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Register a new userLOFAR 150-MHz observations of SS 433 and W 50
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We present LOFAR high-band data over the frequency range 115-189 MHz for the X-ray binary SS 433, obtained in an observing campaign from 2013 February - 2014 May. Our results include a deep, wide-field map, allowing a detailed view of the surrounding supernova remnant W 50 at low radio frequencies, as well as a light curve for SS 433 determined from shorter monitoring runs. The complex morphology of W 50 is in excellent agreement with previously published higher-frequency maps; we find additional evidence for a spectral turnover in the eastern wing, potentially due to foreground free-free absorption. Furthermore, SS 433 is tentatively variable at 150 MHz, with both a debiased modulation index of 11 per cent and a $chi^2$ probability of a flat light curve of $8.2 times 10^{-3}$. By comparing the LOFAR flux densities with contemporaneous observations carried out at 4800 MHz with the RATAN-600 telescope, we suggest that an observed $sim$0.5-1 Jy rise in the 150-MHz flux density may correspond to sustained flaring activity over a period of approximately six months at 4800 MHz. However, the increase is too large to be explained with a standard synchrotron bubble model. We also detect a wealth of structure along the nearby Galactic plane, including the most complete detection to date of the radio shell of the candidate supernova remnant G 38.7-1.4. This further demonstrates the potential of supernova remnant studies with the current generation of low-frequency radio telescopes.
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We present the first wide area (19 deg$^2$), deep ($approx120-150$ {mu}Jy beam$^{-1}$), high resolution ($5.6 times 7.4$ arcsec) LOFAR High Band Antenna image of the Bootes field made at 130-169 MHz. This image is at least an order of magnitude deeper and 3-5 times higher in angular resolution than previously achieved for this field at low frequencies. The observations and data reduction, which includes full direction-dependent calibration, are described here. We present a radio source catalogue containing 6276 sources detected over an area of $19$,deg$^2$, with a peak flux density threshold of $5sigma$. As the first thorough test of the facet calibration strategy, introduced by van Weeren et al., we investigate the flux and positional accuracy of the catalogue. We present differential source counts that reach an order of magnitude deeper in flux density than previously achieved at these low frequencies, and show flattening at 150 MHz flux densities below 10 mJy associated with the rise of the low flux density star-forming galaxies and radio-quiet AGN.
Four mosaics of deep, continuum-subtracted, CCD images have been obtained over the extensive galactic radio continuum shell, W 50, which surrounds the remarkable stellar system SS 433. Two of these mosaics in the Halpha+[N II] and [O III] 5007 A emission lines respectively cover a field of ~2.3 x 2.5 degr^2 which contains all of W 50 but at a low angular resolution of 5 arcsec. The third and fourth mosaics cover the eastern (in [O III] 5007 A) and western (in Halpha+[N II]) filamentary nebulosity respectively but at an angular resolution of 1 arcsec. These observations are supplemented by new low dispersion spectra and longslit, spatially resolved echelle spectra. The [O III] 5007 A images show for the first time the distribution of this emission in both the eastern and western filaments while new Halpha+[N II] emission features are also found in both of these regions. Approaching flows of faintly emitting material from the bright eastern filaments of up 100 km/s in radial velocity are detected. The present observations also suggest that the heliocentric systemic radial velocity of the whole system is 56+-2 km/s. Furthermore, very deep imagery and high resolution spectroscopy of a small part of the northern radio ridge of W 50 has revealed for the first time the very faint optical nebulosity associated with this edge. It is suggested that patchy foreground dust along the ~5 kpc sightline is inhibiting the detection of all of the optical nebulosity associated with W 50. The interaction of the microquasar jets of SS 433 with the W 50 shell is discussed.
The detection of two sources of gamma rays towards the microquasar SS 433 has been recently reported. The first source can be associated with SS 433s eastern jet lobe, whereas the second source is variable and displays significant periodicity compatible with the precession period of the binary system, of about 160 days. The location of this variable component is not compatible with the location of SS 433 jets. To explain the observed phenomenology, a scenario based on the illumination of dense gas clouds by relativistic protons accelerated at the interface of the accretion disk envelope has been proposed. Energetic arguments strongly constrain this scenario, however, as it requires an unknown mechanism capable to periodically channel a large fraction of SS 433s kinetic energy towards an emitter located 36 parsec away from the central binary system.
We study the optical variability of the peculiar Galactic source SS 433 using the observations made with the Russian Turkish 1.5-m telescope (RTT150). A simple technique which allows to obtain high-quality photometric measurements with 0.3-1 s time resolution using ordinary CCD is described in detail. Using the test observations of nonvariable stars, we show that the atmospheric turbulence introduces no significant distortions into the measured light curves. Therefore, the data obtained in this way are well suited for studying the aperiodic variability of various objects. The large amount of SS 433 optical light curve measurements obtained in this way allowed us to obtain the power spectra of its flux variability with a record sensitivity up to frequencies of ~0.5 Hz and to detect its break at frequency =~2.4e-3 Hz. We suggest that this break in the power spectrum results from the smoothing of the optical flux variability due to a finite size of the emitting region. Based on our measurement of the break frequency in the power spectrum, we estimated the size of the accretion-disk photosphere as 2e12 cm. We show that the amplitude of the variability in SS 433 decreases sharply during accretion-disk eclipses, but it does not disappear completely. This suggests that the size of the variable optical emission source is comparable to that of the normal star whose size is therefore R_O approx 2e12 cm approx 30 R_sun. The decrease in flux variability amplitude during eclipses suggests the presence of a nonvariable optical emission component with a magnitude m_R=~13.2.
The extended jets of the microquasar SS 433 have been observed in optical, radio, X-ray, and recently very-high-energy (VHE) $gamma$-rays by HAWC. The detection of HAWC $gamma$-rays with energies as great as 25 TeV motivates searches for high-energy $gamma$-ray counterparts in the Fermi-LAT data in the 100 MeV--300 GeV band. In this paper, we report on the first-ever joint analysis of Fermi-LAT and HAWC observations to study the spectrum and location of $gamma$-ray emission from SS~433. Our analysis finds common emission sites of GeV-to-TeV $gamma$-rays inside the eastern and western lobes of SS 433. The total flux above 1 GeV is $sim 1times10^{-10},rm cm^{-2},s^{-1}$ in both lobes. The $gamma$-ray spectrum in the eastern lobe is consistent with inverse-Compton emission by an electron population that is accelerated by jets. To explain both the GeV and TeV flux, the electrons need to have a soft intrinsic energy spectrum, or undergo a quick cooling process due to synchrotron radiation in a magnetized environment.
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