Optical detection of pulsars constitute a critical part of an expanding set of multiwavelength observations of isolated neutron stars that together aid in the development and constraining of theoretical models of pulsar electromagnetic radiation. To detect optical pulsations, it is necessary to unambiguously identify the optical counterpart of a pulsar that has been observed in other bands, say radio or gamma-rays. Because of its high spin-down energy loss and relative proximity to Earth, the radio pulsar PSR1706-44 has been a prime candidate for observation in many bands of the electromagnetic spectrum. (See Table 1 for a description of PSR1706-44, and a comparison with the Vela pulsar, which is of similar age.) The recent observation by the Very Large Telescope (VLT) -UT1 in its Science Verification (SV) phase, of the field containing this pulsar has allowed the determination of a magnitude limit in the V-band.
Table 1. Properties of PSR1706-44 and PSR0833-45 (Vela)
A number of rotation powered pulsars are found to emit high energy radiation (from optical to gamma-ray bands) which is a combination of differing amounts of three spectral components: 1) power-law emission, resulting from non-thermal radiation of particles accelerated in the pulsar magnetosphere, 2) soft blackbody emission from surface cooling of the neutron star, 3) a hard thermal component from heated polar caps. In addition, there is often a background of unpulsed emission from a surrounding synchrotron nebula.
PSR1706-44 belongs to the set of seven -ray pulsars detected by EGRET (Thompson et al. 1996). It has been detected as an unpulsed point source by ROSAT (Becker et al. 1995). While it has not yet been seen as a pulsed X-ray source, strong upper limits to its pulsed X-ray flux from the Rossi X-ray Timing Explorer (RXTE) and other satellites have been used to constrain the level of the thermal component from the heated polar caps. In the optical, PSR1706-44 has not been detected. Deep optical observations like those in this work are needed to meaningfully test the outer gap model's prediction of optical emission (Ray et al. 1999).
At present, it is not clear how and where in the pulsar magnetosphere the pulsed non-thermal high energy emission originates. Similarly, the relationship between optical pulsed emission and those in the X-ray or gamma-ray bands are unclear. Qualitatively, high energy radiation is believed to occur from incoherent curvature radiation in the outer magnetosphere or by synchrotron emission by energetic electrons near the light cylinder. So far optical pulsations have been detected from the Crab (Cocke et al. 1969) and Vela (Wallace et al. 1977) pulsars, PSR0540-69 (Middleditch & Pennypacker 1985), PSR0656+14 (Shearer et al. 1997) and (possibly) Geminga (Shearer et al. 1998), while ultraviolet pulsations were seen only from the Crab pulsar using the Hubble Space Telescope (Gull et al. 1998). The existing models of optical pulsed radiation (Pacini & Salvati 1987) underpredict the observed fluxes of middle aged pulsars like Geminga and PSR0656+14 by several orders of magnitude. A phenomenological analysis of the optical efficiencies (fraction of spin down power radiated in the optical bands) on pulsar parameters show that for the five observed so far (Goldoni et al. 1995). However, the overall consistency of the models with observed data like phase relationship and the correlation between optical and higher energy bands is not very compelling.
Here we report on the VLT -UT1 SV phase observations in the optical of the field including the position of the radio emission from PSR1706-44. The results are discussed in Sect. 2, and their implications in Sect. 3.
© European Southern Observatory (ESO) 1999
Online publication: March 1, 1999