Pulsar wind nebulae are a prominent class of very high energy (E > 0.1 TeV) Galactic sources. Their Gamma-ray spectra are interpreted as due to inverse Compton scattering of ultrarelativistic ...electrons on the ambient photons, whereas the X-ray spectra are due to synchrotron emission. We investigate the relation between the Gamma- and-X-ray emission and the pulsars' spin-down luminosity and characteristic age. We find that the distance-independent Gamma- to X-ray flux ratio of the nebulae is inversely proportional to the spin-down luminosity, (\propto \dot{E}^-1.9), while it appears proportional to the characteristic age, (\propto tau_c^2.2), of the parent pulsar. We interpret these results as due to the evolution of the electron energy distribution and the nebular dynamics, supporting the idea of so-called relic pulsar wind nebulae. These empirical relations provide a new tool to classify unidentified diffuse Gamma-ray sources and to estimate the spin-down luminosity and characteristic age of rotation powered pulsars with no detected pulsation from the X- and Gamma-ray properties of the associated pulsar wind nebulae. We apply these relations to predict the spin-down luminosity and characteristic age of four (so far unpulsing) candidate pulsars associated to wind nebulae.
We have analyzed the new deep XMM-Newton and Chandra observations of the energetic, radio-quiet pulsar J1813-1246. The X-ray spectrum is nonthermal, very hard, and absorbed. Based on spectral ...considerations, we propose that J1813 is located at a distance further than 2.5 kpc. J1813 is highly pulsed in the X-ray domain, with a light curve characterized by two sharp, asymmetrical peaks, separated by 0.5 in phase. We detected no significant X-ray spectral changes during the pulsar phase. We extended the available Fermi ephemeris to five years. We found two glitches. The gamma -ray light curve is characterized by two peaks, separated by 0.5 in phase, with a bridge in between and no off-pulse emission. The spectrum shows clear evolution in phase, being softer at the peaks and hardening toward the bridge. Surprisingly, both X-ray peaks lag behind the gamma -ray ones by a quarter of phase. We found a hint of detection in the 30-500 keV band with INTEGRAL, which is consistent with the extrapolation of both the soft X-ray and gamma -ray emission of J1813. The unique X-ray and gamma -ray phasing suggests a singular emission geometry. We discuss some possibilities within the current pulsar emission models. Finally, we develop an alternative geometrical model where the X-ray emission comes from polar cap pair cascades.
Aims. Supernova remnants (SNRs) are believed to be the main sources of Galactic cosmic rays. Molecular clouds associated with SNRs can produce gamma-ray emission by means of the interaction of ...accelerated particles with the concentrated gas. The middle-aged SNR W28, because of its associated system of dense molecular clouds, provides an excellent opportunity to test this hypothesis. Methods. We present the AGILE/GRID observations of SNR W28, and compare them with observations at other wavelengths (TeV and 12CO (J=1$\rightarrow$0) molecular line emission). Results. The gamma-ray flux detected by AGILE from the dominant source associated with W28 is (14 ± 5) × 10-8 ph cm-2 s-1 for E > 400 MeV. This source is positionally well correlated with the TeV emission observed by the HESS telescope. The local variations in the GeV to TeV flux ratio imply that there is a difference between the CR spectra of the north-west and south molecular cloud complexes. A model based on a hadronic-induced interaction and diffusion with two molecular clouds at different distances from the W28 shell can explain both the morphological and spectral features observed by both AGILE in the MeV-GeV energy range and the HESS telescope in the TeV energy range. The combined set of AGILE and H.E.S.S. data strongly support a hadronic model for the gamma-ray production in W28.
Isolated neutron stars are seen in x-rays through their nonthermal and/or surface thermal emissions. X-ray Multimirror Mission-Newton observations of the Geminga pulsar show a 43-electron volt ...spectrum from the whole neutron star surface, as well as a power-law component above 2 kiloelectron volts. In addition, we have detected a hot (170 electron volts) thermal emission from an ~60-meter-radius spot on the pulsar's surface. Such a thermal emission, only visible at selected phase intervals, may be coming from polar hot spot(s), long thought to exist as a result of heating from magnetospheric accelerated particles. It may provide the missing link between the x-ray and gamma-ray emission of the pulsar.
The optical spectrum of the Vela pulsar Mignani, R. P.; Zharikov, S.; Caraveo, P. A.
