Investigating the mysterious nature of 1LHAASO J1740+0948u through deep XMM-Newton observations

TL;DR

We combine ~532 ks of XMM-Newton data to characterize the diffuse X-ray surroundings of PSR J1740+1000 and its tail, aiming to identify the origin of the ultra-high-energy source 1LHAASO J1740+0948u. The X-ray tail is well described by a hard spectrum with $Γ\approx1.76$, while no robust diffuse X-ray counterpart is detected within the LHAASO region, yielding stringent upper limits. SED modelling with naima shows that the TeV emission is most consistent with an older, X-ray-faint electron population in a weak magnetic field ($B\leq1.2\ \mu$G), whereas the X-ray tail requires $B\sim6.8\ \mu$G and cannot alone account for the TeV flux. Collectively, the results favour a relic PWN or pulsar halo interpretation for 1LHAASO J1740+0948u, though confirmation requires deeper, multi-wavelength follow-up and broader spatial coverage.

Abstract

1LHAASO J1740+0948u is a very-high-energy (VHE) source reported by LHAASO, with no counterpart at other wavelengths. It is located at 0.2° from PSR J1740+1000, a radio and gamma-ray pulsar placed well above the Galactic plane, which displays an X-ray tail. Despite the offset, the association between the two sources is likely. We aim to study the diffuse X-ray emission around PSR J1740+1000 and its tail to investigate the origin of 1LHAASO J1740+0948u through a multi-wavelength SED fitting, testing different scenarios. We analysed ~500 ks of XMM-Newton observations and studied for the first time the diffuse emission around the pulsar. We also analysed the tail and how its emission evolves as a function of distance. We then performed a fit of the SED, including the spectrum of 1LHAASO J1740+0948u and the X-ray data obtained from either the analysis of the tail or the diffuse emission, to understand whether one of the two X-ray sources could be related to the TeV emission and attempt a source classification. The diffuse X-ray emission analysis resulted in upper limits in the 0.5-10 keV range. The tail is best fitted with a power law with $Γ=1.76\pm0.06$ in 0.5-8 keV, with no significant detection of spectral variations with distance. We do not find a good SED fit that can explain both the X-ray tail and the LHAASO spectrum with reasonable parameters, suggesting that the TeV emission likely comes from an older X-ray-faint electron population. We then performed an SED fitting of the VHE spectrum combined with the upper limits on the diffuse emission, constraining the magnetic field to be as low as $B\leq1.2 μ$G. We suggest that 1LHAASO J1740+0948u could represent either the relic PWN of PSR J1740+1000 or its pulsar halo. Our energy density results hint at a halo-like nature for 1LHAASO J1740+0948u, but deeper multi-wavelength observations are required to confirm this hypothesis.

Figures (6)

• Figure 1: (a) ROSAT all-sky map showing the large-scale diffuse emission of the North Polar Spur and the location of the XMM-Newton pointings, i.e. the small green circle. The red, green and blue colours represent the 0.11--0.28 keV, 0.47--1.21 keV and 0.76--2.04 keV energy bands, respectively. (b) Image of the XMM-Newton FoV, showing both Region 1 and Region 2 (solid circles), and 1LHAASO J1740+0948u (dashed circles). The plotted radii of the source are taken from the 1$\sigma$ extension of the Gaussian ($R_{G, LHAASO}$) and the diffuse ($R_{D, LHAASO}$) models for photon energy above 25 TeV, shown in LHAASO_2025. The 0.5--1 keV band is shown in red, the green emission corresponds to the range 1--2 keV, and the range 2--4.5 keV is represented in blue. Note that the observed point sources were later removed during the diffuse emission analysis.
• Figure 2: XMM-Newton image of the tail region in the energy range 0.5--8 keV. The solid ellipse represents the region where the overall tail emission has been extracted, while the dashed ellipses mark the borders of the sub-tail regions. The dashed circle is the background extraction region. We also display the Galactic coordinates grid.
• Figure 3: Spectra of the single tails in the energy range 0.5--6 keV, excluding the 1.3--1.8 keV band due to the contamination of instrumental lines. Tail 1 is the innermost with respect to the pulsar, Tail 5 is the outermost.
• Figure 4: Evolution of the spectral parameters as a function of the angular distance from the pulsar position. The uncertainties on the index and surface brightness values are shown at 1$\sigma$ level. Left panel: the trend of the spectral index. Right panel: the trend of the normalisation of the power law at 1 keV considering the AREASCAL parameter.
• Figure 5: SED of the tail-like PWN of J1740, including the X-ray points derived in this work and the LHAASO-KM2A spectrum along with the LHAASO-WCDA extrapolation of LHAASO_2025. The electron distribution is modelled as an ECPL with fixed $p=1.6$, $\beta=2$, $E_0=1$ TeV and free $E_{cut}$ and normalisation. We included ICS, considering all the photon fields obtained from Popescu_2017, and Synchrotron, where the magnetic field value was left free to vary. We also report the VERITAS upper limit of Benbow_2021 obtained for an extraction region of 0.1$^\circ$. The model residuals, obtained as (data - model)/error, are shown in the bottom panel.