1,671 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
Low-Luminosity AGN as analogues of Galactic Black Holes in the low/hard state: Evidence from X-ray timing of NGC 4258
We present a broadband power spectral density function (PSD) measured from
extensive RXTE monitoring data of the low-luminosity AGN NGC 4258, which has an
accurate, maser-determined black hole mass of 3.9+/-0.1 * 10^7 solar masses. We
constrain the PSD break timescale to be greater than 4.5 d at >90% confidence,
which appears to rule out the possibility that NGC 4258 is an analogue of black
hole X-ray binaries (BHXRBs) in the high/soft state. In this sense, the PSD of
NGC 4258 is different to those of some more-luminous Seyferts, which appear
similar to the PSDs of high/soft state X-ray binaries. This result supports
previous analogies between LLAGN and X-ray binaries in the low/hard state based
on spectral energy distributions, indicating that the AGN/BHXRB analogy is
valid across a broad range of accretion rates.Comment: Accepted for publication in The Astrophysical Journal Letters; 6
pages; 3 figure
The curious time lags of PG 1244+026: Discovery of the iron K reverberation lag
High-frequency iron K reverberation lags, where the red wing of the line
responds before the line centroid, are a robust signature of relativistic
reflection off the inner accretion disc. In this letter, we report the
discovery of the Fe K lag in PG 1244+026 from ~120 ks of data (1 orbit of the
XMM-Newton telescope). The amplitude of the lag with respect to the continuum
is 1000 s at a frequency of ~1e-4 Hz. We also find a possible
frequency-dependence of the line: as we probe higher frequencies (i.e. shorter
timescales from a smaller emitting region) the Fe K lag peaks at the red wing
of the line, while at lower frequencies (from a larger emitting region) we see
the dominant reflection lag from the rest frame line centroid. The mean energy
spectrum shows a strong soft excess, though interestingly, there is no
indication of a soft lag. Given that this source has radio emission and it has
little reported correlated variability between the soft excess and the hard
band, we explore one possible explanation in which the soft excess in this
source is dominated by the steep power-law like emission from a jet, and that a
corona (or base of the jet) irradiates the inner accretion disc, creating the
blurred reflection features evident in the spectrum and the lag. General
Relativistic ray-tracing models fit the Fe K lag well, with the best-fit giving
a compact X-ray source at a height of 5 gravitational radii and a black hole
mass of 1.3e7 Msun.Comment: 6 pages, 6 figures, resubmitted to MNRAS after moderate revisions.
This paper focuses on the discovery of the Fe K reverberation lag in PG
1244+026. We point the interested reader to Alston, Done & Vaughan (See
today: arXiv:submit/0851673), which focuses on the soft lags in this sourc
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
Short term X-ray rms variability of Cyg X-1
A linear dependence of the amplitude of broadband noise variability on flux
for GBHC and AGN has been recently shown by Uttley & McHardy (2001). We present
the long term evolution of this rms-flux-relation for Cyg X-1 as monitored from
1998-2002 with RXTE. We confirm the linear relationship in the hard state and
analyze the evolution of the correlation for the period of 1996-2002. In the
intermediate and the soft state, we find considerable deviations from the
otherwise linear relationship. A possible explanation for the rms-flux-relation
is a superposition of local mass accretion rate variations.Comment: 3 pages, 3 figures, Proceedings of the 4th Microquasar Workshop, eds.
Ph Durouchoux, Y. Fuchs and J. Rodriguez, published by the Center for Space
Physics: Kolkat
Discovery of high-frequency iron K lags in Ark 564 and Mrk 335
We use archival XMM-Newton observations of Ark 564 and Mrk 335 to calculate
the frequency dependent time-lags for these two well-studied sources. We
discover high-frequency Fe K lags in both sources, indicating that the red wing
of the line precedes the rest frame energy by roughly 100 s and 150 s for Ark
564 and Mrk 335, respectively. Including these two new sources, Fe K
reverberation lags have been observed in seven Seyfert galaxies. We examine the
low-frequency lag-energy spectrum, which is smooth, and shows no feature of
reverberation, as would be expected if the low-frequency lags were produced by
distant reflection off circumnuclear material. The clear differences in the low
and high frequency lag-energy spectra indicate that the lags are produced by
two distinct physical processes. Finally, we find that the amplitude of the Fe
K lag scales with black hole mass for these seven sources, consistent with a
relativistic reflection model where the lag is the light travel delay
associated with reflection of continuum photons off the inner disc.Comment: 10 pages, 12 figures, accepted for publication in MNRA
The swansong in context: long-timescale X-ray variability of NGC 4051
On 9-11 May 1998, the highly-variable, low luminosity Seyfert 1 galaxy
NGC4051 was observed in an unusual low flux state by BeppoSAX (Guainazzi et al.
