55 research outputs found
Simultaneous EUV and X-ray variability of NGC 4051
We present a flux variability study of simultaneous RXTE and EUVE
observations of the highly variable Seyfert galaxy NGC4051. We find a strong
correlation between variability in the EUV and medium energy X-ray
bands,indicating that both are sampling the same power-law continuum. The lag
between the two bands is less than 20 ks and, depending on model assumptions,
may be <1 ks. We examine the consequences of such a small lag in the context of
simple Comptonisation models for the production of the power-law continuum. A
lag of <1 ks implies that the size of the Comptonising region is less than 20
Schwarzschild radii for a black hole of mass >1E6 solar masses.Comment: 8 pages, accepted for publication in MNRA
A possible feature of thermal matter in relativistic jets of radio-loud quasars
It has been suggested that relativistic jets in quasars may contain a
considerable amount of thermal matter. In this paper, we explore the
possibility that the K-alpha line from the thermal matter may appear at tens of
keV due to a high Doppler blue-shift. In the jet comoving frame, the energy
density of photons originally emitted by the accretion disk and reflected off
the broad line region clouds dominates over that of photons of other origin. We
discuss the photoionization states of the thermal matter and find that the
irons elements are neutral. The high metallicity in quasars enhances the
possibility to detect the thermal matter in the relativistic jet in some
radio-loud quasars. A highly Doppler blue-shifted K-alpha line may be detected.
We make a rediction for 3C 273, in which the K-alpha line luminosity might be
of the order erg/s with an equivalent width of 2.4 keV.
Such a line could be detected in a future mission.Comment: 4 pages, 1 Figur
The luminosities of backsplash galaxies in constrained simulations of the Local Group
We study the differences and similarities in the luminosities of bound,
infalling and the so-called backsplash (Gill et al. 2005) galaxies of the Milky
Way and M31 using a hydrodynamical simulation performed within the Constrained
Local UniversE Simulation (CLUES) project. The simulation models the formation
of the Local Group within a self-consistent cosmological framework. We find
that even though backsplash galaxies passed through the virial radius of their
host halo and hence may have lost a (significant) fraction of their mass, their
stellar populations are hardly affected. This leaves us with comparable
luminosity functions for infalling and backsplash galaxies and hence little
hope to decipher their past (and different) formation and evolutionary
histories by luminosity measurements alone. Nevertheless, due to the tidal
stripping of dark matter we find that the mass-to-light ratios have changed
when comparing the various populations against each other: they are highest for
the infalling galaxies and lowest for the bound satellites with the backsplash
galaxies in-between.Comment: 9 pages, 10 figures, 1 table, accepted for publication in MNRA
Measuring the Sources of the Intergalactic Ionizing Flux
We use a wide-field (0.9 square degree) X-ray sample with optical and GALEX
ultraviolet observations to measure the contribution of Active Galactic Nuclei
(AGNs) to the ionizing flux as a function of redshift. Our analysis shows that
the AGN contribution to the metagalactic ionizing background peaks around z=2.
