218 research outputs found
On the Evidence for Axion-like Particles from Active Galactic Nuclei
Burrage, Davis, and Shaw recently suggested exploiting the correlations
between high and low energy luminosities of astrophysical objects to probe
possible mixing between photons and axion-like particles (ALP) in magnetic
field regions. They also presented evidence for the existence of ALP's by
analyzing the optical/UV and X-ray monochromatic luminosities of AGNs. We
extend their work by using the monochromatic luminosities of 320 unobscured
Active Galactic Nuclei from the Sloan Digital Sky Survey/Xmm-Newton Quasar
Survey (Young et al., 2009), which allows the exploration of 18 different
combinations of optical/UV and X-ray monochromatic luminosities. However, we do
not find compelling evidence for the existence of ALPs. Moreover, it appears
that the signal reported by Burrage et al. is more likely due to X-ray
absorption rather than to photon-ALP oscillation.Comment: 16 pages, 12 figures. Updated to reflect the minor changes introduced
in the published versio
Time Variability in the X-ray Nebula Powered by Pulsar B1509-58
We use new and archival Chandra and ROSAT data to study the time variability
of the X-ray emission from the pulsar wind nebula (PWN) powered by PSR B1509-58
on timescales of one week to twelve years. There is variability in the size,
number, and brightness of compact knots appearing within 20" of the pulsar,
with at least one knot showing a possible outflow velocity of ~0.6c (assuming a
distance to the source of 5.2 kpc). The transient nature of these knots may
indicate that they are produced by turbulence in the flows surrounding the
pulsar. A previously identified prominent jet extending 12 pc to the southeast
of the pulsar increased in brightness by 30% over 9 years; apparent outflow of
material along this jet is observed with a velocity of ~0.5c. However, outflow
alone cannot account for the changes in the jet on such short timescales.
Magnetohydrodynamic sausage or kink instabilities are feasible explanations for
the jet variability with timescale of ~1.3-2 years. An arc structure, located
30"-45" north of the pulsar, shows transverse structural variations and appears
to have moved inward with a velocity of ~0.03c over three years. The overall
structure and brightness of the diffuse PWN exterior to this arc and excluding
the jet has remained the same over the twelve year span. The photon indices of
the diffuse PWN and possibly the jet steepen with increasing radius, likely
indicating synchrotron cooling at X-ray energies.Comment: accepted to ApJ, 14 pages, 8 figure
Search for low instability strip variables in the young open cluster NGC 2516
In this paper we revise and complete the photometric survey of the
instability strip of the southern open cluster NGC 2516 published by Antonello
and Mantegazza (1986). No variable stars with amplitudes larger than
were found. However by means of an accurate analysis based on a new statistical
method two groups of small amplitude variables have been disentangled: one with
periods (probably Scuti stars) and one with periods
. The position in the HR diagram and the apparent time-scale may
suggest that the stars of the second group belong to a recently discovered new
class of variables, named Dor variables. They certainly deserve
further study. We also present a comparison between the results of the
photometric survey and the available pointed ROSAT observations of this
cluster.Comment: 7 pages, 2 ps figures. Accepted for P.A.S.
Temporal Variability of the X-ray Emission of the Crab Nebula Torus
We have analyzed five ROSAT HRI images of the Crab Nebula spanning the years
1991 to 1997 and have found significant changes in the emission structure of
the X-ray torus surrounding the pulsar. Certain regions increase in brightness
by about 20% over the six years, while others show decreases in surface
brightness. The origin of these changes is unclear, but a possible explanation
is that the bulk velocity of the synchrotron radiating electrons has decreased
on the order of 20% as well.Comment: 15 pages plus 6 figures, figure 1 and figure 6 are in color, to
appear in The Astrophysical Journal, Jan 1, 1999, Vol. 510, #
The first optical light from the supernova remnant G182.4+4.3 located in the Galactic anti-centre region
We report the discovery of optical filamentary and diffuse emission from
G182.4+4.3 using 1.5-m Russian-Turkish telescope. We present the optical CCD
images obtained with Halpha filter revealing the presence of mainly filamentary
structure at the northwest, filamentary and diffuse structure at the centre,
south and north regions of the remnant. The bright optical filaments located in
northwest and south regions are well correlated with the prominent radio shell
of the remnant strongly suggesting their association. From the flux-calibrated
CCD imaging, the average [SII]/Halpha ratio is found to be ~0.9 and ~1.1 for
south and northwest regions, which clearly indicates that the emission
originates from the shock heated gas. We also present the results of X-ray data
obtained from XMM-Newton that show diffuse emission with a very low luminosity
of ~7.3x10^31 erg s^-1 at a distance of 3 kpc in 0.3-10 keV energy band.