1998) RXTE and EUVE. We present fits of the 4-15 keV RXTE spectrum and BeppoSAX
MECS spectrum obtained during this observation, which are consistent with the
interpretation that the source had switched off, leaving only the spectrum of
pure reflection from distant cold matter. We place this result in context by
showing the X-ray lightcurve of NGC4051 obtained by our RXTE monitoring
campaign over the past two and a half years, which shows that the low state
lasted for ~150 days before the May observations (implying that the reflecting
material is > 10^17 cm from the continuum source) and forms part of a
lightcurve showing distinct variations in long-term average flux over
timescales > months. We show that the long-timescale component to X-ray
variability is intrinsic to the primary continuum and is probably distinct from
the variability at shorter timescales, possibly associated with variations in
the accretion flow of matter onto the central black hole. As the source
approaches the low state, the variability process becomes non-linear. NGC4051
may represent a microcosm of all X-ray variability in radio quiet active
galactic nuclei (AGNs), displaying in a few years a variety of flux states and
variability properties which more luminous AGNs may pass through on timescales
of decades to thousands of years.Comment: 5 pages, accepted for publication in MNRA
X-ray reverberation around accreting black holes
Luminous accreting stellar mass and supermassive black holes produce
power-law continuum X-ray emission from a compact central corona. Reverberation
time lags occur due to light travel time-delays between changes in the direct
coronal emission and corresponding variations in its reflection from the
accretion flow. Reverberation is detectable using light curves made in
different X-ray energy bands, since the direct and reflected components have
different spectral shapes. Larger, lower frequency, lags are also seen and are
identified with propagation of fluctuations through the accretion flow and
associated corona. We review the evidence for X-ray reverberation in active
galactic nuclei and black hole X-ray binaries, showing how it can be best
measured and how it may be modelled. The timescales and energy-dependence of
the high frequency reverberation lags show that much of the signal is
originating from very close to the black hole in some objects, within a few
gravitational radii of the event horizon. We consider how these signals can be
studied in the future to carry out X-ray reverberation mapping of the regions
closest to black holes.Comment: 72 pages, 32 figures. Accepted for publication in The Astronomy and
Astrophysics Review. Corrected for mostly minor typos, but in particular
errors are corrected in the denominators of the covariance and rms spectrum
error equations (Eqn. 14 and 15
The Closest Look at 1H0707-495: X-ray Reverberation Lags with 1.3 Ms of Data
Reverberation lags in AGN were first discovered in the NLS1 galaxy,
1H0707-495. We present a follow-up analysis using 1.3 Ms of data, which allows
for the closest ever look at the reverberation signature of this remarkable
source. We confirm previous findings of a hard lag of ~100 seconds at
frequencies v ~ [0.5 - 4] e-4 Hz, and a soft lag of ~30 seconds at higher
frequencies, v ~ [0.6 - 3] e-3 Hz. These two frequency domains clearly show
different energy dependences in their lag spectra. We also find evidence for a
signature from the broad Fe K line in the high frequency lag spectrum. We use
Monte Carlo simulations to show how the lag and coherence measurements respond
to the addition of Poisson noise and to dilution by other components. With our
better understanding of these effects on the lag, we show that the lag-energy
spectra can be modelled with a scenario in which low frequency hard lags are
produced by a compact corona responding to accretion rate fluctuations
propagating through an optically thick accretion disc, and the high frequency
soft lags are produced by short light-travel delay associated with reflection
of coronal power-law photons off the disc.Comment: 11 pages, 10 figures. Accepted for publication in MNRA
Stabilization of radiation pressure dominated accretion disks by viscous fluctuations
The standard thin accretion disk model has been successfully used to explain
the soft X-ray spectra of Galactic black hole systems and perhaps the UV
emission of Active Galactic Nuclei. However, radiation pressure dominated disks
are known to be viscously unstable and should produce large amplitude
oscillations that are typically not observed. Instead, these sources exhibit
stochastic variability which may naturally arise due to viscous fluctuations in
a turbulent disk. Here we investigate whether these aperiodic viscous
fluctuations can stabilize the inner radiation pressure dominated disks and
hence maybe the answer to a forty year old problem in accretion disk theory.Comment: 6 pages, 6 figures; accepted to Astronomy and Astrophysic
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