130 research outputs found
On reverberation and cross-correlation estimates of the size of the broad-line region in active galactic nuclei
It is known that the dependence of the emission-line luminosity of a typical
cloud in the active galactic nuclei broad-line regions (BLRs) upon the incident
flux of ionizing continuum can be nonlinear. We study how this nonlinearity can
be taken into account in estimating the size of the BLR by means of the
"reverberation" methods. We show that the BLR size estimates obtained by
cross-correlation of emission-line and continuum light curves can be much (up
to an order of magnitude) less than the values obtained by reverberation
modelling. This is demonstrated by means of numerical cross-correlation and
reverberation experiments with model continuum flares and emission-line
transfer functions and by means of practical reverberation modelling of the
observed optical spectral variability of NGC 4151. The time behaviour of NGC
4151 in the H_alpha and H_beta lines is modelled on the basis of the
observational data by Kaspi et al. (1996, ApJ, 470, 336) and the theoretical
BLR model by Shevchenko (1984, Sov. Astron. Lett., 10, 377; 1985, Sov. Astron.
Lett., 11, 35). The values of the BLR parameters are estimated that allow to
judge on the size and physical characteristics of the BLR. The small size of
the BLR, as determined by the cross-correlation method from the data of Kaspi
et al. (1996, ApJ, 470, 336), is shown to be an artifact of this method. So,
the hypothesis that the BLR size varies in time is not necessitated by the
observational data.Comment: 26 pages, including 11 figure
V4641Sgr - Super-Eddington source enshrouded by an extended envelope
An optical spectroscopy of an unusual fast transient V4641 Sgr constrains its
mass to 8.7-11.7M_sun (9.6M_sun is the best fit value) and the distance to
7.4--12.3 kpc (Orosz et al. 2001). At this distance the peak flux of 12 Crab in
the 2--12 keV energy band, measured by ASM/RXTE, implies the X-ray luminosity
exceeding 2-3e39 erg/s, i.e. near or above the Eddington limit for a 9.6M_sun
black hole. An optical photometry shows that at the peak of the optical
outburst the visual magnitude increased by Delta m_V > 4.7^m relative to the
quiescent level and reached m_V < 8.8^m. An assumption that this optical
emission is due to irradiated surface of an accretion disk or a companion star
with the the black body shape of the spectrum would mean that the bolometric
luminosity of the system exceeds L>3e41 erg/s > 300 L_Edd.
We argue that the optical data strongly suggest presence of an extended
envelope surrounding the source which absorbs primary X-rays flux and reemits
it in optical and UV. The data also suggests that this envelope should be
optically thin in UV, EUV and soft X-rays. The observed properties of V4641 Sgr
at the peak of an optical flare are very similar to those of SS433. This
envelope is likely the result of near or super Eddington rate of mass accretion
onto the black hole and it vanishes during subsequent evolution of the source
when apparent luminosity drops well below the Eddington value. Thus this
transient source provides us direct proof of the dramatic change in the
character of an accretion flow at the mass accretion rate near or above the
critical Eddington value as predicted long time ago by the theoretical models.Comment: 4 pages, 2 figures. Submitted to A&A Letter
Possible 38 day X-ray period of KS1731-260
We report the detection of a 38 day period in the X-ray flux of the transient
burster KS1731-260. The narrow peak of periodicity was detected during ~TJD
10150--11050 when the source had a high and relatively stable X-ray flux. After
\~TJD 11100 the source became strongly variable on a time scale of months that
contaminates the search for the 38 day periodicity. The detected period can not
be a binary period. The binary with Roche lobe overflow has in this case large
radii of the secondary and of the accretion disk. Disk and secondary star
illumination by X-ray flux from luminous neutron star would lead to high
infrared brightness of the binary. That clearly contradicts the infrared data
even for the brightest infrared sources within CHANDRA error box of KS1731-260.
