683 research outputs found
Catching NGC4051 in the low state with XMM-Newton
The Narrow Line Seyfert 1 galaxy NGC4051 shows unusual low flux states,
lasting several months, when the 2-10 keV X-ray spectrum becomes unusually hard
(photon index<1) while the spectrum at lower X-ray energies is dominated by a
large soft excess. A Chandra TOO of the low state has shown that the soft
excess and hard components are variable and well-correlated. The variability of
the hard component rules out an origin in a distant reflector. Here we present
results from a recent XMM-Newton TOO of NGC4051 in the low state, which allows
a much more detailed examination of the nature of the hard and soft spectral
components in the low state. We demonstrate that the spectral shape in the low
state is consistent with the extrapolation of the spectral pivoting observed at
higher fluxes. The XMM-Newton data also reveals the warm absorbing gas in
emission, as the drop in the primary continuum flux unmasks prominent emission
lines from a range of ion species.Comment: 4 pages, 4 figures. Proc. of the meeting: "The Restless High-Energy
Universe" (Amsterdam, The Netherlands), E.P.J. van den Heuvel, J.J.M. in 't
Zand, and R.A.M.J. Wijers Ed
Revealing the X-ray source in IRAS 13224-3809 through flux-dependent reverberation lags
IRAS 13224-3809 was observed in 2011 for 500 ks with the XMM-Newton
observatory. We detect highly significant X-ray lags between soft (0.3 - 1 keV)
and hard (1.2 - 5 keV) energies. The hard band lags the soft at low frequencies
(i.e. hard lag), while the opposite (i.e. soft lag) is observed at high
frequencies. In this paper, we study the lag during flaring and quiescent
periods. We find that the frequency and absolute amplitude of the soft lag is
different during high-flux and low-flux periods. During the low flux intervals,
the soft lag is detected at higher frequencies and with smaller amplitude.
Assuming that the soft lag is associated with the light travel time between
primary and reprocessed emission, this behaviour suggests that the X-ray source
is more compact during low-flux intervals, and irradiates smaller radii of the
accretion disc (likely because of light bending effects). We continue with an
investigation of the lag dependence on energy, and find that isolating the
low-flux periods reveals a strong lag signature at the Fe K line energy,
similar to results found using 1.3 Ms of data on another well known Narrow-Line
Seyfert I galaxy, 1H0707-495.Comment: 6 pages, 8 figures, accepted for publication in MNRA
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
X-ray spectral variability of the Seyfert galaxy NGC 4051
We report on the X-ray spectral variability of the Seyfert 1 galaxy NGC 4051
observed with the Rossi X-ray Timing Explorer (RXTE) during a 1000 day period
between May 1996 and March 1999. The spectra were obtained as part of
monitoring observations and from two long observations using the RXTE
Proportional Counter Array (PCA). During the monitoring period the 2-10 keV
flux of NGC 4051 varied between 10E-12 and 7x 10E11 (cgs). We re-analysed RXTE
PCA observations from a distinct low state in May 1998 using the latest
background and detector response models. The RXTE and BeppoSAX observations of
NGC 4051 during the low state show a very hard spectrum with a strong
unresolved fluorescence line. This emission, probably due to reflection from a
molecular torus, is likely to be constant over long time-scales and is
therefore assumed as an underlying component at all flux states. By subtracting
the torus component we are able to determine the spectral variability of the
primary continuum. In the variable component we observe a strong
anti-correlation of X-ray flux and spectral hardness in the PCA energy band. We
show that the changes in hardness are caused by slope variability of the
primary power law spectrum rather than by changing reflection or variable
photoelectric absorption. The primary spectral index varies between Gamma=1.6
for the faintest states and Gamma=2.3 during the brightest states, at which
level the spectral index approaches an asympotic value. We find that the
response of the flux of the 6.4 keV iron fluorescence line to changes in the
continuum flux depends on the timescale of the observation. The profile of the
line is very broad and indicates an origin in the innermost regions of the
accretion disk.Comment: accepted for publication in MNRA
The swan song: the disappearance of the nucleus of NGC 4051 and the echo of its past glory
BeppoSAX observed the low-luminous Seyfert 1 Galaxy NGC4051 in a ultra-dim
X-ray state. The 2-10 keV flux (1.26 x 10^{-12} erg/cm^2/s) was about 20 times
fainter than its historical average value, and remained steady along the whole
observation (~2.3 days). The observed flat spectrum (\Gamma ~ 0.8) and intense
iron line (EW ~600 eV) are best explained assuming that the active nucleus has
switched off, leaving only a residual reflection component visible.Comment: 5 pages, Latex, 3 Postscript figures, accepted for publication in
MNRA
Discovery of a relation between black hole mass and soft X-ray time lags in active galactic nuclei
We carried out a systematic analysis of time lags between X-ray energy bands
in a large sample (32 sources) of unabsorbed, radio quiet active galactic
