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

TL;DR

This work addresses whether a large-scale X-ray halo associated with the Geminga pulsar exists by performing a wide-field scan with SRG/ART-XC in the 4–12 keV band, covering a region ≈3.5° on a side. It adopts a benchmark halo model tied to GeV–TeV observations, incorporating particle injection, diffusion with D(E)=D0 (E/1 GeV)^{δ}, energy losses, and a uniform ambient magnetic field B, to predict synchrotron X-ray emission described by sky maps M_halo. The one-day ART-XC data yield no significant halo detection, placing a 68% confidence upper limit on B of about 6 μG and flux upper limits weaker by a factor of ≈3 than previous narrow-FOV constraints, while demonstrating the method’s potential for longer observations. Simulations indicate that a 10–20 day exposure could tighten limits by ≈2× and possibly detect the halo for B ≈ 3 μG, highlighting the value of wide-field X-ray searches for pulsar halos and informing future missions like AXIS.

Abstract

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.

Figures (6)

• Figure 1: Model sky map $M_{\rm halo}$ of the Geminga X-ray halo flux coming from synchrotron emission as integrated in the 4–12 keV band, for $B=3\mu$G. The field of view of the SRG/ART-XC observation is reported with a white square, and compared to the XMM-Newton field of view from Manconi:2024wlq (green dashed circle), as well as with the extension of the TeV gamma rays observed by HAWC (dotted yellow circle).
• Figure 2: Model prediction of the surface brightness of the Geminga X-ray halo flux from synchrotron emission, integrated over the 4$-$12 keV band within four degrees of the center. The magnetic field strength varies from $2~\mu$G (dashed blue line) to $6~\mu$G (dotted black line).
• Figure 3: Sky image of the Geminga pulsar region in the 4$-$12 keV obtained with ART-XC, shown in units of counts pix$^{-1}$. The image is smoothed with the dmimgadapt task from CIAO-4.15 using a Gaussian kernel. The corresponding exposure map is highly uniform, with ${\sim}1800$ seconds per pixel. The position of the Geminga pulsar is marked by a white circle of $1^{\circ}$ diameter. The grid is displayed in Galactic coordinates. The green circle denotes the ART-XC FOV, $36'$ in diameter.
• Figure 4: Geminga halo emission model scaling factor, $S_{\rm halo}$, obtained from one-day ART-XC data, shown as a function of magnetic field strength.
• Figure 5: Background count rate measured by ART-XCin the 60$-$120 keV band as a function of time in years. The vertical dashed line indicates the date of the SRG/ART-XC scanning observation of Geminga on 16 April 2023. The figure is based on data obtained from the SRG/ART-XC environment monitor, available at https://monitor.srg.cosmos.ru/.