PSR J2021+3651 is a 17 kyr old rotation powered pulsar detected in the radio, X-rays, and γ-rays. It powers a torus-like pulsar wind nebula with jets, dubbed the Dragonfly, which is very similar to that of the Vela pulsar. The Dragonfly is likely associated with the extended TeV source VER J2019+368 and extended radio emission. We conducted first deep optical observations with the GTC in the Sloan r ′ band to search for optical counterparts of the pulsar and its nebula. No counterparts were detected down to r ′ 27.2 and 24.8 for the point-like pulsar and the compact X-ray nebula, respectively. We also reanalyzed Chandra archival X-ray data taking into account an interstellar extinction -distance relation, constructed by us for the Dragonfly line of sight using the red-clump stars as standard candles. This allowed us to constrain the distance to the pulsar, D = 1.8 +1.7 −1.4 kpc at 90% confidence. It is much smaller than the dispersion measure distance of ∼12 kpc but compatible with a γ-ray "pseudo-distance" of 1 kpc. Based on that and the optical upper limits, we conclude that PSR J2021+3651, similar to the Vela pulsar, is a very inefficient nonthermal emitter in the optical and X-rays, while its γ-ray efficiency is consistent with an average efficiency for γ-pulsars of similar age. Our optical flux upper limit for the pulsar is consistent with the longwavelength extrapolation of its X-ray spectrum while the nebula flux upper limit does not constrain the respective extrapolation.

We have analyzed archival XMM-Newton and Chandra observations of the γ-ray radioquiet pulsar J1826−1256 and its pulsar wind nebula. The pulsar spectrum can be described by a power-law model with a photon index Γ ≈ 1. We find that the nebular spectrum softens with increasing distance from the pulsar, implying synchrotron cooling. The empirical interstellar absorption-distance relation gives a distance of ≈ 3.5 kpc to J1826−1256. We also discuss the nebula geometry and association between the pulsar, the very high energy source HESS J1826−130, the supernova remnant candidate G18.45−0.42 and the open star cluster Bica 3.

We analysed Chandra observations of the bright Fermi pulsar J0633+0632 and found evidence of an absorption feature in its spectrum at 804 +42 −26 eV (the errors are at 90% confidence) with equivalent width of 63 +47 −36 eV. In addition, we analysed in detail the X-ray spectral continuum taking into account correlations between the interstellar absorption and the distance to the source. We confirm early findings that the spectrum contains non-thermal and thermal components. The latter is equally well described by the blackbody and magnetised atmosphere models and can be attributed to the emission from the bulk of the stellar surface in both cases. The distance to the pulsar is constrained in a range of 1-4 kpc from the spectral fits. We infer the blackbody surface temperature of 108 +22 −14 eV, while for the atmosphere model, the temperature, as seen by a distant observer, is 53 +12 −7 eV. In the latter case, J0633+0632 is one of the coldest middle-aged isolated neutron stars. Finally, it powers an extended pulsar wind nebula whose shape suggests a high pulsar proper motion. Looking backwards the direction of the presumed proper motion, we found a likely birthplace of the pulsar-the Rosette nebula, a 50-Myr-old active star-forming region located at about 1.5 • from the pulsar. If true, this constrains the distance to the pulsar in the range of 1.2-1.8 kpc.

Using archival X-ray data we have found point-like X-ray counterpart candidates positionally coincident with six γ-ray pulsars discovered recently in the Fermi Gammaray Space Telescope data by the Einstein@Home project. The candidates for PSRs J0002+6216, J0554+3107, J1844−0346 and J1105−6037 are detected with Swift, and those for PSRs J0359+5414 and J2017+3625 are detected with Chandra. Despite a low count statistics for some candidates, assuming plausible constraints on the absorbing column density towards the pulsars, we show that X-ray spectral properties for all of them are consistent with those observed for other pulsars. J0359+5414 is the most reliably identified object. We detect a nebula around it, whose spectrum and extent suggest that this is a pulsar wind nebula powered by the pulsar. Associations of J0002+6216 and J1844−0346 with supernova remnants CTB 1 and G28.6−0.1 are proposed.