” was used.

With such hypothetical (except for the case of the Crab pulsar) additional power-law tails all 46 pulsars were then considered as targets for 50 h observations with the CTA configurations: “B”, “C” and “E”. We found that 20 pulsars would be detected with the configurations “B” and “E”. This number reduces to 12 for the configuration “C”. This indicates that configurations “B” and “E” are better suited for pulsar studies than “C” (due to the higher number of LSTs). Fig. 4.9 shows how the detectability with configuration “B” depends on the exponential cutoff energy value (as determined by Fermi -LAT) and the photon flux density at this energy. In conclusion, it seems that under the hypothesis of the existence of the VHE Crab-like energy tails, a large fraction (up to ∼ 40% for configuration “B” and “E”) of the presently known brightest Fermi pulsars might be detectable with CTA.

On a second step, the possibility of power-law tails with different slopes was investigated. To test such cases, broken power-law spectral shapes were used, in the form proposed by VERITAS [26]. The key parameters in this form are: the break energy E0, the slope α of the photon flux spectrum in the Fermi -LAT range

well below E0 and the slope β of the photon flux spectrum in the VHE tail, i.e. well

Figure 4.10: Pulsars detectable by the Prod-2 “2A” layout in 50 hours assuming their spectra are extrapolated with a variable power-law tail index. Several site locations of different altitudes were tested: “Aar” (1640 m), “Leoncito” (2662 m) and “SAC” (3600 m).

Using the previous conditions, detectability was tested for Prod-1 configurations. As expected, configuration “B” is the optimal one for all the possible values of β; the second best is configuration “E” and the worst one is “C”.

To study the impact of the construction altitude, Prod-2 “2A” layout was also used for the 3 sites previously introduced. The studied β values, shown in Fig. 4.10, range between 2 (very hard VHE emission) and 6.5 (very soft VHE emission, almost identical to the exponential cut-off from the 1FGL). High construction altitudes would allow the detection of more pulsars if their emission was consistent with an exponential cut-off, while lower site altitudes would increase the detected VHE γ-ray pulsars population if they had behaviours similar to the one of the Crab pulsar in the VHE range.

Needless to say, there is no assurance that γ-ray pulsars will cooperate in the way described above. However, some theoretical models of young and energetic pulsars as well as old millisecond pulsars speak in favor of pulsed spectral compo- nents located in the VHE domain [158]. CTA will be the only facility in the near future capable of solving this problem.