X‐Ray Emission from PSR J1809−1917 and Its Pulsar Wind Nebula, Possibly Associated with the TeV Gamma‐Ray Source HESS J1809−193

Abstract

We detected X-ray emission from the 50-kyr-old pulsar J1809‐1917 and resolved its pulsar wind nebula (PWN) with the Chandra X-ray Observatory. The pulsar’s observed flux is Fpsr = (1: 8� 0: 2)� 10 -14 ergs cm -2 s -1 in the 1‐6 keV band. A two-component blackbody+power-law (BB+PL) fit of the pulsar’s spectrum yields the photon index psr = 1: 2� 0: 6 and luminosity L psr = (4� 1)� 10 31 ergs s -1 of the PL component, in the 0.5‐8 keV band, for a plausible distance d = 3: 5 kpc and nH = 0: 7� 10 22 cm -2 . The BB component corresponds to the temperature T � 2 MK, and bolometric luminosity L bol � 1� 10 32 ergs s -1 . The bright inner PWN component of a 3 00 � 12 00 size is elongated in the north-south direction, with the pul sar close to its south end. This component is immersed in a larger (� 20 00 � 40 00 ), similarly elongated outer PWN component of lower surface brightness. The elongated shape of the compact PWN can be explained by the ram pressure confinement of the pulsar wind due to the supersonic motion of the pulsar. The observed flux of the compact PWN, including both components, is Fpwn ’ (1: 5� 0: 1)� 10 -13 ergs cm -2 s -1 in the 1‐6 keV band. The PWN spectrum can be fitted with a PL model with nH � 0: 7� 10 22 cm -2 and photon index pwn = 1: 4� 0: 1, corresponding to the 0.5‐8 keV luminosity Lpwn � 4� 10 32 ergs s -1 . The compact PWN appears to be inside a more extended (� 4 0 � 4 0 ) emission with the total observed flux Fext � 5� 10 -13 ergs s -1 in the 0.8‐7 keV band. This large-scale emission is more extended to the south of the pulsar, i.e. in the direction of the alleged pulsar motion. To explain the extended X-ray emission ahead of the moving pulsar, one has to invoke strong intrinsic anisotropy of the pulsar wind or assume that this e mission comes from a relic PWN swept by the asymmetrical reverse SNR shock. The pulsar and its PWN are located within the extent of the unidentified TeV source HESS J1809‐193. The brightest part of the TeV source is offset by � 8 0 to the south of the pulsar, i.e. in the same direction as the large-scale X-ray emission. Altho ugh the association between the PSR J1809‐1917 and HESS J1809‐193 is plausible, an alternative source of relativistic electrons powering HESS J1809‐193 might be the serendipitously discovered X-ray source CXOU J180940.7-192544. In addition to the CMBR or Galactic starlight background, the low-frequency seed p hotons for Compton upscattering to TeV energies might be supplied by bright infrared emission from dust-molecular clouds seen within HESS J1809‐193. Subject headings: pulsars: individual (PSR J1809‐1917) — X-rays: individual (CXOU J180940.7-192544, CXOU 180933.3-192959) — gamma-rays: individual (HESS J1809‐193) — ISM: individual (IRAS 18067‐1927, IRAS 18067‐1921)