988 research outputs found
An absorption event in the X-ray lightcurve of NGC 3227
We have monitored the Seyfert galaxy NGC 3227 with the Rossi X-ray Timing
Explorer (RXTE) since January 1999. During late 2000 and early 2001 we observed
an unusual hardening of the 2-10 keV X-ray spectrum which lasted several
months. The spectral hardening was not accompanied by any correlated variation
in flux above 8 keV. We therefore interpret the spectral change as transient
absorption by a gas cloud of column density 2.6 10^23 cm^-2 crossing the line
of sight to the X-ray source. A spectrum obtained by XMM-Newton during an early
phase of the hard-spectrum event confirms the obscuration model and shows that
the absorbing cloud is only weakly ionised. The XMM-Newton spectrum also shows
that ~10% of the X-ray flux is not obscured, but this unabsorbed component is
not significantly variable and may be scattered radiation from a large-scale
scattering medium. Applying the spectral constraints on cloud ionisation
parameter and assuming that the cloud follows a Keplerian orbit, we constrain
the location of the cloud to be R~10-100 light-days from the central X-ray
source, and its density to be n_H~10^8cm^-3, implying that we have witnessed
the eclipse of the X-ray source by a broad line region cloud.Comment: 5 pages, 6 figures, accepted for publication in MNRAS letter
Simultaneous Ultraviolet and X-ray Observations of the Seyfert Galaxy NGC 4151. II. Physical Conditions in the UV Absorbers
We present a detailed analysis of the intrinsic absorption in the Seyfert 1
galaxy NGC 4151 using UV spectra from the HST/STIS and FUSE, obtained 2002 May
as part of a set of contemporaneous observations that included Chandra/HETGS
spectra. In our analysis of the Chandra spectra, we determined that the soft
X-ray absorber was the source of the saturated UV lines of O VI, C IV, and N V
associated with the absorption feature at a radial velocity of ~ -500 km/sec,
which we referred to as component D+E. In the present work, we have derived
tighter constrains on the the line-of-sight covering factors, densities, and
radial distances of the absorbers. We find that the Equivalent Widths (EWs) of
the low-ionization lines associated with D+E varied over the period from 1999
July to 2002 May. The drop in the EWs of these lines between 2001 April and
2002 May are suggestive of bulk motion of gas out of our line-of-sight. If
these lines from these two epochs arose in the same sub-component, the
transverse velocity of the gas is ~ 2100 km/sec. Transverse velocities of this
order are consistent with an origin in a rotating disk, at the roughly radial
distance we derived for D+E.Comment: 51 pages, including 12 figures. Accepted for publication in ApJ
Supplement
A Spectroscopic and Photometric Study of Short-Timescale Variability in NGC5548
Results of a ground-based optical monitoring campaign on NGC5548 in June 1998
are presented. The broad-band fluxes (U,B,V), and the spectrophotometric
optical continuum flux F_lambda(5100 A) monotonically decreased in flux while
the broad-band R and I fluxes and the integrated emission-line fluxes of Halpha
and Hbeta remained constant to within 5%. On June 22, a short continuum flare
was detected in the broad band fluxes. It had an amplitude of about ~18% and it
lasted only ~90 min. The broad band fluxes and the optical continuum
F_lambda(5100 A) appear to vary simultaneously with the EUV variations. No
reliable delay was detected for the broad optical emission lines in response to
the EUVE variations. Narrow Hbeta emission features predicted as a signature of
an accretion disk were not detected during this campaign. However, there is
marginal evidence for a faint feature at lambda = 4962 A with FWHM=~6 A
redshifted by Delta v = 1100 km/s with respect to Hbeta_narrow.Comment: 12 pages, 7 figures, accepted for publishing in A&
Intensive HST, RXTE and ASCA Monitoring of NGC 3516: Evidence Against Thermal Reprocessing
During 1998 April 13-16, NGC 3516 was monitored almost continuously with HST
for 10.3 hr in the UV and 2.8 d in the optical, and simultaneous RXTE and ASCA
monitoring covered the same period. The X-rays were strongly variable with the
soft (0.5-2 keV) showing stronger variations (~65% peak-to-peak) than the hard
(2-10 keV; ~50% peak-to-peak). The optical continuum showed much smaller but
highly significant variations: a slow ~2.5% rise followed by a faster ~3.5%
decline. The short UV observation did not show significant variability.
The soft and hard X-ray light curves were strongly correlated with no
significant lag. Likewise, the optical continuum bands (3590 and 5510 A) were
also strongly correlated with no measurable lag above limits of <0.15 d.
However no significant correlation or simple relationship could be found for
the optical and X-ray light curves. These results appear difficult to reconcile
with previous reports of correlations between X-ray and optical variations and
of measurable lags within the optical band for some other Seyfert 1s.
