Multi-wavelength insights into the pulsar wind nebula candidate near 1LHAASO J0343+5254u: an obscured merging galaxy cluster?

Abstract

The advent of the Large High Altitude Air Shower Observatory (LHAASO) accelerated the detection of TeV and PeV gamma-ray sources. Some of these are associated with pulsar wind nebulae (PWNe) and other Galactic objects, while others are yet to be connected to sources at other wavelengths. Recently, the discovery of an extended X-ray source within the unidentified PeV source 1LHAASO J0343+5254u was reported, this source was claimed as a candidate PWN based on its X-ray spectrum. We will revisit the interpretation of the extended X-ray source based on multi-wavelength observations. We present new LOFAR continuum radio imaging at observing frequencies of 54 and 144 MHz, an alternative X-ray modeling and archival near-infrared (NIR) data. We discover several radio sources with morphologies and spectra suggestive of a radio halo, a radio relic and tailed radio galaxies, all of which are typically found in (merging) galaxy clusters. Furthermore, we show that the X-ray data can be modeled as thermal emission from the intracluster medium (ICM), with our best-fitting thermal ICM model being slightly preferred to a non-thermal power-law fit. We further find a 9.7$σ$ over-density in red NIR sources in the surrounding region, among them possible hosts of the tailed radio sources. Our results favor an interpretation of the X-ray source as a massive, merging galaxy cluster located in a highly extinct region of the Galactic plane, unrelated to 1LHAASO J0343+5254u. Future observations in the hard X-ray regime will be able to conclusively settle the discussion on the nature of the X-ray emission.

Figures (8)

• Figure 1: Observed source spectrum (black=MOS1, red=MOS2, green=pn) with APEC best-fit model reported. The spectra of the different cameras and observations were fitted jointly, but the results of a single ObsID (0923400401) were reported for clarity. For energy ranges 1.2-1.9 keV (for the MOS detectors) and 1.2-1.7 keV and 7-9.2 keV (for the pn detector) were excluded during the fit. Each spectral bin was rebinned to a minimum significance of 5$\sigma$ for visualization purposes. Residuals in the pn spectrum at 2.1 keV are instrumental 2008AA...486..359L.
• Figure 2: LoTSS DR3 of the wider area surrounding the LHAASO sources in Galactic coordinates. The central frequency is 144 MHz and the noise level is $\sigma_\mathrm{rms}=100\,\mathrm{\mu{Jy}\,beam^{-1}}$ at a resolution of $6"$. The green circles show the 39% containment radius of the LHAASO sources, and the red contours mark the $0.5-7$ keV emission around XMMU J034124.2+525720 smoothed to 20$"$ resolution, starting at $5\times10^{-6}\,\mathrm{cts\,s^{-1}\,arcsec^{-2}}$ and increasing in factors of two.
• Figure 3: Radio map at 144 MHz, the noise level is $\sigma_\mathrm{rms}=100\,\mathrm{\mu{Jy}\,beam^{-1}}$ at $5"\times8"$ resolution. In cyan we show the $0.5-7$ keV X-ray contours. The insets show the NIR image with the radio contours of the tailed sources overlaid; red circles mark UGS galaxies with red $(J-K)$ color.
• Figure 4: Radio spectral index map at a resolution of $20"\times13"$. Grey contours correspond to $1\,\mathrm{Jy\,beam^{-1}}\times[1,2,4,8]$ in the 144 MHz map at the same resolution, black crosses mark the location of candidate IR host galaxies.
• Figure 5: UKIDSS GPS galaxies, the marker color corresponds to the difference in $(J-K)$ color from a value of 2.2. The black circle marks the target and the dashed circular annulus the background region. In cyan, we show the X-ray contours.