1,739 research outputs found
Four Years of Extreme Ultraviolet Observations of Markarian 421: II. Temporal Analysis
The Extreme Ultraviolet Explorer (EUVE) satellite accumulated ~one million
seconds of public data between 1994 and 1997 for the BL Lacertae object
Markarian 421. This is the second of two papers in which we present the results
of spectral and temporal analysis of this EUVE data set. We analyze in the
present paper the imaging data by means of power spectrum and structure
function techniques, while the spectral analysis is presented in a companion
paper. We find for MRK 421 a power spectrum with slope -2.14 +- 0.28 with a
break at ~3 days. This is the first time that a break in the power spectrum of
a BL Lacertae object has been found. We also find evidence of non-stationarity
for MRK 421 EUV emissionComment: Accepted for publication in the Astrophysical Journal. 16 pages, 14
Postscript figures, 3 Table
Generating artificial light curves: Revisited and updated
The production of artificial light curves with known statistical and
variability properties is of great importance in astrophysics. Consolidating
the confidence levels during cross-correlation studies, understanding the
artefacts induced by sampling irregularities, establishing detection limits for
future observatories are just some of the applications of simulated data sets.
Currently, the widely used methodology of amplitude and phase randomisation is
able to produce artificial light curves which have a given underlying power
spectral density (PSD) but which are strictly Gaussian distributed. This
restriction is a significant limitation, since the majority of the light curves
e.g. active galactic nuclei, X-ray binaries, gamma-ray bursts show strong
deviations from Gaussianity exhibiting `burst-like' events in their light
curves yielding long-tailed probability distribution functions (PDFs). In this
study we propose a simple method which is able to precisely reproduce light
curves which match both the PSD and the PDF of either an observed light curve
or a theoretical model. The PDF can be representative of either the parent
distribution or the actual distribution of the observed data, depending on the
study to be conducted for a given source. The final artificial light curves
contain all of the statistical and variability properties of the observed
source or theoretical model i.e. same PDF and PSD, respectively. Within the
framework of Reproducible Research, the code, together with the illustrative
example used in this manuscript, are both made publicly available in the form
of an interactive Mathematica notebook.Comment: Accepted for publication in MNRAS. The paper is 23 pages long and
contains 21 figures and 2 tables. The Mathematica notebook can be found in
the web as part of this paper (Online Material) or at
http://www.astro.soton.ac.uk/~de1e08/ArtificialLightCurves
Measuring the broadband power spectra of active galactic nuclei with RXTE
We have developed a Monte Carlo technique to test models for the true power
spectra of intermittently sampled lightcurves against the noisy, observed power
spectra, and produce a reliable estimate of the goodness of fit of the given
model. We apply this technique to constrain the broadband power spectra of a
sample of four Seyfert galaxies monitored by the Rossi X-ray Timing Explorer
(RXTE) over three years. We show that the power spectra of three of the AGN in
our sample (MCG-6-30-15, NGC5506 and NGC3516) flatten significantly towards low
frequencies, while the power spectrum of NGC5548 shows no evidence of
flattening. We fit two models for the flattening, a `knee' model, analogous to
the low-frequency break seen in the power spectra of BHXRBs in the low state
(where the power-spectral slope flattens to \alpha=0) and a `high-frequency
break' model (where the power-spectral slope flattens to \alpha=1), analogous
to the high-frequency break seen in the high and low-state power spectra of the
classic BHXRB Cyg X-1. Both models provide good fits to the power spectra of
all four AGN. For both models, the characteristic frequency for flattening is
significantly higher in MCG-6-30-15 than in NGC 3516 (by factor ~10) although
both sources have similar X-ray luminosities, suggesting that MCG-6-30-15 has a
lower black hole mass and is accreting at a higher rate than NGC 3516. Assuming
linear scaling of characteristic frequencies with black hole mass, the high
accretion rate implied for MCG-6-30-15 favours the high-frequency break model
for this source and further suggests that MCG-6-30-15 and possibly NGC 5506,
may be analogues of Cyg X-1 in the high state [ABRIDGED].Comment: 23 pages, accepted for publication in MNRA
The nature of X-ray spectral variability in MCG-6-30-15
The flux-flux plot (FFP) method can provide model-independent clues regarding
the X-ray variability of active galactic nuclei. To use it properly, the bin
size of the light curves should be as short as possible, provided the average
counts in the light curve bins are larger than . We apply the FFP
method to the 2013, simultaneous XMM-Newton and NuSTAR observations of the
Seyfert galaxy MCG6-30-15, in the 0.3-40 keV range. The FFPs above keV are well-described by a straight line. This result rules out spectral
slope variations and the hypothesis of absorption driven variability. Our
results are fully consistent with a power-law component varying in
normalization only, with a spectral slope of , plus a variable,
relativistic reflection arising from the inner accretion disc around a rotating
black hole. We also detect spectral components which remain constant over days (at least). At energies above keV, the stable component is
consistent with reflection from distant, neutral material. The constant
