35 research outputs found
Stable accretion from a cold disc in highly magnetized neutron stars
The aim of this paper is to investigate the transition of a strongly
magnetized neutron star into the accretion regime with very low accretion rate.
For this purpose we monitored the Be-transient X-ray pulsar GRO J1008-57
throughout a full orbital cycle. The current observational campaign was
performed with the Swift/XRT telescope in the soft X-ray band (0.5-10 keV)
between two subsequent Type I outbursts in January and September 2016. The
expected transition to the propeller regime was not observed. However, the
transitions between different regimes of accretion were detected. In
particular, after an outburst the source entered a stable accretion state
characterised by the accretion rate of ~10^14-10^15 g/s. We associate this
state with accretion from a cold (low-ionised) disc of temperature below ~6500
K. We argue that a transition to such accretion regime should be observed in
all X-ray pulsars with certain combination of the rotation frequency and
magnetic field strength. The proposed model of accretion from a cold disc is
able to explain several puzzling observational properties of X-ray pulsars.Comment: 8 pages, 3 figures, 1 table, accepted by A&
The supercritical accretion disk in SS433 and ultraluminous X-ray sources
SS433 is the only known persistent supercritical accretor, it may be very
important for understanding ultraluminous X-ray sources (ULXs) located in
external galaxies. We describe main properties of the SS433 supercritical
accretion disk and jets. Basing on observational data of SS433 and published 2D
simulations of supercritical accretion disks we estimate parameters of the
funnel in the disk/wind of SS 433. We argue that the UV radiation of the SS433
disk (~50000 K, ~10^{40}erg/s) is roughly isotropic, but X-ray radiation (~10^7
K, ~10^{40}erg/s) of the funnel is midly anisotropic. A face-on SS433 object
has to be ultraluminous in X-rays (10^{40-41}erg/s). Typical time-scales of the
funnel flux variability are estimated. Shallow and very broad (0.1-0.3c) and
blue-shifted absorption lines are expected in the funnel X-ray spectrum.Comment: 4 pages, 2 figures; Proceedings of IAUS23
Broad band variability of SS433: Accretion disk at work?
We present broad band power spectra of variations of SS433 in radio, optical and X-ray spectral bands. We show that at frequencies lower than 10 -5 Hz the source demonstrates the same variability pattern in all these bands. The broad band power spectrum can be fitted by one power law down to frequencies âŒ10-7 Hz with flattening afterwards. Such a flattening means that on time scales longer than âŒ107 s the source variability becomes uncorrelated. This naturally leads to the appearance of quasi-poissonian flares in the source light curve, which have been regularly observed in radio and optical spectral bands. The radio flux power spectrum appears to have a second break at Fourier frequencies âŒ10-5 Hz which can be caused by the smearing of the intrinsic radio variability on timescale of the light-crossing time of the radio emitting region. We find a correlation of the radio and optical fluxes of SS433 and the radio flux is delayed by about âŒ2 days with respect to the optical one. Power spectra of optical and X-ray variabilities continue with the same power law from 10 -7 Hz up to âŒ0.01-0.05 Hz. The broad band power spectrum of SS433 can be interpreted in terms of self-similar accretion rate modulations in the accretion disk proposed by Lyubarskii (1997, MNRAS, 292, 679) and elaborated by Churazov et al. (2001, MNRAS, 321, 759). We discuss a viscous time-scale in the accretion disk of SS433 with reference to the observed broad band power spectrum. © ESO 2006
Two Active States of the Narrow-Line Gamma-Ray-Loud AGN GB 1310 + 487
Context. Previously unremarkable, the extragalactic radio source GB1310 487 showed gamma-ray flare on 2009 November 18, reaching a daily flux of approximately 10(exp -6) photons cm(exp -2) s(exp -1) at energies E greater than 100MeV and became one of the brightest GeV sources for about two weeks. Its optical spectrum shows strong forbidden-line emission while lacking broad permitted lines, which is not typical for a blazar. Instead, the spectrum resembles those of narrow emission-line galaxies. Aims. We investigate changes in the object's radio-to-GeV spectral energy distribution (SED) during and after the prominent gamma-ray flare with the aim of determining the nature of the object and of constraining the origin of the variable high-energy emission. Methods. The data collected by the Fermi and AGILE satellites at gamma-ray energies; Swift at X-ray and ultraviolet (UV); the Kanata, NOT, and Keck telescopes at optical; OAGH and WISE at infrared (IR); and IRAM30m, OVRO 40m, Effelsberg 100m, RATAN-600, and VLBA at radio are analyzed together to trace the SED evolution on timescales of months. Results. The gamma-ray radio-loud narrow-line active galactic nucleus (AGN) is located at redshift z = 0.638. It shines through an unrelated foreground galaxy at z = 0.500. The AGN light is probably amplified by gravitational lensing. The AGN SED shows a two-humped structure typical of blazars and gamma-ray-loud narrow-line