Remaining possibility is that observed periodicity is connected with the
accretion disk precession, similar to that was observed for SS 433, Her X-1,
Cyg X-1 etc.Comment: 4 pages, 5 figures. Submitted to A&A Letter
Variability of Active Galactic Nuclei from the Optical to X-ray Regions
Some progress in understanding AGN variability is reviewed. Reprocessing of
X-ray radiation to produce significant amounts of longer-wavelength continua
seems to be ruled out. In some objects where there has been correlated X-ray
and optical variability, the amplitude of the optical variability has exceeded
the amplitude of X-ray variability. We suggest that accelerated particles
striking material could be linking X-ray and optical variability (as in
activity in the solar chromosphere). Beaming effects could be significant in
all types of AGN. The diversity in optical/X-ray relationships at different
times in the same object, and between different objects, might be explained by
changes in geometry and directions of motion relative to our line of sight.
Linear shot-noise models of the variability are ruled out; instead there must
be large-scale organization of variability. Variability occurs on
light-crossing timescales rather than viscous timescales and this probably
rules out the standard Shakura-Sunyaev accretion disk. Radio-loud and
radio-quiet AGNs have similar continuum shapes and similar variability
properties. This suggests similar continuum origins and variability mechanisms.
Despite their extreme X-ray variability, narrow-line Seyfert 1s (NLS1s) do not
show extreme optical variability.Comment: Invited talk given at Euro Asian Astronomical Society meeting in
Moscow, June 2002. 20 pages, 4 figures. References update
One Hundred Years of Observations of the Be Star HDE 245770 (the X-ray Binary A0535+26/V725 Tau): The End of an Active Phase
UBV observations of the X-ray binary system A0535+26/V725 Tau at the Crimean
Station of the Sternberg Astronomical Institute in 1980-1998 are presented.
Based on our and published data, we analyze the photometric history of the star
from 1898.Comment: Translated from Pis'ma Astronomicheskii Zhurnal, Vol. 26, No. 1,
2000, pp. 13-2
Observational evidence for matter propagation in accretion flows
We study simultaneous X-ray and optical observations of three intermediate
polars EX Hya, V1223 Sgr and TV Col with the aim to understand the propagation
of matter in their accretion flows. We show that in all cases the power spectra
of flux variability of binary systems in X-rays and in optical band are similar
to each other and the majority of X-ray and optical fluxes are correlated with
time lag <1 sec. These findings support the idea that optical emission of
accretion disks, in these binary systems,largely originates as reprocessing of
X-ray luminosity of their white dwarfs. In the best obtained dataset of EX Hya
we see that the optical lightcurve unambiguously contains some component, which
leads the X-ray emission by ~7 sec. We interpret this in the framework of the
model of propagating fluctuations and thus deduce the time of travel of the
matter from the innermost part of the truncated accretion disk to the white
dwarf surface. This value agrees very well with the time expected for matter
threaded onto the magnetosphere of the white dwarf to fall to its surface. The
datasets of V1223 Sgr and TV Col in general confirm these findings,but have
poorer quality.Comment: 7 pages, 6 figures. Accepted for publication in MNRA
Aql X-1 in Outburst and Quiescence
We present photometry and spectroscopy of the soft x-ray transient Aql X-1.