These results also present serious problems for "reprocessing" models in
which the X-ray source heats a stratified accretion disk which then reemits in
the optical/ultraviolet: the synchronous variations within the optical would
suggest that the emitting region is <0.3 lt-d across, while the lack of
correlation between X-ray and optical variations would indicate, in the context
of this model, that any reprocessing region must be >1 lt-d in size. It may be
possible to resolve this conflict by invoking anisotropic emission or special
geometry, but the most natural explanation appears to be that the bulk of the
optical luminosity is generated by some other mechanism than reprocessing.Comment: 23 pages including 6 figures, accepted for publication in Ap
New constraints on the continuum-emission mechanism of AGN: Intensive monitoring of NGC 7469 in the X-ray and ultraviolet
We have undertaken near-continuous monitoring of the Seyfert 1 galaxy NGC
7469 in the X-ray with RXTE over a ~30d baseline. The source shows strong
variability with a root-mean-square (rms) amplitude of ~16 per cent, and
peak-to-peak variations of a factor of order 2. Simultaneous data over this
period were obtained in the ultraviolet (UV) using IUE, making this the most
intensive X-ray UV/X-ray variability campaign performed for any active galaxy.
Comparison of the continuum light curves reveals very similar amplitudes of
variability, but different variability characteristics, with the X-rays showing
much more rapid variations. The data are not strongly correlated at zero lag.
The largest absolute value of the correlation coefficient occurs for an
anticorrelation between the two bands, with the X-ray variations leading the UV
by ~4d. The largest positive correlation is for the ultraviolet to lead the
X-rays by ~4d. Neither option appears to be compatible with any simple
interband transfer function. The peak positive correlation at ~4d occurs
because the more prominent peaks in the UV light curve appear to lead those in
the X-rays by this amount. However, the minima of the light curves are
near-simultaneous. These observations provide new constraints on theoretical
models of the central regions of active galactic nuclei. Models in which the
observed UV emission is produced solely by re-radiation of absorber X-rays are
ruled out by our data, as are those in which the X-rays are produced solely by
Compton upscattering of the observed UV component by a constant distribution of
particles.Comment: 33 pages, 8 figures. LaTeX with encapsulated postscript. To appear in
the Astrophysical Journal. Also available via
http://lheawww.gsfc.nasa.gov/users/nandra/pubs/7469/abstract.htm
Multiwavelength Monitoring of the Narrow-Line Seyfert 1 Galaxy Akn 564. II. Ultraviolet Continuum and Emission-line Variability
We present results of an intensive two-month campaign of approximately daily
spectrophotometric monitoring of the narrow-line Seyfert 1 galaxy Akn 564 with
HST. The fractional variability amplitude of the continuum variations between
1365-3000 A is ~6%, about a factor 3 less than that found in typical Seyfert 1
galaxies over a similar period of time. However, large amplitude, short
time-scale flaring behavior is evident, with trough-to-peak flux changes of
about 18% in approximately 3 days. We present evidence for wavelength-dependent
continuum time delays, with the variations at 3000 A lagging behind those at
1365 A by about 1 day. These delays may be interpreted as evidence for a
stratified continuum reprocessing region, possibly an accretion-disk structure.
The Lyman-alpha 1216 emission-line exhibits flux variations of about 1%
amplitude.Comment: 27 pages, 14 figures. Accepted by Astrophysical Journa
Multiwavelength Monitoring of the Narrow-Line Seyfert 1 Galaxy Akn 564. I. ASCA Observations and the Variability of the X-ray Spectral Components
We present a 35 day ASCA observation of the NLS1 Akn 564, which was part of a
multiwavelength AGN Watch monitoring campaign. Akn 564 shows a photon index
varying across the range 2.45--2.72. The presence of the soft hump component
below 1 keV, previously detected in ASCA data, is confirmed. Time-resolved
spectroscopy with ~daily sampling reveals a distinction in the variability of
the soft hump and power-law components over a timescale of weeks, with the hump
varying by a factor of 6 across the 35-day observation compared to a factor 4
in the power-law. Flux variations in the power-law component are measured down
to a timescale of ~1000s and accompanying spectral variability suggests the
soft hump is not well-correlated with the power-law on such short timescales.
We detect Fe Ka and a blend of Fe Kb plus Ni Ka, indicating an origin in highly
ionized gas. Variability measurements constrain the bulk of the Fe Ka to
originate within a light week of the nucleus. The large EW of the emission
lines may be due to high metallicity in NLS1s, supporting some evolutionary
models for AGN.Comment: 41 pages, 15 figures. Accepted for publication in the Astrophysical
Journal (v3 has final fixes for publication
Confirmation of a correlation between the X-ray luminosity and spectral slope of AGNs in the Chandra Deep Fields
We present results from a statistical analysis of 173 bright radio-quiet AGNs
selected from the Chandra Deep Field-North and Chandra Deep Field-South surveys
(hereafter, CDFs) in the redshift range of 0.1 < z < 4. We find that the X-ray
power-law photon index (Gamma) of radio-quiet AGNs is correlated with their
2-10 keV rest-frame X-ray luminosity (L_X) at the > 99.5 percent confidence
level in two redshift bins, 0.3 < z < 0.96, and 1.5 < z < 3.3 and is slightly
less significant in the redshift bin 0.96 < z < 1.5. We investigate the
redshift evolution of the correlation between the power-law photon index and
the hard X-ray luminosity and find that the slope and offset of a linear fit to
the correlation change significantly (at the > 99.9 percent confidence level)
between redshift bins of 0.3 < z < 0.96 and 1.5 < z < 3.3. We explore physical
scenarios explaining the origin of this correlation and its possible evolution
with redshift in the context of steady corona models focusing on its dependency
on variations of the properties of the hot corona with redshift.Comment: Comments: 20 pages, includes 17 figures, Accepted for publication in
A
- …