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Register a new userMultiwavelength spectroscopy of PSR B0656+14
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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-Jeans 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.
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(abridged version) We present a detailed spectroscopic and timing analysis of X-ray observations of the bright radio-to-gamma-ray emitting pulsar PSR B0656+14, which were obtained simultaneously with eROSITA and XMM-Newton during the Calibration and Performance Verification phase of the Spektrum-Roentgen-Gamma mission (SRG) for 100 ks. Using XMM-Newton and NICER we firstly established an X-ray ephemeris for the time interval 2015 to 2020, which connects all X-ray observations in this period without cycle count alias and phase shifts. The mean eROSITA spectrum clearly reveals an absorption feature originating from the star at 570 eV with a Gaussian sigma of about 70 eV, tentatively identified earlier in a long XMM-Newton observation (Arumugasamy et al. 2018). A second absorption feature, described here as an absorption edge, occurs at 260-265 eV. It could be of atmospheric or of instrumental origin. These absorption features are superposed on various emission components, phenomenologically described as the sum of hot (120 eV) and cold (65 eV) blackbody components, both of photospheric origin, and a power-law with photon index Gamma=2. The phase-resolved spectroscopy reveals that the Gaussian absorption line at 570 eV is clearly present throughout ~60% of the spin cycle. The visibility of the line strength coincides in phase with the maximum flux of the hot blackbody. We also present three families of model atmospheres: a magnetised atmosphere, a condensed surface, and a mixed model, which were applied to the mean observed spectrum and whose continuum fit the observed data well. The atmosphere model, however, predicts too short distances. For the mixed model, the Gaussian absorption may be interpreted as proton cyclotron absorption in a field as high as 10^14 G, which is significantly higher than that derived from the moderate observed spin-down.
PSR B0656+14 is a middle-aged pulsar with a characteristic age $tau_c=110$ kyr and spin-down power $dot{E}= 3.8times 10^{34}$ erg s$^{-1}$. Using Chandra data, we searched for a pulsar wind nebula (PWN) and found evidence of extended emission in a 3.5-15 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.
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, with 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.
We report detection of PSR B0656$+$14 with the Gran Telescopio Canarias in narrow optical $F657$, $F754$, $F802$, and $F902$ and near-infrared $JHK_s$ bands. The pulsar detection in the $K_s$ band extends its spectrum to 2.2 $mu$m and confirms its flux increase towards the infrared. We also present a thorough analysis of the optical spectrum obtained by us with the VLT. For a consistency check, we revised the pulsar near-infrared and narrow-band photometry obtained with the textit{HST}. We find no narrow spectral lines in the optical spectrum. We compile available near-infrared-optical-UV and archival 0.3-20keV X-ray data and perform a self-consistent analysis of the rotation phase-integrated spectrum of the pulsar using unified spectral models. The spectrum is best fitted by the four-component model including two blackbodies, describing the thermal emission from the neutron star surface and its hot polar cap, the broken power-law, originating from the pulsar magnetosphere, and an absorption line near $sim$0.5 keV detected previously. The fit provides better constraints on the model parameters than using only a single spectral domain. The derived surface temperature is $T_{NS}^{infty}=7.9(3)times10^5$K. The intrinsic radius (7.8-9.9 km) of the emitting region is smaller than a typical neutron star radius (13km) and suggests a nonuniform temperature distribution over the star surface. In contrast, the derived radius of the hot polar cap is about twice as large as the `canonical one. The spectrum of the nonthermal emission steepens from the optical to X-rays and has a break near 0.1 keV. The X-ray data suggest the presence of another absorption line near 0.3keV.
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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