Using HAWC to discover invisible pulsars

Abstract

Observations by HAWC and Milagro have detected bright and spatially extended TeV γ-ray sources surrounding the Geminga and Monogem pulsars. We argue that these observations, along with a substantial population of other extended TeV sources coincident with pulsar wind nebulae, constitute a new morphological class of spatially extended TeV halos. We show that HAWCs wide field-of-view unlocks an expansive parameter space of TeV halos not observable by atmospheric Cherenkov telescopes. Under the assumption that Geminga and Monogem are typical middle-aged pulsars, we show that ten-year HAWC observations should eventually observe 37 +17 −13 middle-aged TeV halos that correspond to pulsars whose radio emission is not beamed towards Earth. Depending on the extrapolation of the TeV halo efficiency to young pulsars, HAWC could detect more than 100 TeV halos from mis-aligned pulsars. These pulsars have historically been difficult to detect with existing multiwavelength observations. TeV halos will constitute a significant fraction of all HAWC sources, allowing follow-up observations to efficiently find pulsar wind nebulae and thermal pulsar emission. The observation and subsequent multi-wavelength follow-up of TeV halos will have significant implications for our understanding of pulsar beam geometries, the evolution of PWN, the diffusion of cosmic-rays near energetic pulsars, and the contribution of pulsars to the cosmic-ray positron excess. I. INTRODUCTIONRecent observations by the High Altitude Water Cherenkov Observatory (HAWC) [1], along with earlier results from Milagro [2], have detected diffuse TeV emission surrounding the Geminga and B0656+14 (hereafter referred to as Monogem [3]) pulsars. While it is difficult to constrain the exact morphology of this emission, both systems are well-fit by Gaussian distributions with an angular extension of ∼2 • . These observations are intriguing for several reasons. First, the short cooling times of very high energy electrons imply that even middle-aged pulsars accelerate e + e − to energies exceeding ∼50 TeV. Second, the angular size of these "TeV halos" indicates that the propagation of cosmic rays near pulsars is significantly more constrained than typical for the interstellar medium [4,5]. Third, the intensity of this emission indicates that a significant fraction of the total pulsar spin-down luminosity is converted into e + e − pairs, providing evidence in support of pulsar interpretations of the rising cosmic-ray *

TeV halos are everywhere: Prospects for new discoveries

“…• TeV halo observations would allow us to detect pulsars with radio emission not aligned toward Earth and hence which have been missed in previous blind searches [1]. Further, the angular size of halos could provide useful distance estimations for Galactic pulsars.…”

“…The lower limits on the number of beamed and unbeamed TeV halos would become much stronger if the TeV halo candidates that were first identified by Ref. [1] are confirmed by subsequent observations. Ref.…”

“…Ref. [1] finds three additional 2HWC sources that are consistent with the position of middle-aged radio pulsars, and twelve additional 2HWC sources that are consistent with the positions of younger pulsars. They estimate that only 2.6 chance coincidences would be expected if the 2HWC sources were not associated with pulsar activity.…”

“…Theoretically, the creation of halos requires strongly inhabited diffusion around pulsars, which might be expected for all pulsars if a steep cosmic-ray gradient around them efficiently excites selfgenerated turbulence [25]. Observationally, Linden et al [1] listed seven middle-aged pulsars that should be detected by HAWC, under the assumption that every pulsar has a Geminga-like TeV halo, and find that five are in fact associated with the 2HWC sources. These are consistent with the expectation that a significant fraction of pulsars have TeV halos.…”

TASI Lectures on Indirect Searches For Dark Matter

“…In recent years, Fermi's detection of gamma-ray emission from both non-blazar AGN [167] and star-forming galaxies [168], combined with the observed correlations of the emission at gamma-ray and radio/infrared wavelengths, has revealed that these source classes each contribute significantly to the IGRB. Even more recent studies have shown that the combination of these source classes dominates the observed IGRB [169,170], leaving relatively little room for the presence of dark matter annihilation products.…”