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Unmasking LHAASO J2108+5157: Near Infrared Insights into a Mysterious TeV Source

Josep Martí, Pedro L. Luque-Escamilla, Josep M. Paredes, José Martínez Aroza

TL;DR

LHAASO J2108+5157 remains an enigmatic ultra-high-energy gamma-ray source with no secure low-energy counterpart. The authors perform a dedicated near-infrared survey using CAHA/OMEGA2000 to search for signatures of shocked gas, SNR, or a pulsar wind nebula, and to test the microquasar scenario, complemented by archival Spitzer IRAC and radio data. They find no evidence of shocked gas, extended nebulae, or accreting compact objects in the observed field, and the radio GMRT core appears as a faint, extended NIR/IRAC source incompatible with a Galactic microquasar or a nearby FR I galaxy at TeV-relevant distances. Archival data suggest the GMRT source is more consistent with a background FR I radio galaxy, with host luminosity constraints and lack of X-ray emission disfavoring a Galactic origin, leaving the LHAASO–J2108+5157 counterpart unidentified. The study underscores the continued mystery of this TeV emitter and highlights the need for deeper, multiwavelength campaigns to reveal its nature and potential hadronic or leptonic acceleration mechanisms at play.

Abstract

LHAASO J2108+5157 is one of the few ultra-high energy gamma-ray sources in the LHAASO catalogue without secure counterpart at longer wavelengths. Several Galactic scenarios have been proposed, including an evolved supernova remnant and a pulsar wind nebula. Yet, no shocked gas, shell-like structure, or compact pulsar candidate has been identified. Follow-up observations with VERITAS and the LST-1 prototype have not firmly clarified its nature. A recent microquasar candidate from GMRT radio data remains uncertain. Here we present the first dedicated near-infrared study of the field, combining deep JHKs imaging with narrow band observations targeting the H2 v=1-0 S(1) line. Our observations were initially planned to encompass the full source region, but now only partially cover the latest updated position and size of LHAASO J2108+5157. We find no evidence of shocked emission, extended nebular structures, or an accreting compact object signature in the covered field. The GMRT radio source, despite its jet-like morphology, exhibits near-infrared properties incompatible with both a Galactic microquasar and a nearby radio galaxy, discouraging an association with the gamma-ray emission. Our analysis reveals no convincing counterpart consistent within the positional uncertainty, leaving LHAASO J2108+5157 as an enigmatic ultra-high energy emitter that requires deeper observations.

Unmasking LHAASO J2108+5157: Near Infrared Insights into a Mysterious TeV Source

TL;DR

LHAASO J2108+5157 remains an enigmatic ultra-high-energy gamma-ray source with no secure low-energy counterpart. The authors perform a dedicated near-infrared survey using CAHA/OMEGA2000 to search for signatures of shocked gas, SNR, or a pulsar wind nebula, and to test the microquasar scenario, complemented by archival Spitzer IRAC and radio data. They find no evidence of shocked gas, extended nebulae, or accreting compact objects in the observed field, and the radio GMRT core appears as a faint, extended NIR/IRAC source incompatible with a Galactic microquasar or a nearby FR I galaxy at TeV-relevant distances. Archival data suggest the GMRT source is more consistent with a background FR I radio galaxy, with host luminosity constraints and lack of X-ray emission disfavoring a Galactic origin, leaving the LHAASO–J2108+5157 counterpart unidentified. The study underscores the continued mystery of this TeV emitter and highlights the need for deeper, multiwavelength campaigns to reveal its nature and potential hadronic or leptonic acceleration mechanisms at play.

Abstract

LHAASO J2108+5157 is one of the few ultra-high energy gamma-ray sources in the LHAASO catalogue without secure counterpart at longer wavelengths. Several Galactic scenarios have been proposed, including an evolved supernova remnant and a pulsar wind nebula. Yet, no shocked gas, shell-like structure, or compact pulsar candidate has been identified. Follow-up observations with VERITAS and the LST-1 prototype have not firmly clarified its nature. A recent microquasar candidate from GMRT radio data remains uncertain. Here we present the first dedicated near-infrared study of the field, combining deep JHKs imaging with narrow band observations targeting the H2 v=1-0 S(1) line. Our observations were initially planned to encompass the full source region, but now only partially cover the latest updated position and size of LHAASO J2108+5157. We find no evidence of shocked emission, extended nebular structures, or an accreting compact object signature in the covered field. The GMRT radio source, despite its jet-like morphology, exhibits near-infrared properties incompatible with both a Galactic microquasar and a nearby radio galaxy, discouraging an association with the gamma-ray emission. Our analysis reveals no convincing counterpart consistent within the positional uncertainty, leaving LHAASO J2108+5157 as an enigmatic ultra-high energy emitter that requires deeper observations.
Paper Structure (4 sections, 3 figures, 1 table)

This paper contains 4 sections, 3 figures, 1 table.

Table of Contents

  1. Introduction
  2. NIR observations and results
  3. Mining the archives and results
  4. Discussion and conclusions

Figures (3)

  • Figure 1: CAHA near-IR field around LHAASO J2108+5157 in $K_s$ filter. Overlaid are the partial confidence regions of the Fermi-LAT (white ellipse), HAWC (green ellipse), and LHAASO (white and yellow dashed ellipses for KM2A and WCDA detectors, respectively) detections. The position of the radio candidate microquasar Mahanta2024_uGMRT, consistent in position with all VHE/UHE detections, is marked with a green cross.
  • Figure 2: Left:Spitzer/IRAC 2 image and a zoom-in of the NIR counterpart of the candidate microquasar proposed by Mahanta2024_uGMRT. The green line indicates the path of the brightness profile (slice) used in Fig. 3. The accurate VLASS position of the radio core is marked in the zoomed view with a green circle representing the 95% confidence radius. Right: Same as the left, but with the CAHA $K_s$-band image.
  • Figure 3: Spitzer IRAC 2 normalized brightness profile of the proposed LHAASO J2108+5157 NIR counterpart candidate to the GMRT radio source, sliced along the east–west direction so as to pass through the brightness peaks of both our target and the nearby star located immediately to the east. The GMRT candidate clearly exhibits an extended morphology. Dashed lines show the best-fitting 2D elliptical Gaussian models projected along the cut direction. The source profile is significantly broader than the stellar PSF, indicating that the infrared emission is spatially resolved. The uncertainty of the normalized profile is negligible compared to the observed broadening.