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Astron. Astrophys. 342, L5-L8 (1999)

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3. PSR B0656+14

An extensive literature exists devoted to the analysis of EXOSAT, ROSAT and EUVE observations of the pulsar PSR B0656+14. The consensus is of thermal emission from the surface, modulated by emission from a hot polar cap and from nonthermal activity in the magnetosphere. However, its distance estimate is by no means exact ranging from [FORMULA] 100 to 760 pc (Caraveo et al. 1994) - the latter based upon a DM fit to the uncertain galactic electron model in this vicinity (Taylor et al. 1993). Applying a common distance estimate of [FORMULA] 500 pc, a range of solutions are possible in ([FORMULA], [FORMULA]) space assuming the canonical model, and this is reflected in the literature, with [FORMULA] = (3-9)[FORMULA] K and [FORMULA] = (0.5-2.0)[FORMULA] (Finley et al. 1992, Anderson et al. 1993, Greiveldinger et al. 1996). Early limited observations using the NTT (Caraveo et al. 1994) and the HST (Pavlov et al. 1996) indicated that the optical counterpart's emission was predominately nonthermal in nature, and a subsequent two-component model fit to detailed HST and ground-based photometry spanning the UBVRI regime by [FORMULA] substantiated this assessment - although the detection of highly pulsed emission in the B band (Shearer et al. 1997) from the optical counterpart unequivocally confirmed this hypothesis. The upper limit on the unpulsed flux from the resulting lightcurve was estimated to be [FORMULA]. Fig. 2. shows the [FORMULA] model fit with these pulsed/unpulsed fluxes.

[FIGURE] Fig. 2. HST & 6m BTA based photometry and best-fit two component model of PSR B0656+14 for the colour excess [FORMULA] = 0.03 (Pavlov et al. 1997), with TRIFFID/BTA pulsed and unpulsed flux estimates in B superimposed (Shearer et al. 1997)

[FORMULA] fitted the observed UBVRI spectrum with a two-component model following the formalism of (2) and (3). Applying the interstellar extinctions determined for the three estimated colour excesses towards the pulsar of [FORMULA] = 0.01, 0.03 and 0.05 yielded best fit values of G = 3.0, 3.7 and 4.3 respectively. Taking [FORMULA] = [FORMULA], (2) may be rearranged in terms of G, and solved for the three [FORMULA] estimates, concluding with G = 4.4, 4.8 and 5.2 (all [FORMULA]). respectively. An increase of this G parameter is consistent with either an increase in the expected [FORMULA], emission area or a decrease in the pulsar's distance.

In terms of previous model fits, G ranges from 0.9 (Finley et al. 1992), 2.1 (Greiveldinger et al. 1996) to 2.6 (Anderson et al. 1993), the latter incorporating a magnetised ([FORMULA] G) H atmosphere. As observations have accrued, and uncertainties in X-ray detector sensitivities have been addressed, the trend has been a decrease in the derived surface temperature, from [FORMULA]K to [FORMULA]K. Recent work by Edelstein et al. (1998), which substituted EUVE DS data in place of the uncertain low energy ROSAT PSPC channels, has yielded a ([FORMULA],[FORMULA]) space differing markedly from earlier ROSAT results alone. Combining this new parameter space with independent model fits incorporating the B unpulsed upper limit suggests [FORMULA] [FORMULA] 5.0[FORMULA] K (Golden 1998). It is possible to constrain [FORMULA] for B0656+14 in two ways - firstly by determining apparent expected radii using both column density and DM derived distances, and secondly by applying canonical or otherwise derived radius limits to yield optimum distance scales. [FORMULA] estimates towards PSR B0656+14, although by no means certain, suggest that the pulsar is [FORMULA] 250-280 pc (Anderson et al. 1993, Edelstein et al. 1998), rather closer than the DM derived distance of [FORMULA] pc. By manipulation of (3) with [FORMULA] [FORMULA] 5.0[FORMULA] K and the range of G parameters determined from the unpulsed upper limit, the expected radial estimates for the [FORMULA] distances are 14.7 [FORMULA] [FORMULA] [FORMULA] 17.7 km, and substantially greater in the case of the radio derived distance. We note that such radii estimates are in excess of the 14 km upper limit determined by Walter et al. (1997) for the old INS RXJ185635-3754. Alternatively, applying the ideal canonical [FORMULA] [FORMULA] 13km, manipulation of (3) as above suggests that 205 [FORMULA] d [FORMULA] 227 pc based upon the range of [FORMULA] estimates. This supports the conclusions of [FORMULA] although placing the pulsar somewhat in closer proximity than had been originally thought. Indeed if one was to consider the proposed estimate of [FORMULA] [FORMULA] [FORMULA] km for Geminga as a working upper limit, this would place PSR B0656+14 at a distance of no less than d = [FORMULA] for the suggested optimum colour index of [FORMULA] = 0.03 (PWC97)

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© European Southern Observatory (ESO) 1999

Online publication: December 22, 1998
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