The nature of UHE source 1LHAASO J1740+0948u and its connection to PSR J1740+1000

TL;DR

The study investigates the origin of the ultra-high-energy TeV source 1LHAASO J1740+0948u, examining its potential link to the middle-aged pulsar PSR J1740+1000. Using XMM-Newton X-ray imaging and spectroscopy, plus Fermi-LAT gamma-ray constraints, the authors rule out nearby X-ray sources and favor a leptonic, PWN-origin scenario. They show that inverse-Compton emission from electrons accelerated in the pulsar wind, reaching energies near the polar-cap potential drop, can explain the TeV emission if rapid advective transport along an extended tail or a pulsar-filament channel is invoked. The results provide a framework to test UHE particle acceleration in evolved PWNe and highlight the need for deeper radio/X-ray mapping and future CTA observations to distinguish transport geometries and refine acceleration limits.

Abstract

We present multi-wavelength analysis of 1LHAASO J1740+0948u and its surroundings including the pulsar wind nebula of middle-aged pulsar PSR J1740+1000. Although a dozen X-ray sources are found within the UHE emission site, careful analysis shows that they are unlikely to produce the observed UHE emission. The most likely particle accelerator is pulsar J1740+1000 which if offset by 13' north of the UHE source but appears to be connected to it by an extended feature seen in X-rays. For a plausible pulsar distance of 1.2 kpc, 1LHAASO J1740+0948u must be located about 5 pc away which requires rapid transport of electrons along the feature to avoid radiative losses. This poses several challenges for standard pulsar theory. Firstly, being produced $\lesssim$ 10 kyrs ago, particles must have been accelerated to the energy corresponding to a large fraction of the pulsar's full potential drop across the polar cap. Secondly, due to the lack of TeV emission extension toward the pulsar, particles must be accumulating in the UHE region. In this context, we discuss two possible scenarios: a tail filled with pulsar wind and confined by the bow-shock due to the fast pulsar's motion and an ISM filament filled by the most energetic pulsar wind particles escaping from the apex of the bow-shock.

Figures (7)

• Figure 1: X-ray images of LHAASO J1740+0948u field. The image on the left is from XMM-Newton EPIC MOS1+2 (0.5 -- 10 keV) and on the right is from CXO ACIS (0.5 -- 7 keV). X-ray sources in the LHAASO field are labeled in each image, and correspond to entries in Table \ref{['tab:combine_table']}. The regions used for spectral extraction of the pulsar tail in the XMM data are shown in green with solid (source) and dashed (background) lines. The inset on the left is a zoomed-in portion of the XMM image showing the pulsar and its tail.
• Figure 2: X-ray to optical flux ratios versus X-ray hardness ratios for 1LHAASO J1740+0948u field X-ray sources alongside 4XMM-DR13 training dataset X-ray sources 2024RNAAS...8...74L. We performed cross-matching using XMM-Newton positions to PanSTARRS and only retained X-ray sources with a single PanSTARRS match to avoid confusion. Optical fluxes are obtained from the PanSTARRS g-band magnitude with the exception of PSO J264.9676+09.7932, the counterpart to 2CXO J173952.1+094733, where the r-band magnitude is used instead due to the lack of g-band magnitude. X-ray to optical flux ratios are defined as $R_{\mathrm{x}/\mathrm{o}}=F_{0.2-12.0 \mathrm{keV}}/F_g$ for XMM-Newton sources and $R_{\mathrm{x}/\mathrm{o}}=F_{0.5-7.0 \mathrm{keV}}/F_g$ for CXO sources. 1LHAASO J1740+0948u field sources without PanSTARRS counterparts are shown as red circles and with black triangles representing the lower limits. The three sources with lower limits, src7x, src8x, and src8c have $R_{\mathrm{x}/\mathrm{o}}$ of 1.04, 1.24, 11.50, respectively. The expected region for isolated pulsars is shaded in gray (based on optically detected pulsars from 2013MNRAS.436..401M). Hardness ratio is defined as $\textrm{HR}=(F_{2.0-12.0 \textrm{ keV}}-F_{0.2-2.0 \textrm{ keV}})/(F_{2.0-12.0 \textrm{ keV}}+F_{0.2-2.0 \textrm{ keV}})$ for XMM-Newton sources and $\textrm{HR}=(F_{2.0-7.0 \textrm{ keV}}-F_{0.5-2.0 \textrm{ keV}})/(F_{2.0-7.0 \textrm{ keV}}+F_{0.5-2.0 \textrm{ keV}})$ for CXO sources. For X-ray sources from the 1LHAASO J1740+0948u site, XMM-Newton flux is used instead of CXO flux when available because the former is more accurately measured.
• Figure 3: Pan-STARRS optical images images (left) of the regions around src2 (top), src7x (middle), and src8x (bottom). The XMM-Newton sources are labeled with green circles, the CXO sources are labeled with purple circles, the 2-sigma error circles are shown with dash lines respectively ( error_ellipse_r0 is used for the CXO error circle). The Gaia sources are labeled with cyan squares, the CatWISE sources are labeled with yellow crosses, and the ALLWISE sources are labeled with orange crosses. The corresponding XMM-Newton MOS2 spectra (right).
• Figure 4: Test Statistic (TS) map centered on the source 1LHAASO J1740+0948u from 5--50 GeV showing the significance of the LHAASO source. Contributions of all other GeV sources within the region shown are subtracted out. The color scale spans TS values from 0 up to 9 (an approximately $3\sigma$ detection), highlighting regions of potential $\gamma$-ray excess. The white cross marks the catalog position of PSR J1740+1000 and an arrow pointing to the LHAASO position of the TeV source.
• Figure 5: MW SED for the outflow from J7140 together with SED of LHAASO J1740+0948u. Shown are the data from XMM-Newton (black; this paper), Fermi-LAT (red upper limits; this paper), VERITAS (magenta upper limt; 2021ApJ...916..117B), LHAASO (blue; 2024ApJS..271...25C), and HAWC (grey; 2020ApJ...905...76A). The red curve shows the spectrum based on the tail model from 2021ApJ...916..117B with synchrotron emission (dashed green), the IC emission (dashed cyan). The model parameters are: $z_0=6.2\times 10^{17}$ cm, $E_{\rm max}=400$ TeV, $\beta=-0.3$, $\gamma=1$, $\alpha=-0.7$, $B_0=1~\mu$ G, $v_0=50000$ km s$^{-1}$ (see 2021ApJ...916..117B for the model details). Note that the X-ray spectrum was extracted from the smaller volume then the extent of the tail from pulsar to the UHE emission cite used for the spatial integration in the model. If some of the very faint X-ray is unaccounted for due to the smaller extraction region, X-ray flux may be slightly higher and spectrum may be somewhat different.