A wide-field X-ray search for the Geminga pulsar halo with SRG/ART-XC

Searches for the putative large-scale X-ray halo around the Geminga pulsar have been extensively performed using various narrow field-of-view X-ray telescopes. In this paper, we present wide-field scanning observation of Geminga with SRG/ART-XC. Our X-ray analysis provides, for the first time, direct imaging of a $3.5^\circ \times 3.5^\circ$ region in the $4-12$ keV energy band, comparable in extent to the expected Geminga emission. The ART-XC observation provides a highly uniform sky coverage without strong vignetting effects. The synchrotron X-ray halo flux was predicted using a physical model based on particle injection, diffusion, and cooling over the pulsar’s lifetime, as well as the spectral and spatial properties of the synchrotron X-ray and inverse-Compton gamma-ray emissions. The model is tuned to reproduce existing multiwavelength data from X-ray upper limits and GeV to TeV gamma-ray observations. After accounting for the high particle background and its uncertainties, no significant emission is found in the assumed source region, and X-ray flux upper limits are derived. These limits are less constraining by up to a factor of three with respect to existing results obtained with narrow field-of-view telescopes and longer exposure times. Nonetheless, we place direct and independent constraints on Geminga’s ambient magnetic field strength, which are compatible with other studies. Our methodology, including simulation for longer observation times, is applied for the first time to the wide field-of-view search for pulsar halos. Using extensive simulations, we also show that a 68% probability of detecting the Geminga pulsar halo can be achieved with a 20-day SRG/ART-XC exposure for a 3 $μG$ magnetic field.

💡 Research Summary

This paper presents the first wide-field X-ray search for the synchrotron halo of the Geminga pulsar using the ART-XC telescope aboard the SRG observatory. Pulsar halos are extended regions of gamma-ray emission observed around middle-aged pulsars like Geminga, believed to be produced by high-energy electron-positron pairs diffusing away from the pulsar and upscattering ambient photons via inverse Compton scattering. A synchrotron X-ray counterpart, generated by the same particle population interacting with the ambient magnetic field, is predicted but has remained undetected, primarily because its expected angular size (several degrees) exceeds the field of view of most existing X-ray telescopes.

The study addresses this limitation by utilizing ART-XC’s wide field of view (36 arcminutes) and a dedicated scanning strategy. The observations, conducted on April 16, 2023, covered a 3.5° × 3.5° region centered on Geminga with a uniform exposure of ~1800 seconds per point in the 4-12 keV energy band. This energy range was chosen to minimize Galactic absorption and contamination from soft diffuse emission from the nearby Monogem Ring supernova remnant.

To interpret the data, the authors employed a detailed physical model for the Geminga halo. The model assumes continuous particle injection following the pulsar’s spin-down history, a power-law spectrum with an exponential cutoff for the accelerated pairs, and their subsequent propagation through the interstellar medium under a scenario of “suppressed diffusion” – where the diffusion coefficient is hundreds of times lower than the Galactic average. This model is tuned to successfully reproduce the existing multiwavelength data from GeV to TeV gamma-rays. The synchrotron X-ray flux and morphology in the 4-12 keV band were then predicted as a function of the assumed uniform ambient magnetic field strength (B).

The analysis of the one-day ART-XC observation revealed no statistically significant extended emission coincident with the predicted halo region. After carefully accounting for the high particle background and its associated uncertainties, the authors derived upper limits on the X-ray flux. While these limits are up to a factor of three less constraining than previous results from narrow-field telescopes with longer exposures (like XMM-Newton), they represent the first direct and independent constraints obtained from imaging the full expected angular extent of the halo.

The non-detection allows the authors to place constraints on the magnetic field around Geminga, consistent with other indirect estimates. Furthermore, recognizing the exposure limitation of the current dataset, the paper includes a prospective analysis using extensive simulations. These simulations demonstrate that with a significantly longer exposure of 20 days, ART-XC would have a 68% probability of detecting the Geminga pulsar halo for an ambient magnetic field of 3 μG. This finding highlights the potential of future wide-field hard X-ray observations to finally uncover the synchrotron counterpart of TeV pulsar halos, which would provide crucial independent insights into particle acceleration, transport, and the magnetic environment around these ancient cosmic powerhouses.