TeV gamma-ray emission initiated by the population or individual millisecond pulsars within globular clusters

Abstract

Two energetic millisecond pulsars (MSPs) within globular clusters (GC), J1823-3021A in NGC 6624 and PSR B1821-24 in M28, have been recently discovered to emit pulsed GeV gamma-rays. These MSPs are expected to eject energetic leptons. Therefore, GCs have been proposed to produce GeV-TeV gamma-rays as a result of the comptonization process of the background radiation within a GC. We develop this general scenario by taking into account not only the diffusion process of leptons within a GC but also their advection with the wind from the GC. Moreover, we consider distribution of MSP within a GC and the effects related to the non-central location of the dominating, energetic MSP. Such more complete scenario is considered for the modelling of the GeV-TeV gamma-ray emission from the core collapsed GC M15 and also for GCs which contain recently discovered energetic MSPs within NGC 6624 and M28. The confrontation of the modelling of the gamma-ray emission with the observations with the present Cherenkov telescopes and the future Cherenkov Telescope Array (CTA) allows to constrain more reliably the efficiency of lepton production within the inner magnetosphere of the MSPs and re-accelerated in their vicinity. We discuss the expected limits on this parameter in the context of expectations from the pulsar models. we conclude that deep observations of GCs, even with the present sensitivity of Cherenkov telescopes (H.E.S.S., MAGIC, VERITAS), should start to constrain the models for the acceleration and radiation processes of leptons within the inner pulsar magnetosphere and its surrounding.

Deep observations of the globular cluster M15 with the MAGIC telescopes

“…It is expected that the winds of MSPs mix within the GC with the matter ejected by the ambient RG stars. For the expected massloss rates of specific RGs, estimated in the range 10 −9 -3 × 10 −7 M yr −1 (Boyer et al 2008;Meszaros, Avrett & Dupree 2009), and assuming the number of RGs within M15 to be of the order of 100 (for another GC, NGC 2808 137 RGs have been observed, Cacciari et al 2004), it is possible to estimate the velocity of the mixed MSP and RG winds (see equation 1 in Bednarek et al 2016). Assuming that MSPs provide luminosities of the order of L rot derived above, and that the MSP winds mix with the RG material with a mixing efficiency of 0.5, we estimate the velocity of the resulting mixed wind in the range v w ≈ (0.2-3.5) × 10 8 cm s −1 .…”

“…Two general models for the injection of energetic leptons were considered, either monoenergetic injection directly from the MSPs (e.g. Bednarek & Sitarek 2007;Venter et al 2009;Cheng et al 2010;Zajczyk, Bednarek & Rudak 2013;Bednarek, Sitarek & Sobczak 2016) or with a powerlaw distribution as a result of re-acceleration in the pulsar wind collision regions (e.g. Bednarek & Sitarek 2007;Kopp et al 2013;Bednarek et al 2016;Ndiyavala, Krüger & Venter 2018).…”

“…Bednarek & Sitarek 2007;Venter et al 2009;Cheng et al 2010;Zajczyk, Bednarek & Rudak 2013;Bednarek, Sitarek & Sobczak 2016) or with a powerlaw distribution as a result of re-acceleration in the pulsar wind collision regions (e.g. Bednarek & Sitarek 2007;Kopp et al 2013;Bednarek et al 2016;Ndiyavala, Krüger & Venter 2018). These works assumed that TeV γ -rays originate in the Inverse Compton (IC) scattering process of low-energy radiation (optical from the GC, Cosmic Microwave Background (CMB), or the infrared and optical radiation from the Galactic disc) by leptons accelerated by MSPs.…”

“…These works assumed that TeV γ -rays originate in the Inverse Compton (IC) scattering process of low-energy radiation (optical from the GC, Cosmic Microwave Background (CMB), or the infrared and optical radiation from the Galactic disc) by leptons accelerated by MSPs. The most recent developments of numerical codes computing the γray emission from GCs take into account effects related to different diffusion scenarios for energetic particles (Ndiyavala et al 2018), the advection of leptons from GCs due to Red Giant (RG) winds and non-homogeneous injection of leptons into the GCs (Bednarek et al 2016). Modelling results, if confronted with observations, should allow us to constrain the processes occurring in the MSP magnetospheres, acceleration of leptons within GCs, and their transport within and around GCs.…”

Millisecond pulsars, TeV halos, and implications for the Galactic Center gamma-ray excess

“…In particular, only a handful of MSPs within HAWC's fieldof-view have a spindown flux (which we define throughout this study as the spindown power divided by the distance squared) high enough to produce a TeV halo that would currently be detectable by HAWC. Furthermore, it is generally expected that MSPs do indeed generate such emission [8][9][10], as the modeling of their light curves favor the abundant production of multi-TeV electron-positron pairs [8]. On the other hand, the high-surface brightness of TeV halos indicates that cosmic-ray diffusion is inhibited in the vicinity of these sources, and it is unknown whether MSPs are capable of generating such conditions.…”