Optical photometry during an active state shows a strong (0.6 mag peak-to-peak)
modulation at a period of 19 hours. Infrared (K'-band) photometry during a
quiescent state limits any ellipsoidal variations to <0.07 mag (peak-to-peak),
which implies an inclination i<31 (90% limit). Spectroscopy in a quiescent
state shows at most very small radial velocity variations, which implies a very
low inclination of i<12 (90% limit). The low inclination is rather unexpected
given the large photometric modulation seen in the active state. The upper
limit to the equivalent width of the anomalous Li 6707A line is <0.3A, which is
similar to the measured strength of this line in several other x-ray
transients.Comment: Accepted for publication in ApJ, 12 pages, 5 figure
Line Shifts, Broad-Line Region Inflow, and the Feeding of AGNs
Velocity-resolved reverberation mapping suggests that the broad-line regions
(BLRs) of AGNs can have significant net inflow. We use the STOKES radiative
transfer code to show that electron and Rayleigh scattering off the BLR and
torus naturally explains the blueshifted profiles of high-ionization lines and
the ionization dependence of the blueshifts. This result is insensitive to the
geometry of the scattering region. If correct, this model resolves the
long-standing conflict between the absence of outflow implied by
velocity-resolved reverberation mapping and the need for outflow if the
blueshifting is the result of obscuration. The accretion rate implied by the
inflow is sufficient to power the AGN. We suggest that the BLR is part of the
outer accretion disk and that similar MHD processes are operating. In the
scattering model the blueshifting is proportional to the accretion rate so
high-accretion-rate AGNs will show greater high-ionization line blueshifts as
is observed. Scattering can lead to systematically too high black hole mass
estimates from the C IV line. We note many similarities between narrow-line
region (NLR) and BLR blueshiftings, and suggest that NLR blueshiftings have a
similar explanation. Our model explains the higher blueshifts of broad
absorption line QSOs if they are more highly inclined. Rayleigh scattering from
the BLR and torus could be more important in the UV than electron scattering
for predominantly neutral material around AGNs. The importance of Rayleigh
scattering versus electron scattering can be assessed by comparing line
profiles at different wavelengths arising from the same emission-line region.Comment: 10 pages, 7 figures, Astrophysical Journal in press. The only changes
from the previous version are to include some additional discussion of the
plausibility of supersonic inflow velocities (see section 5.2) and some
additional reference
Resolved Mid-IR Emission in the Narrow Line Region of NGC 4151
We present subarcsecond resolution mid infrared images of NGC 4151 at 10.8
micron and 18.2 micron. These images were taken with the University of Florida
mid-IR camera/spectrometer OSCIR at the Gemini North 8-m telescope. We resolve
emission at both 10.8 micron and 18.2 micron extending ~ 3.5" across at a P.A.
of ~ 60 degrees. This coincides with the the narrow line region of NGC 4151 as
observed in [OIII] by the Hubble Space Telescope. The most likely explanation
for this extended mid-IR emission is dust in the narrow line region heated by a
central engine. We find no extended emission associated with the proposed torus
and place an upper limit on its mid-IR size of less than or equal to ~ 35 pc.Comment: accepted for publication in the Astrophysical Journal, 19 pages
including 5 figure
A0620-00 revisited: a black-hole transient case-study
For the first time we have performed a detailed study of the X-ray, optical
and infra-red light curves of the 1975/1976 outburst of the famous black-hole
transient A0620-00 (Nova Mon 1975, V616 Mon). During the various stages of its
outburst the X-rays lag the optical by ~5-20 days. Moreover, the activity
associated with the secondary maximum started even earlier in the infra-red.
This suggests that most of the outburst processes occur in the outer parts of
the disk. We also find various drops in the X-ray intensity (lasting ~1 day or
more). Near one of these a local maximum in the optical light curve occurs.
This maximum appears ~30 days after the secondary maximum, i.e. close to the
viscous time scale of an irradiated disk. We suggest this feature to be an
`echo' of the secondary maximum. At the end of the outburst a similar local
optical maximum occurs, which might be an `echo' of the tertiary maximum. We
find that the drops in the optical and X-rays near the tertiary maximum are
also present in other SXTs. They always occur ~150 days after the start of the
outburst. We find for the first time that the X-ray spectrum of A0620-00 starts
to harden ~100-150 days after the start of its outburst, similar to GS2000+25
and GS1124-68. This suggests we witness the transition from the so-called high
to low state at that time. We show that the optical outburst light curve of
A0620-00 closely resembles that of the cataclysmic variable AL Com. This
strengthens the similarity in the properties of the SXTs and TOADs (or WZ Sge
stars), and shows that the optical outburst light curves of both groups are
governed by the disk properties and not by the compact object. Since
irradiation provides a natural mechanism to prolong the outburst of SXTs, we
suggest this could be of influence as well during TOAD outbursts.Comment: 38 pages, 10 figures, accepted for New Astronomy Review
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