Context. Optical observations of pulsars are crucial to studying the neutron star properties from the structure and composition of the interior to the properties and geometry of the magnetosphere. Historically, X and γ-ray observations have paved the way to pulsar optical identifications. The launch of the Fermi Gamma-ray Space Telescope opened new perspectives in the optical-to-γ-ray studies of neutron stars with the detection of more than 80 γ-ray pulsars. Aims. Here, we aim to search for optical emission from two Fermi pulsars that are interesting targets on the basis of their spin-down age, energetics, and distance. PSR J1357−6429 is a Vela-like pulsar (P = 166.1 ms; τ = 7.31 kyr), at a distance of ∼2.4 kpc with a rotational energy loss rateĖ ∼ 3 × 10 36 erg s −1 . PSR J1048−5832 is also a Vela-like (P = 123.6 ms; τ = 20.3 kyr) pulsar at a distance of ∼2.7 kpc and with aĖ ∼ 2 × 10 36 erg s −1 . The two pulsars and their pulsar wind nebulae (PWNe) are also detected in X-rays by Chandra and XMM-Newton. No deep optical observations of these two pulsars have been reported so far. Methods. We used multi-band optical images (V, R, I) taken with the Very Large Telescope (VLT) and available in the European Southern Observatory (ESO) archive to search for, or put tight constraints on, their optical emission. Results. We re-assessed the positions of the two pulsars from the analyses of all the available Chandra observations and the comparison with the published radio coordinates. For PSR J1357−6429, this yielded a tentative proper motion μ = 0. 17 ± 0. 055 yr −1 (70 • ± 15 • position angle). We did not detect any candidate counterparts to PSR J1357−6429 and PSR J1048−5832 down to V ∼ 27 and ∼27.6, respectively, although for the former we found possible evidence of a faint, unresolved object at the Chandra position. Our limits imply an efficiency in converting spin-down power into optical luminosity < ∼ 7 × 10 −7 and < ∼ 6 × 10 −6 , possibly close to that of the Vela pulsar. Conclusions. Observations with the Hubble Space Telescope (HST) are required to identify PSR J1357−6429 against nearby field stars. Owing to the high extinction (A V ∼ 5) and the presence of a molecular cloud complex, near-infrared observations of PSR J1048−5832 are better suited to spotting its candidate counterpart.
The Large Area Telescope (LAT) onboard the Fermi satellite opened a new era for pulsar astronomy, detecting γ-ray pulsations from more than 60 pulsars, ∼ 40% of which are not seen at radio wavelengths. One of the most interesting sources discovered by LAT is PSR J0357+3205, a radio-quiet, middle-aged (τ C ∼ 0.5 Myr) pulsar standing out for its very low spin-down luminosity (Ė rot ∼ 6×10 33 erg s −1 ), indeed the lowest among non-recycled γ-ray pulsars. A deep X-ray observation with Chandra (0.5-10 keV), coupled with sensitive optical/infrared ground-based images of the field, allowed us to identify PSR J0357+3205 as a faint source with a soft spectrum, consistent with a purely non-thermal emission (photon index Γ = 2.53 ± 0.25). The absorbing column (N H = 8 ± 4 × 10 20 cm −2 ) is consistent with a distance of a few hundred parsecs. Moreover, the Chandra data unveiled a huge (9 arcmin long) extended feature apparently protruding from the pulsar. Its non-thermal X-ray spectrum points to synchrotron emission from energetic particles from the pulsar wind, possibly similar to other elongated X-ray tails associated with rotation-powered pulsars and explained as bow-shock pulsar wind nebulae (PWNe). However, energetic arguments, as well as the peculiar morphology of the diffuse feature associated with PSR J0357+3205 make the bow-shock PWN interpretation rather challenging.