Seyfert 1 galaxies, with the high-energy (inverse-Compton) emission dominating by more than an order of magnitude over the low-energy (synchrotron) emission during gamma-ray flares. The difference between the two SED humps is smaller during the low-activity state. Fermi observations reveal a strong correlation between the gamma-ray flux and spectral index, with the hardest spectrum observed during the brightest gamma-ray state. The gamma-ray flares occurred before and during a slow rising trend in the radio, but no direct association between gamma-ray and radio flares could be established. Conclusions. If the gamma-ray flux is a mixture of synchrotron self-Compton (SSC) and external Compton (EC) emission, the observed GeV spectral variability may result from varying relative contributions of these two emission components. This explanation fits the observed changes in the overall IR to gamma-ray SED
Unravelling a simple method for the low temperature synthesis of silicon nanocrystals and monolithic nanocrystalline thin films
In this work, we present new results on the plasma processing and structure of hydrogenated polymorphous silicon (pm-Si:H) thin films. pm-Si:H thin films consist of a low volume fraction of silicon nanocrystals embedded in a silicon matrix with medium range order, and they possess this morphology as a significant contribution to their growth comes from the impact on the substrate of silicon clusters and nanocrystals synthesized in the plasma. Quadrupole mass spectrometry, ion flux measurements, and material characterization by transmission electron microscopy (TEM) and atomic force microscopy all provide insight on the contribution to the growth by silicon nanocrystals during PECVD deposition. In particular, cross-section TEM measurements show for the first time that the silicon nanocrystals are uniformly distributed across the thickness of the pm-Si:H film. Moreover, parametric studies indicate that the best pm-Si:H material is obtained at the conditions after the transition between a pristine plasma and one containing nanocrystals, namely a total gas pressure around 2 Torr and a silane to hydrogen ratio between 0.05 to 0.1. From a practical point of view these conditions also correspond to the highest deposition rate achievable for a given RF power and silane flow rate.ope
Broad band variability of SS433: Accretion disk at work?
We present broad band power spectra of variations of SS433 in radio, optical
and X-ray spectral bands. We show that at frequencies lower than 1e-5 Hz the
source demonstrates the same variability pattern in all these bands. The broad
band power spectrum can be fitted by one power law down to frequencies ~1e-7 Hz
with flattening afterwards. Such a flattening means that on time scales longer
than ~1e7 sec the source variability becomes uncorrelated. This naturally leads
to the appearance of quasi-poissonian flares in the source light curve, which
have been regularly observed in radio and optical spectral bands. The radio
flux power spectrum appears to have a second break at Fourier frequencies ~1e-5
Hz which can be caused by the smearing of the intrinsic radio variability on
timescale of the light-crossing time of the radio emitting region. We find a
correlation of the radio and optical fluxes of SS433 and the radio flux is
delayed by about ~2 days with respect to the optical one. Power spectra of
optical and X-ray variabilities continue with the same power law from 1e-7 Hz
up to ~0.01-0.05 Hz. The broad band power spectrum of SS433 can be interpreted
in terms of self-similar accretion rate modulations in the accretion disk
proposed by Lyubarskii (1997) and elaborated by Churazov et al. (2001). We
discuss a viscous time-scale in the accretion disk of SS433 in implication to
the observed broad band power spectrum.Comment: 8 pages, 2 figures. Submitted to A&
Spectroscopy of Optical Counterparts of Ultraluminous X-ray Sources
Here we present the results of panoramic and long-slit observations of eight
ULX nebular counterparts held with the 6m SAO telescope. In two ULXNe we
detected for the first time signatures of high excitation ([OIII]5007 / H\beta
> 5). Two of the ULXs were identified with young (T ~ 5-10 Myr) massive star
clusters. Four of the eight ULX Nebulae (ULXNe) show bright high-excitation
lines. This requires existence of luminous (~ 10^{38} .. 10^{40} erg/s) UV/EUV
sources coinciding with the X-ray sources. Other 4 ULXNe require shock
excitation of the gas with shock velocities of 20-100km/s. However, all the
studied ULXN spectra show signatures of shock excitation, but even those ULXNe
where the shocks are prevailing show presence of a hard ionizing source with
the luminosity at least ~10^{38} erg/s. Most likely shock waves, X-ray and EUV
ionization act simultaneously in all the ULXNe, but they may be roughly
separated in two groups, shock-dominated and photoionization-dominated ULXNe.
The ULXs have to produce strong winds and/or jets powering their nebulae with
\~10^{39} erg/s. Both the wind/jet activity and the EUV source needed are
consistent with the suggestion that ULXs are high-mass X-ray binaries with the
supercritical accretion disks of the SS433 type.Comment: submitted to Astrophysical Bulletin (Bull. Special Astrophys. Obs.