The measured values of the ionizing background from the AGNs are lower than
previous estimates and confirm that ionization from AGNs is insufficient to
maintain the observed ionization of the intergalactic medium (IGM) at z>3. We
show that only sources with broad lines in their optical spectra have
detectable ionizing flux and that the ionizing flux seen in an AGN is not
correlated with its X-ray color. We also use the GALEX observations of the
GOODS-N region to place a 2-sigma upper limit of 0.008 on the average
ionization fraction fnu(700 A)/fnu(1500 A) for 626 UV selected galaxies in the
redshift range z=0.9-1.4. We then use this limit to estimate an upper bound to
the galaxy contribution in the redshift range z=0-5. If the z~1.15 ionization
fraction is appropriate for higher redshift galaxies, then contributions from
the galaxy population are also too low to account for the IGM ionization at the
highest redshifts (z>4).Comment: 15 pages, Accepted by The Astrophysical Journa
Measurements of the UV background at 4.6 < z < 6.4 using the quasar proximity effect
We present measurements of the ionising ultraviolet background (UVB) at z ~
5-6 using the quasar proximity effect. The fifteen quasars in our sample cover
the range 4.6 < z_q < 6.4, enabling the first proximity effect measurements of
the UVB at z > 5. The metagalactic hydrogen ionisation rate, Gamma_bkg, was
determined by modelling the combined ionisation field from the quasar and the
UVB in the proximity zone on a pixel-by-pixel basis. The optical depths in the
spectra were corrected for the expected effect of the quasar until the mean
flux in the proximity region equalled that in the average Ly-alpha forest, and
from this we make a measurement of Gamma_bkg. A number of systematic effects
were tested using synthetic spectra. Noise in the flux was found to be the
largest source of bias at z ~ 5, while uncertainties in the mean transmitted
Ly-alpha flux are responsible for the largest bias at z ~ 6. The impacts of
large-scale overdensities and Lyman limit systems on Gamma_bkg were also
investigated, but found to be small at z > 5. We find a decline in Gamma_bkg
with redshift, from log(Gamma_bkg) = -12.15 0.16 at z ~ 5 to
log(Gamma_bkg) = -12.84 0.18 at z ~ 6 (1 sigma errors). Compared to UVB
measurements at lower redshifts, our measurements suggest a drop of a factor of
five in the HI photoionisation rate between z ~ 4 and z ~ 6. The decline of
Gamma_bkg appears to be gradual, and we find no evidence for a sudden change in
the UVB at any redshift that would indicate a rapid change in the attenuation
length of ionising photons. Combined with recent measurements of the evolution
of the mean free path of ionising photons, our results imply decline in the
emissivity of ionising photons by roughly a factor of two from z ~ 5 to 6,
albeit with significant uncertainty due to the measurement errors in both
Gamma_bkg and the mean free path.Comment: 22 pages, 19 figures, 5 tables; accepted for publication in MNRA
The impact of baryonic physics on the shape and radial alignment of substructures in cosmological dark matter haloes
We use two simulations performed within the Constrained Local UniversE
Simulation (CLUES) project to study both the shape and radial alignment of (the
dark matter component of) subhaloes; one of the simulations is a dark matter
only model while the other run includes all the relevant gas physics and star
formation recipes. We find that the involvement of gas physics does not have a
statistically significant effect on either property -- at least not for the
most massive subhaloes considered in this study. However, we observe in both
simulations including and excluding gasdynamics a (pronounced) evolution of the
dark matter shapes of subhaloes as well as of the radial alignment signal since
infall time. Further, this evolution is different when positioned in the
central and outer regions of the host halo today; while subhaloes tend to
become more aspherical in the central 50% of their host's virial radius, the
radial alignment weakens in the central regime while strengthening in the outer
parts. We confirm that this is due to tidal torquing and the fact that
subhaloes at pericentre move too fast for the alignment signal to respond.Comment: 10 pages, 8 figures, 2 tables, accepted for publication in MNRAS,
replaced with proof-corrected version (minor typos
Constrained simulations of the Local Group: on the radial distribution of substructures
We examine the properties of satellites found in high resolution simulations
of the local group. We use constrained simulations designed to reproduce the
main dynamical features that characterize the local neighborhood, i.e. within
tens of Mpc around the Local Group (LG). Specifically, a LG-like object is
found located within the 'correct' dynamical environment and consisting of
three main objects which are associated with the Milky Way, M31 and M33. By
running two simulations of this LG from identical initial conditions - one with
and one without baryons modeled hydrodynamically - we can quantify the effect
of gas physics on the population of subhaloes in an environment similar
to our own. We find that above a certain mass cut, subhaloes in hydrodynamic simulations are more
radially concentrated than those in simulations with out gas. This is caused by
the collapse of baryons into stars that typically sit in the central regions of
subhaloes, making them denser. The increased central density of such a subhalo,
results in less mass loss due to tidal stripping than the same subhalo
simulated with only dark matter. The increased mass in hydrodynamic subhaloes
with respect to dark matter ones, causes dynamical friction to be more
effective, dragging the subhalo towards the centre of the host. This results in
these subhaloes being effectively more radially concentrated then their dark
matter counterparts.Comment: 12 pages, 9 figure
Averaging the AGN X-ray spectra from deep Chandra fields
The X-ray spectra of Active Galactic Nuclei (AGN) carry the signatures of the
emission from the central region, close to the Super Massive Black Hole (SMBH).