Furthermore, we find a surprisingly young age of ~4400 yr for this remnant with
such a large radius of ~22 pc.Comment: 9 pages, 6 figures and 1 table, Accepted for publication in MNRA
XMM-Newton Observations of High Redshift Quasars
We report on our XMM observations of the high redshift quasars BR 2237--0607
(z=4.558) and BR 0351--1034 (z=4.351), together with 14 other z>4 objects found
in the XMM public archive. Contrary to former reports, we do not find high
redshift radio-loud quasars to be more absorbed than their radio-quiet
counterparts. We find that the optical to X-ray spectral index alpha-ox is
correlated with the luminosity density at 2500 A, but does not show a
correlation with redshift. The mean 2-10 keV power-law slope of the 9 high
redshift radio-quiet quasars in our sample for which a spectral analysis can be
performed is alpha-x1.23+-0.48, similar to alpha-x=1.19 found from the ASCA
observations of low redshift Narrow-Line Seyfert 1 galaxies (NLS1s), and
significantly different from alpha-x=0.78 found for low redshift Broad-Line
Seyfert galaxies. While the optical/UV spectra of low to high redshift quasars
look remarkably similar, we find a first indication of a difference in their
X-ray spectrum. The steep X-ray spectral index suggests high Eddington ratios
L/L_Edd. These observations give credence to the hypothesis of Mathur (2000)
that NLS1s are low luminosity cousins of high redshift quasars, both likely to
be in their early evolutionary stage.Comment: 25 pages, AJ, in press (Jan 2006
Long term X-ray spectral variability of the nucleus of M81
We have analysed the soft X-ray emission from the nuclear source of the
nearby spiral galaxy M81, using the available data collected with ROSAT, ASCA,
BeppoSAX and Chandra. The source flux is highly variable, showing (sometimes
dramatic: a factor of 4 in 20 days) variability at different timescales, from 2
days to 4 years, and in particular a steady increase of the flux by a factor of
>~ 2 over 4 years, broken by rapid flares. After accounting for the extended
component resolved by Chandra, the nuclear soft X-ray spectrum (from
ROSAT/PSPC, BeppoSAX/LECS and Chandra data) cannot be fitted well with a single
absorbed power-law model. Acceptable fits are obtained adding an extra
component, either a multi-color black body (MCBB) or an absorption feature. In
the MCBB case the inner accretion disk would be far smaller than the
Schwartzchild radius for the 3-60X 10^6 solar masses nucleus requiring a
strictly edge-on inclination of the disk, even if the nucleus is a rotating
Kerr black hole. The temperature is 0.27 keV, larger than expected from the
accretion disk of a Schwartzchild black hole, but consistent with that expected
from a Kerr black hole. In the power-law + absorption feature model we have
either high velocity (0.3 c) infalling C_v clouds or neutral C_i absorption at
rest. In both cases the C:O overabundance is a factor of 10.Comment: 30 pages with 15 figures. Accepted for publication in the
Astrophysical Journa
The X-ray Warm Absorber in NGC3516
The Seyfert 1 galaxy, NGC3516 has been the subject of many absorption line
studies at both ultraviolet and X-ray wavelengths. In the UV, strong, broad,
variable associated metal line absorption with velocity width \kms\
is thought to originate in gas with nh larger than about cm
lying between 0.01 and 9 pc from the central active nucleus. The Ginga X-ray
data are consistent with several possibilities: a warm absorber and a cold
absorber combined either with partial covering or an unusually strong
reflection spectrum. We present ROSAT observations of NGC3516 which show a
strong detection of a warm absorber dominated by a blend of OVII/OVIII edges at
0.8 keV with nh cm and U: 8--12. We argue
that NGC3516 contains an outflowing `XUV' absorber showing the presence of
X-ray absorption edges, that are consistent with the presence of broad
absorption lines in the old IUE spectra and their disappearance in the new UV
observations. Our dynamical model suggests that the OVII absorption edge will
continue to weaken compared to the OVIII edge, an easily testable prediction
with future missions like AXAF. Eventually the source would be transparent to
the X-rays unless a new absorption system is produced.Comment: 24 pages with 5 figures. To appear in the Ap.J. Requires AAS macro
The Nuclear Spectral Energy Distribution of NGC 4395, The Least Luminous Type 1 Seyfert Galaxy
We present X-ray (ROSAT), infrared, and radio observations of NGC 4395, which
harbors the optically least luminous type 1 Seyfert nucleus discovered thus
far. In combination with published optical and ultraviolet spectra, we have
used these data to assemble the broadband spectral energy distribution (SED) of
the galaxy's nucleus. Interestingly, the SED of NGC 4395 differs markedly from
the SEDs of both quasars and typical low-luminosity active galactic nuclei,
which may be a manifestation of the different physical conditions (i.e., black
hole masses, accretion rates, and/or accretion modes) that exist in these
objects. The nuclear X-ray source in NGC 4395 is variable and has an observed
luminosity of just ~ 10^38 ergs/s. Although this emission could plausibly be
associated with either a weak active nucleus or a bright stellar-mass binary
system, the optical and ultraviolet emission-line properties of the nucleus
strongly suggest that the X-rays arise from a classical AGN.Comment: Accepted for publication in PASP (July 1999), 17 pages, including 4
Postscript figure
The X-ray Properties of z>4 Quasars
We report on a search for X-ray emission from quasars with redshifts greater
than four using the ROSAT public database. Our search has doubled the number of
z>4 quasars detected in X-rays from 6 to 12. Most of those known prior to this
work were radio-loud and X-ray selected sources; our study increases the number
of X-ray detected, optically selected z>4 quasars from one to seven. We present
their basic X-ray properties and compare these to those of lower redshift
quasars. We do not find any evidence for strong broad-band spectral differences
between optically selected z>4 quasars and those at lower redshifts.Comment: 7 pages, 4 figures included, LaTeX emulateapj.sty, accepted for
publication in the Astronomical Journa
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