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تسجيل مستخدم جديدOptical photometry of the PSR B0656+14 and its neighborhood
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A. B. Koptsevich
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We present the results of the broad-band photometry of the nearby middle-aged radio pulsar PSR B0656+14 and its neighborhood obtained with the 6-meter telescope of the SAO RAS and with the Hubble Space Telescope. The broad-band spectral flux $F_ u$ of the pulsar decreases with increasing frequency in the near-IR range and increases with frequency in the near-UV range. The increase towards UV can be naturally interpreted as the Rayleigh-Jeans tail of the soft thermal component of the X-ray spectrum emitted from the surface of the cooling neutron star. Continuation of the power-law component, which dominates in the high-energy tail of the X-ray spectrum, to the IR-optical-UV frequencies is consistent with the observed fluxes. This suggests that the non-thermal pulsar radiation may be of the same origin in a broad frequency range from IR to hard X-rays. We also studied 4 objects detected in the pulsars 5 neighborhood.
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We have observed the optical pulse profile of PSR B0656+14 in 10 phase bins at a high signal-to-noise ratio, and have measured the linear polarization profile over 30% of the pulsar period with some significance. The pulse profile is double-peaked, w
ith a bridge of emission between the two peaks, similar to gamma-ray profiles observed in other pulsars. There is no detectable unpulsed flux, to a 1-sigma limit of 16% of the pulse-averaged flux. The emission in the bridge is highly (~ 100%) polarized, with a position angle sweep in excellent agreement with the prediction of the Rotating Vector Model as determined from radio polarization observations. We are able to account for the gross features of the optical light curve (i.e., the phase separation of the peaks) using both polar cap and outer gap models. Using the polar cap model, we are also able to estimate the height of the optical emission regions.
Using high-quality Hubble Space Telescope observations, we construct the near infra-red (NIR) to far ultra-violet (FUV) spectral energy distribution (SED) of PSR B0656+14. The SED is non-monotonic. Fitting it with a simple combination of a Rayleigh-J
eans spectrum (UV) and non-thermal power-law (optical/NIR) leaves significant residuals, strongly hinting at one or more spectral features. We consider various models (combination of continuum components, and absorption/emission lines) with possible interpretations, and place them in the context of the broader spectral energy distribution. Surprisingly, the extrapolation of the best-fit X-ray spectral model roughly match the NIR-FUV data, and the power-law component is also consistent with the gamma-ray fluxes. We compare the multiwavelength SED of B0656+14 with those of other optical, X-ray and gamma-ray detected pulsars, and notice that a simple power-law spectrum crudely accounts for most of the non-thermal emission.
We observed the middle-aged radio pulsar 0656+14 with a CCD detector at the 6-m telescope. Broadband BVRI images show the following magnitudes of the pulsar counterpart: B = 24.85 (+0.19, -0.16), V = 24.90 (+0.16, -0.14), R = 24.52 (+0.12, -0.11), I
= 23.81 (+0.27, -0.21). We fitted the UV-optical (space + ground-based) data with a two-component model which combines a power law (non-thermal component) with a thermal spectrum emitted by the neutron star surface. The power law component, with the energy power-law index alpha=1.5 (+1.1,-1.2), dominates in the observed range. Constraints on the thermal component correspond to the Rayleigh-Jeans parameter Gequiv T_6(R_{10}/d_{500})^2=4.1 (+2.1, -4.1), where $T=10^6T_6$ K is the brightness temperature, R_infty = 10 R_{10}$ km is the neutron star radius as seen by a distant observer, and $d=500 d_{500}$ pc is the distance. The shape of the optical-UV spectrum of PSR 0656+14 differs considerably from those observed from other pulsars: the middle-aged Geminga and young Crab, Vela.
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l and the statistical significance is 4.5 sigma. The pulsed fraction was estimated 0.64+/-0.17. This pulsation was detected also at energies greater than 2.5 keV suggesting either a non-thermal origin or a quite high temperature region on the neutron star surface. Spectral analysis showed that only the X-ray spectrum of PSR B1706-44 can be fitted by a single power-law component, while that of PSR B1055-52 requires also a blackbody component (kT = 0.075 keV) and that of PSR B0656+14 two blackbody components (kT_1 = 0.059, kT_2 = 0.12 keV).
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5-15 arcsec annulus around the pulsar, with a luminosity $L_{rm 0.5-8,keV}^{rm ext} sim 8times 10^{28}$ erg s$^{-1}$ (at the distance of 288 pc), which is a fraction of $sim 0.05$ of the non-thermal pulsar luminosity. If the extended emission is mostly due to a PWN, its X-ray efficiency, $eta_{rm pwn} = L_{rm 0.5-8,keV}^{rm ext}/dot{E} sim 2times 10^{-6}$, is lower than those of most other known PWNe but similar to that of the middle-aged Geminga pulsar. The small radial extent and nearly round shape of the putative PWN can be explained if the pulsar is receding (or approaching) in the direction close to the line of sight. The very soft spectrum of the extended emission ($Gammasim 8$), much softer than those of typical PWNe, could be explained by a contribution from a faint dust scattering halo, which may dominate in the outer part of the extended emission.
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