[Paper Review] Pulsar Emission above the Spectral Break - A Stacked Approach

A. McCann|arXiv (Cornell University)|Mar 5, 2015

Astrophysics and Cosmic Phenomena2 references3 citations

TL;DR

This study performs a stacked analysis of 115 Fermi-LAT gamma-ray pulsars over 4.2 years of average exposure to search for cumulative emission above 50 GeV, finding no significant excess. The results constrain the average flux from the pulsar population to less than 7% of the Crab pulsar's flux in the 56–100 GeV band, suggesting that non-exponentially suppressed emission (e.g., from inverse-Compton scattering) is not common across the population, though it cannot be ruled out entirely.

ABSTRACT

NASA's Fermi space telescope has provided us with a bountiful new population of gamma-ray sources following its discovery of 150 new gamma-ray pulsars. One common feature exhibited by all of these pulsars is the form of their spectral energy distribution, which can be described by a power law followed by a spectral break occurring between $\sim$1 and $\sim$8 GeV. The common wisdom is that the break is followed by an exponential cut-off driven by radiation/reaction-limited curvature emission. The discovery of pulsed gamma rays from the Crab pulsar, the only pulsar so far detected at very high energies (E$>$100GeV), contradicts this "cutoff" picture. Here we present a new stacked analysis with an average of 4.2 years of data on 115 pulsars published in the 2nd LAT catalog of pulsars. This analysis is sensitive to low-level $\sim$100 GeV emission which cannot be resolved in individual pulsars but can be detected from an ensemble.

Motivation & Objective

• To test whether non-exponentially suppressed gamma-ray emission—such as from inverse-Compton scattering—is common in gamma-ray pulsars above the GeV spectral break.
• To improve sensitivity to low-level very-high-energy (VHE; E > 50 GeV) emission by stacking data from 115 pulsars, overcoming individual pulsar detection limits.
• To assess the spectral shape of the ensemble pulsar population in the 100 MeV to 50 GeV range, particularly whether it deviates from exponential cutoff models.
• To inform galactic diffuse emission models and indirect dark matter searches by constraining pulsar spectral shapes.

Proposed method

• A stacked analysis combines Fermi-LAT data from 115 gamma-ray pulsars using aperture photometry, applying phase-resolved on-source and off-source event selection.
• The analysis uses a modified on-minus-off method to extract excess flux in energy bins, with exposure corrections applied via gtexposure.
• Spectral energy distributions (SEDs) are fitted with power-law times sub-exponential cutoff functions, with the cutoff index b allowed to float.
• The analysis compares fits with fixed b=1 (exponential cutoff) and floating b to assess preference for sub-exponential forms.
• Statistical uncertainties are used in fitting, though they likely underestimate true uncertainties due to systematic effects.
• The method is applied separately to young pulsars, millisecond pulsars, and sub-samples to test for spectral differences.

Experimental results

Research questions

• RQ1 Is there cumulative VHE emission from the Fermi-LAT pulsar population above 50 GeV, indicating a non-exponentially suppressed component in individual pulsars?
• RQ2 Do the stacked SEDs of young and millisecond pulsars prefer sub-exponential cutoffs over exponential cutoffs, suggesting collective emission mechanisms like inverse-Compton scattering?
• RQ3 Can the observed spectral shape above the GeV break be explained by curvature radiation models with radiation-reaction limits, or does it require alternative emission processes?
• RQ4 What is the upper limit on the average flux from the pulsar population in the 56–100 GeV band, and how does it constrain individual pulsar brightness?
• RQ5 How do the results affect the interpretation of unassociated gamma-ray excesses in the context of dark matter searches?

Key findings

• No significant cumulative emission is detected above 50 GeV in the stacked analysis of 115 pulsars, with no excess observed in any energy bin.
• The average flux from the entire pulsar sample is constrained to less than 7% of the Crab pulsar's flux in the 56–100 GeV energy band.
• The stacked SEDs for young and millisecond pulsars are consistent with a power-law times a sub-exponential cutoff, with best-fit b values of 0.59 ± 0.02 and 0.7 ± 0.15, respectively.
• The highest energy spectral point lies at ~2.4σ above the best-fit sub-exponential cutoff model, though this is not strong evidence for non-exponential emission.
• The flux limits from this stacked analysis are approximately three times lower than the best individual pulsar limits from the 2nd Fermi-LAT Pulsar Catalog in the 56–100 GeV range.
• The results do not rule out the presence of a few bright VHE pulsars (e.g., Crab-like) or dozens of dimmer ones, but constrain the average flux across the population.