XMM-Newton Observations of Two Candidate Supernova

Candidate supernova remnants G23.5+0.1 and G25.5+0.0 were observed by XMM-Newton in the course of a snap-shot survey of plerionic and composite SNRs in the Galactic plane. In the field of G23.5+0.1, we detected an extended source, ∼3 ′ in diameter, which we tentatively interpret as a pulsar-wind nebula (PWN) of the middle-aged radio pulsar B1830–08 (J1833–0827; P = 85.3 ms, τ = 147 kyr, E ̇ = 5.8 × 1035 erg s−1, d = 5.7 kpc), with the PWN luminosity L0.2−10 keV ≈ 5 × 1033 erg s−1 ≈ 8 × 10−3 Ė. The pulsar is not resolved in the EPIC images. Our analysis suggests an association between PSR B1830– 08 and the surrounding diffuse radio emission. If the radio emission is due to the SNR, then the pulsar must be significantly younger than its characteristic age. Alternatively, the radio emission may come from a relic PWN. The field also contains SGR 1833–0832 and another middle-aged pulsar B1829–08 [J1832– 0827; P = 647 ms, τ = 161 kyr, E ̇ = 9.3 × 1033 erg s−1, d = 4.7 kpc], none of which are detected in our observation. In the field of G25.5+0.0, which contains the extended TeV source HESS J1837–069, we detected the recently discovered young high-energy pulsar J1838–0655 (P = 70.5 ms, τ = 23 kyr, E ̇ = 5.5 × 1036 erg s−1) embedded in a PWN with extent of 1.3′. The unabsorbed pulsar + PWN luminosity is L2−11 keV ≈ 2 × 1034 erg s−1 ≈ 4 × 10−3 E ̇ at an assumed distance of 7 kpc. We also detected another PWN candidate (AX J1837.3–0652) with an extent of 2 ′ and unabsorbed luminosity L2−10 keV ≈ 4 × 1033 erg s−1 at d = 7 kpc. The third X-ray source, located within the extent of the HESS J1837– 069, has a peculiar extended radio counterpart, possibly a radio galaxy with a double nucleus or a microquasar. We did not find any evidence of the SNR emission in the G25.5+0.0 field. We provide detailed multiwavelength analysis ar X i