For this reason, the study of deep X-ray spectra is a powerful instrument to
investigate the origin of their emission.
The emission line most often observed in the X-ray spectra of AGN is Fe K. It
is known that it can be broadened and deformed by relativistic effects if
emitted close enough to the central SMBH. In recent statistical studies of the
X-ray spectra of AGN samples, it is found that a narrow Fe line is ubiquitous,
while whether the broad features are as common is still uncertain. We present
here the results of an investigation on the characteristics of the Fe line in
the average X-ray spectra of AGN in deep Chandra fields.
The average spectrum of the AGN is computed using Chandra spectra with more
than 200 net counts from the AEGIS, Chandra Deep Field North and Chandra Deep
Field South surveys. The sample spans a broader range of X-ray luminosities
than other samples studied with stacking methods up to z=3.5. We analyze the
average spectra of this sample using our own averaging method, checking the
results against extensive simulations. Subsamples defined in terms of column
density of the local absorber, luminosity and z are also investigated.
We found a very significant Fe line with a narrow profile in all our samples
and in almost all the subsamples that we constructed. The equivalent width (EW)
of the narrow line estimated in the average spectrum of the full sample is 74
eV. The broad line component is significantly detected in the subsample of AGN
with L<1.43 1E44 cgs and z<0.76, with EW=108 eV.
We concluded that the narrow Fe line is an ubiquitous feature of the X-ray
spectra of the AGN up to z=3.5.The broad line component is significant in the
X-ray spectra of the AGN with low luminosity and low z.Comment: 17 pages, 23 figures, accepted for publication in Astronomy &
Astrophysics. Replacements in the bibliography: Iwasawa K., Mainieri V.,
Brusa M., et al., 2011, arXiv:1111.2447v1 (previously: Iwasawa et al 2011, in
preparation); Mateos S., Carrera F.J., Page M.J., et al., 2010, \aap, 510,
A35 (previously: Mateos S., Warwick R.S., Carrera F.J., et al., 2008, \aap,
492, 51
Too small to succeed? Lighting up massive dark matter subhaloes of the Milky Way
Using Constrained Local UniversE Simulations (CLUES) of the formation of the
Local Group in a cosmological context we investigate the recently highlighted
problem that the majority of the most massive dark subhaloes of the Milky Way
are too dense to host any of its bright satellites. In particular, we examine
the influence of baryonic processes and find that they leave a twofold effect
on the relation between the peak of the rotation curve and its position (Vmax
and Rmax). Satellites with a large baryon fraction experience adiabatic
contraction thus decreasing Rmax while leaving Vmax more or less unchanged.
Subhaloes with smaller baryon fractions undergo a decrease in Vmax possibly due
to outflows of material. Furthermore, the situation of finding subhaloes in
simulations that lie outside the confidence interval for possible hosts of the
bright MW dwarf spheroidals, appears to be far more prominent in cosmologies
with a high sigma8 normalisation and depends on the mass of the host. We
conclude that the problem cannot be simply solved by including baryonic
processes and hence demands further investigations.Comment: 6 pages, 3 figures. Accepted as MNRAS Letter 2011 July 21. Received
2011 July 20; in original form 2011 June
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