We observed the young pulsar J1357–6429 with the Chandra and XMM-Newton observatories .
The pulsar spectrum fits well a combination of absorbed power-law model ( \Gamma = 1.7 \pm 0.6 ) and blackbody model ( kT = 140 ^ { +60 } _ { -40 } eV , R \sim 2 km at the distance of 2.5 kpc ) .
Strong pulsations with pulsed fraction of 42 \% \pm 5 \% , apparently associated with the thermal component , were detected in 0.3–1.1 keV .
Surprisingly , pulsed fraction at higher energies , 1.1–10 keV , appears to be smaller , 23 \% \pm 4 \% .
The small emitting area of the thermal component either corresponds to a hotter fraction of the neutron star ( NS ) surface or indicates inapplicability of the simplistic blackbody description .
The X-ray images also reveal a pulsar-wind nebula ( PWN ) with complex , asymmetric morphology comprised of a brighter , compact PWN surrounded by the fainter , much more extended PWN whose spectral slopes are \Gamma = 1.3 \pm 0.3 and \Gamma = 1.7 \pm 0.2 , respectively .
The extended PWN with the observed flux of \sim 7.5 \times 10 ^ { -13 } erg s ^ { -1 } cm ^ { -2 } is a factor of 10 more luminous then the compact PWN .
The pulsar and its PWN are located close to the center of the extended TeV source HESS J1356–645 , which strongly suggests that the VHE emission is powered by electrons injected by the pulsar long ago .
The X-ray to TeV flux ratio , \sim 0.1 , is similar to those of other relic PWNe .
We found no other viable candidates to power the TeV source .
A region of diffuse radio emission , offset from the pulsar toward the center of the TeV source , could be synchrotron emission from the same relic PWN rather than from the supernova remnant .