PSR J0205+6449 is a young (∼ 5400 years), Crab-like pulsar detected in radio and at X and γ-ray energies and has the third largest spin-down flux among known rotationpowered pulsars. It also powers a bright synchrotron nebula detected in the optical and X-rays. At a distance of ∼ 3.2 kpc and with an extinction comparable to the Crab, PSR J0205+6449 is an obvious target for optical observations. We observed PSR J0205+6449 with several optical facilities, including 8m class ground-based telescopes, such as the Gemini and the Gran Telescopio Canarias. We detected a point source, at a significance of 5.5σ, of magnitude i'∼25.5, at the centre of the optical synchrotron nebula, coincident with the very accurate Chandra and radio positions of the pulsar. Thus, we discovered a candidate optical counterpart to PSR J0205+6449. The pulsar candidate counterpart is also detected in the g' (∼27.4) band and weakly in the r' (∼26.2) band. Its optical spectrum is fit by a power law with photon index Γ O = 1.9 ± 0.5, proving that the optical emission if of non-thermal origin, is as expected for a young pulsar. The optical photon index is similar to the X-ray one (Γ X = 1.77±0.03), although the optical fluxes are below the extrapolation of the X-ray power spectrum. This would indicate the presence of a double spectral break between the X-ray and optical energy range, at variance with what is observed for the Crab and Vela pulsars, but similar to the Large Magellanic Cloud pulsar PSR B0540−69.
The Fermi Large Area Telescope (LAT) discovered the time signature of a radio-silent pulsar coincident with RX J0007.0+7302, a plerion-like X-ray source at the centre of the CTA 1 supernova remnant. The inferred timing parameters of the γ-ray pulsar PSR J0007+7303 (P =315.8 ms;Ṗ ∼ 3.6 × 10 −13 s s −1 ) point to a Vela-like neutron star, with an age comparable to that of CTA 1. The PSR J0007+7303 low distance (∼ 1.4 kpc), interstellar absorption (A V ∼ 1.6), and relatively high energy loss rate (Ė ∼ 4.5 × 10 35 erg s −1 ), make it a suitable candidate for an optical follow-up. Here, we present deep optical observations of PSR J0007+7303. The pulsar is not detected in the Gran Telescopio Canarias (GTC) images down to a limit of r ′ ∼ 27.6 (3σ), the deepest ever obtained for this pulsar, while William Herschel Telescope (WHT) images yield a limit of V ∼ 26.9. Our r ′ -band limit corresponds to an optical emission efficiency η opt ≡ L opt /Ė 9.4 × 10 −8 . This limit is more constraining than those derived for other Vela-like pulsars, but is still above the measured optical efficiency of the Vela pulsar. We compared the optical upper limits with the extrapolation of the XMM-Newton X-ray spectrum and found that the optical emission is compatible with the extrapolation of the X-ray power-law component, at variance with what is observed, e.g. in the Vela pulsar.
Abstract.Although optical pulsar studies have been limited to a few favoured objects, the observation of pulsars at optical wavelengths provides an opportunity to derive a number of important pulsar characteristics, including the energy spectrum of the emitting electrons and the geometry of the emission zone. These parameters will be vital for a comprehensive model of pulsar emission mechanisms. Observations of the Crab pulsar with the high-time-resolution photon-tagging photometer IquEYE show an optical-radio delay of ∼178 µs. Incorporating simultaneous Jodrell Bank radio observations suggested a correlation between giant radio pulses and enhanced optical pulses for this pulsar, thus offering possible evidence for the reprocessing of radio photons. Keywords. pulsars: individual (Crab) Multiwavelength High-Time-Resolution AstronomyObservations of the Crab pulsar with the high-time-resolution photometer IquEYE in December 2009 had concurrent radio observations taken on two nights at Jodrell Bank. The radio data were analysed to find so-called 'giant radio pulses' (GRPs): occasional giant pulses with an intensity of up to 1000 times that of a typical pulse (Lorimer and Kramer, 2005.) To check our reduction and timing correction pipelines, the optical and GRP data were folded using a radio ephemeris specially fitted over the observing dates. The mainpulse GRPs identified for the night of 2009 December 14 are approximately normally distributed about the nominal peak radio phase (Fig. 1). 663 GRPs were identified above a 6.0 σ threshold, which also had concurrent optical observations. SummaryThese results, in combination with those of Shearer et al. (2003), have established a clear link between the optical emission and giant radio pulses of the Crab pulsar (Fig. 2.) The delay of 178 µs recorded here between the radio and optical peaks (Fig. 3) points to a phase and spatial separation of the order of 2• and/or 50 kilometres. One possible explanation of the optical emission is the reprocessing of radio photons; see Petrova et al. (2009).We note however that optical polarisation studies (S lowikowska 2009) show an association between the optical polarisation and the arrival phase of the radio precursor. More observations are needed of the linear and circular polarisation during giant radio pulses, as well as a determination of the optical enhancement as a function of GRP phase.
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