component at low energies is consistent with a blackbody spectrum of eV. The fluxes of these components are of the
average continuum flux (in the respective bands). They should always be
included in the models that are used to fit the spectrum of the source. The
FFPs below 1.6 keV are non-linear, which could be due to the variable warm
absorber in this source.Comment: 18 pages, 12 figures, 6 tables. Accepted for publication in MNRA
X-ray and optical counterparts of hard X-ray selected sources from the SHEEP survey: first results
We present followup observations of five hard X-ray sources from the ASCA
5-10 keV SHEEP survey, which has a limiting flux of erg
cm s. Chandra data have been obtained to improve the X-ray
positions from a few arcmin to , which allows unambiguous optical
identification. While the objects almost certainly house AGN based on their
X-ray luminosity, optical spectroscopy reveals a variety of properties. The
identifications indicate that the SHEEP survey samples the same populations as
deeper surveys which probe the origin of the X-ray background, but because the
SHEEP sources are far brighter, they are more amenable to detailed followup
work. We find a variety of classifications and properties, including a type II
QSO, a galaxy undergoing star formation, and a broad-line AGN which has a very
hard X-ray spectrum, indicating substantial absorption in the X-ray but none in
the optical. Two objects have X-ray/optical flux ratios which, were they at an
X-ray flux level typical of objects in Chandra deep surveys, would place them
in the ``optically faint'' category. They are both identified with broad line
QSOs at z. Clearly this survey - which is relatively unbiased against
obscured objects - is revealing a set of remarkable objects quite different to
the familiar classes of AGN found in previous optical and soft X-ray surveys.Comment: 5 pages, 3 figures. MNRAS, in pres
Modelling the variable broad-band optical/UV/X-ray spectrum of PG1211+143: Implications for the ionized outflow
We present the results from a detailed analysis of the 2007 Swift monitoring
campaign of the quasar PG1211+143. We constructed broad-band, optical/UV/X-ray
spectral energy distributions over three X-ray flux intervals. We fitted them
with a model which accounts for the disc and the X-ray coronal emission and the
warm absorber (well established in this source). The three flux spectra are
well fitted by the model we considered. The disc inner temperature remains
constant at ~2 eV, while X-rays are variable both in spectral slope and
normalization. The absorber covers almost 90% of the central source. It is
outflowing with a velocity less than 2.3*10^4 km/s (3sigma upper limit), and
has a column density of ~10^23.2. Its ionization parameter varies by a factor
of 1.6, and it is in photo-ionizing equilibrium with the ionizing flux. It is
located at a distance of less than 0.35 pc from the central source and its
relative thickness, DR/R is less than 0.1. The absorber' s ionization parameter
variations can explain the larger than average amplitude of the X-ray
variations. The absence of optical/UV variations (consistent with the high
black hole mass estimate) argues against the presence of inward propagating
disc fluctuations and strong X-ray illumination of the disc (in agreement with
the low ratio of X-ray over the bolometric luminosity of ~20-35). We estimate
an upper limit for the mass outflow of ~5 solar masses per year (~2.3 times the
Eddington mass accretion rate). If the outflow rate is indeed that high, then
it must be a short-lived episode in the quasar's life time. Finally, we
estimate an upper limit for the kinetic power of the outflow of ~1.4*10^43
ergs/s. This outflow cannot deploy significant mechanical energy to the
surrounding ISM of the quasar's host galaxy, but is sufficient to heat the ISM
to 10^7 K and to produce a fast decline to the star formation rate of the
galaxy.Comment: Accepted for publication by A&
Aperiodic variability of low-mass X-ray binaries at very low frequencies
We have obtained discrete Fourier power spectra of a sample of persistent
low-mass neutron-star X-ray binaries using long-term light curves from the All
Sky Monitor on board the Rossi X-ray Timing Explorer. Our aim is to investigate
their aperiodic variability at frequencies in the range 1 x 10^{-7}-5 x 10^{-6}
Hz and compare their properties with those of the black-hole source Cyg X-1. We
find that the classification scheme that divides LMXBs into Z and atoll sources
blurs at very low frequencies. Based on the long-term (~ years) pattern of
variability and the results of power-law fits (P ~ v^{-a}) to the 1 x 10^{-7}-5
x 10^{-6} Hz power density spectra, low-mass neutron-star binaries fall into
three categories. Type I includes all Z sources, except Cyg X-2, and the atoll
sources GX9+1 and GX13+1. They show relatively flat power spectra (a < 0.9) and
low variability (rms < 20%). Type II systems comprise 4U 1636-53, 4U 1735-44
and GX3+1. They are more variable (20% < rms < 30%) and display steeper power
spectra (0.9 < a < 1.2) than Type I sources. Type III systems are the most
variable (rms > 30%) and exhibit the steepest power spectra (a > 1.2). The
sources 4U 1705-44, GX354-0 and 4U 1820-30 belong to this group. GX9+9 and Cyg
X-2 appear as intermediate systems in between Type I and II and Type II and III
sources, respectively. We speculate that the differences in these systems may
be caused by the presence of different types of mass-donor companions. Other
factors, like the size of the accretion disc and/or the presence of weak
magnetic fields, are also expected to affect their low-frequency X-ray
aperiodic varibility.Comment: 9 pages, 6 figures. To be published in A&
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