nuclei (AGN), observed by XMM-Newton. The analysis of X-ray lags (up to the
highest/shortest frequencies/time-scales), is performed in the
Fourier-frequency domain, between energy bands where the soft excess (soft
band) and the primary power law (hard band) dominate the emission. We report a
total of 15 out of 32 sources displaying a high frequency soft lag in their
light curves. All 15 are at a significance level exceeding 97 per cent and 11
are at a level exceeding 99 per cent. Of these soft lags, 7 have not been
previously reported in the literature, thus this work significantly increases
the number of known sources with a soft/negative lag. The characteristic
time-scales of the soft/negative lag are relatively short (with typical
frequencies and amplitudes of \nu\sim 0.07-4 \times 10^{-3} Hz and \tau\sim
10-600 s, respectively), and show a highly significant (\gsim 4\sigma)
correlation with the black hole mass. The measured correlations indicate that
soft lags are systematically shifted to lower frequencies and higher absolute
amplitudes as the mass of the source increases. To first approximation, all the
sources in the sample are consistent with having similar mass-scaled lag
properties. These results strongly suggest the existence of a mass-scaling law
for the soft/negative lag, that holds for AGN spanning a large range of masses
(about 2.5 orders of magnitude), thus supporting the idea that soft lags
originate in the innermost regions of AGN and are powerful tools for testing
their physics and geometry.Comment: 12 pages, 6 figures. Revised version, accepted for publication in
MNRA
The rms-flux relations in different branches in Cyg X-2
In this paper, the rms-flux (root mean square-flux) relation along the
Z-track of the bright Z-Source Cyg X-2 is analyzed using the observational data
of Rossi X-ray Timing Explorer (RXTE). Three types of rms-flux relations, i.e.
positive, negative, and 'arch'-like correlations are found in different
branches. The rms is positively correlated with flux in normal branch (NB), but
anti-correlated in the vertical horizontal branch (VHB). The rms-flux relation
shows an 'arch'-like shape in the horizontal branch (HB). We also try to
explain this phenomenon using existing models.Comment: Accepted for publication in Astrophysics & Space Scienc
Complex X-ray spectral behaviour of NGC 4051 in the low flux state
The Narrow Line Seyfert 1 galaxy NGC 4051 was observed in one of its
prolonged low-lux states by XMM-Newton in November 2002. Here we present the
results of an analysis of EPIC-pn data obtained during the observation. Within
the low state, the source shows complex spectral variability which cannot
easily be explained by any simple model. However, by making a `flux-flux' plot
which combines the low state data with data obtained during a normal flux
state, we demonstrate that the extremely hard spectrum observed above 2 keV
results from a continuation of the spectral variability seen in the normal
state, which is caused by spectral pivoting of the power-law continuum. The
pivoting power-law appears to be attached to a Comptonised thermal component of
variable flux (blackbody temperature kT~0.1 keV, consistent with the small
black hole mass in NGC 4051) which dominates the soft X-ray band in the low
state, and is probably the source of seed photons for Comptonisation.
Additional constant thermal and reflection components, together with absorption
by ionised gas, seem to be required to complete the picture and explain the
complex X-ray spectral variability seen in the low state of NGC 4051.Comment: Accepted for publication in MNRAS. 13 pages, 10 figs. A higher
resolution eps version of Fig. 8 is included in the source file
Variability of X-ray binaries from an oscillating hot corona
The spectral and timing properties of an oscillating hot thermal corona are
investigated. This oscillation is assumed to be due to a magneto-acoustic wave
propagating within the corona and triggered by an external, non specified,
excitation. A cylindrical geometry is adopted and, neglecting the rotation, the
wave equation is solved in for different boundary conditions. The resulting
X-ray luminosity, through thermal comptonization of embedded soft photons, is
then computed, first analytically, assuming linear dependence between the local
pressure disturbance and the radiative modulation. These calculations are also
compared to Monte-Carlo simulations. The main results of this study are: (1)
the corona plays the role of a low band-pass medium, its response to a white
noise excitation being a at top noise Power Spectral Density (PSD) at low
frequencies and a red noise at high frequency, (2) resonant peaks are present
in the PSD. Their powers depend on the boundary conditions chosen and, more
specifically, on the impedance adaptation with the external medium at the
corona inner boundary. (3) The flat top noise level and break as well as the
resonant peak frequencies are inversely proportional to the external radius rj.
(4) Computed rms and f-spectra exhibit an overall increase of the variability
with energy. Comparison with observed variability features, especially in the
hard intermediate states of X-ray binaries are discussed.Comment: 12 pages, 7 figures, accepted for publication in MNRA
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