The environment of TeV halo progenitors

TL;DR

This work develops a statistical, physically motivated framework to connect TeV haloes around middle-aged pulsars to the environments they traverse from birth to the ISM. By combining Monte Carlo sampling of massive-star progenitors, two ambient-medium models (wind-blown bubbles around isolated stars and superbubbles around clusters), and a thin-shell SNR evolution within those media, the authors estimate the time-dependent likelihood that a pulsar remains inside its parent environment. They find that pulsars typically stay embedded in their birth environments much longer than standard escape-times suggest, with transitions to the ISM occurring around $6\times10^{2}$ kyr in clustered environments and around $3\times10^{2}$ kyr overall for a Galactic mix; this implies TeV haloes are more common for middle-aged pulsars still linked to turbulent progenitor regions. Applying the model to a sample of known halo candidates shows many have high probabilities of remaining in their parent environments, including Geminga, supporting a connection between TeV haloes and progenitor-influenced surroundings. The results have implications for the interpretation of TeV haloes and for the local cosmic-ray positron flux, highlighting the importance of environment-aware propagation near pulsars.

Abstract

TeV haloes are extended sources of very-high-energy gamma rays found around some middle-aged pulsars. The emission spanning several tens of parsecs suggests an efficient confinement of the ultra-relativistic lepton pairs produced by pulsars in their vicinity. The physical mechanism responsible for this suppressed transport has not yet been identified. In some scenarios, pair confinement may be linked to the medium the pulsars are located in. We aim at understanding the type of medium pulsars probe over their lifetime. We developed a model for the environment probed by moving pulsars, from their birth in core-collapse explosions - where they receive a natal kick - until their entry into the interstellar medium. The model involves: (i) a Monte-Carlo sampling of the properties of the massive-star progenitors of pulsars; (ii) a calculation of the structure of the surrounding medium shaped by these progenitors for the two cases of isolated stars and star clusters; and (iii) a computation of the evolution of supernova remnants in these parent environments. Ultimately, from a distribution of neutron star kick velocities, we assess the medium in which pulsars are located as a function of time. We first derived the statistical properties of a fully synthetic Galactic population and then applied the model to a selection of known pulsars to assess the likely nature of their environment. We show that pulsars escape into the interstellar medium at around 300 kyr, significantly later than assumed in the literature. Given our assumptions, all known pulsars with a confirmed TeV halo have high probabilities of still being in their parent environment, which suggests that efficient pair confinement is connected to the region influenced by progenitor stars. To test this, we provide the probability that known pulsars still reside in their parent environment for a list of known pulsars.

Figures (7)

• Figure 1: Evolution of the SNR as a function of time in the WBB scenario, for two values of the stellar progenitor initial mass. For each case, we overplotted the positions of the wind-termination shock (dotted lines, on top of each other, see Tab. \ref{['tab:parameters_WBB']}) and outer bubble radius (dashed lines) at the time of explosion. The purple line indicates the trajectory of a pulsar with kick velocity $350~\mathrm{km/s}$.
• Figure 2: Evolution of the SNR as a function of time in the SB scenario, for a stellar progenitor of initial mass $8~\mathrm{M}_\odot$ and three different masses of the stellar cluster it belongs to. Graphical elements are similar to Fig. \ref{['im:SNR_WBB']}. The SNR curves turn flat when the remnant merges with the cavity interior and disappears.
• Figure 3: Probability for a pulsar to be found inside its parent remnant or bubble environment as a function of time. The red and blue lines correspond to the SB and WBB cases, respectively. The grey-shaded band corresponds to values typically found in the literature for the time at which a pulsar exits into the ISM, thus showing that they are significantly underestimated. The four vertical lines correspond to the characteristic ages of the confirmed TeV haloes.
• Figure 4: Probability for a pulsar to be found inside its parent environment as a function of time, for a Galactic population made of a proper mix of isolated and cluster massive-star progenitors (see text). The different curves correspond to different assumptions on the pulsar kick velocity distribution. Other graphical elements are similar to Fig. \ref{['im:escape_times_galaxy_detail']}.
• Figure 5: Probability for a selection of 30 known pulsars within $2~\mathrm{kpc}$ to be found inside their parent environments, as a function of their characteristic ages. This is based on a Galactic population model made of a proper mix of isolated and cluster massive-star progenitors (see text), and a kick velocity distribution from igoshev_observed_2020. Dashed and dotted lines mark the $80\%$ and $20\%$ probabilities, respectively.