Astronomy and astrophysics (Berlin),
10/2007, Letnik:
473, Številka:
3
Journal Article
Recenzirano
Odprti dostop
Context.Our knowledge of the optical spectra of Isolated Neutron Stars (INSs) is limited by their intrinsic faintness. Among the fourteen optically identified INSs, medium resolution spectra have ...been obtained only for a handful of objects. No spectrum has been published yet for the Vela pulsar (PSR B0833–45), the third brightest ($V=23.6$) INS with an optical counterpart. Optical multi-band photometry underlines a flat continuum. Aims.In this work we present the first optical spectroscopy observations of the Vela pulsar, performed in the 4000-11 000 Å spectral range. Methods.Our observations have been performed at the ESO Very Large Telescope (VLT) using the FORS2 instrument. Results.The spectrum of the Vela pulsar is characterized by a flat power-law $F_{\nu} \propto \nu^{-\alpha}$ with $\alpha = -0.04 \pm0.04 $ (4000-8000 Å), which compares well with the values obtained from broad-band photometry. This confirms, once more, that the optical emission of Vela is entirely of magnetospheric origin. Conclusions.The comparison between the optical spectral indeces of rotation-powered INSs does not show evidence for a spectral evolution, suggesting that, as in the X-rays, the INS aging does not affect the spectral properties of the magnetospheric emission. At the same time, the optical spectral indeces are found to be nearly always flatter then the X-rays ones, clearly suggesting a general spectral turnover at lower energies.
We report the discovery of PSR J1838-0537, a gamma-ray pulsar found through a blind search of data from the Fermi Large Area Telescope (LAT). The pulsar has a spin frequency of 6.9 Hz and a frequency ...derivative of -2.2 x 10 super(-11) Hz s super(-1), implying a young characteristic age of 4970 yr and a large spin-down power of 5.9 x 10 super(36) erg s super(-1). Follow-up observations with radio telescopes detected no pulsations; thus PSR J1838-0537 appears radio-quiet as viewed from Earth. In 2009 September the pulsar suffered the largest glitch so far seen in any gamma-ray-only pulsar, causing a relative increase in spin frequency of about 5.5 x 10 super(-6) After the glitch, during a putative recovery period, the timing analysis is complicated by the sparsity of the LAT photon data, the weakness of the pulsations, and the reduction in average exposure from a coincidental, contemporaneous change in LAT's sky-survey observing pattern. The pulsar's sky position is coincident with the spatially extended TeV source HESS J1841-055 detected by the High Energy Stereoscopic System (H.E.S.S.). The inferred energetics suggest that HESS J1841-055 contains a pulsar wind nebula powered by the pulsar.
We report the results of a deep XMM-Newton observation of the radio-faint gamma -ray pulsar J1741-2054 and its nebula together with the analysis of five years of Fermi Large Area Telescope (LAT) ...data. The X-ray spectrum of the pulsar is consistent with an absorbed power law plus a blackbody, originating at least partly from the neutron star cooling. The nebular emission is consistent with that of a synchrotron pulsar wind nebula, with hints of spatial spectral variation. We extended the available Fermi LAT ephemeris and folded the gamma -ray and X-ray data. We detected X-ray pulsations from the neutron star: both the thermal and non-thermal components are ~35%-40% pulsed, with phase-aligned maxima. A sinusoid fits the thermal-folded profile well. A 10 bin phase-resolved analysis of the X-ray emission shows softening of the non-thermal spectrum during the on-pulse phases. The radio, X-ray, and gamma -ray light curves are single-peaked, not phase-aligned, with the X-ray peak trailing the gamma -ray peak by more than half a rotation. Spectral considerations suggest that the most probable pulsar distance is in the 0.3-1.0 kpc range, in agreement with the radio dispersion measure.
Context. X-ray observations unveiled the existence of enigmatic point-like sources at the centre of young supernova remnants (SNRs). These sources, dubbed central compact objects (CCOs), are thought ...to be neutron stars formed by the supernova explosion. However, their multi-wavelength phenomenology is surprisingly different from that of most young neutron stars. Aims. The aim of this work is to understand the nature of the CCO 1WGA J1713.4-3949 in the G347.3-0.5 SNR through deep optical and infrared (IR) observations, the first ever performed for this source. Methods. By exploiting its derived Chandra X-ray position we carried out optical (BVI) observations with the New Technology Telescope (NTT) and adaptive optics IR (JHKs) observations with the Very Large Telescope (VLT). Results. We detected two faint ($I\approx23.5$, $I\approx 24.3$.) patchy objects close to the Chandra error circle in the NTT images. They were clearly resolved in our VLT images which unveiled a total of six candidate counterparts ($17.8<H<20.3$) with quite red colours ($H-K_{\rm s}$ ~ 0.6). If they are stars, none of them can be associated with 1WGA J1713.4-3949 for the most likely values of distance and hydrogen column density. The identification of the faintest candidate with the neutron star itself can not be firmly excluded, while the identification with a fallback disk is ruled out by its non-detection in the I band. No other candidates are detected down to $B\sim 26$, $V\sim26.2$, $I\sim 24.7$, $H\sim$ 21.3, and $K\sim$ 20.5. Conclusions. Our high-resolution IR imaging unveiled a few objects close/within the Chandra X-ray position of 1WGA J1713.4-3949. However, at present none of them can be firmly identified as the object's likely counterpart.
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