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NAVI: Novel authentication with visual information
Text-based passwords, despite their well-known drawbacks, remain the dominant user authentication scheme implemented. Graphical password systems, based on visual information such as the recognition of photographs and / or pictures, have emerged as a promising alternative to the aggregate reliance on text passwords. Nevertheless, despite the advantages offered they have not been widely used in practice since many open issues need to be resolved. In this paper we propose a novel graphical password scheme, NAVI, where the credentials of the user are his username and a password formulated by drawing a route on a predefined map. We analyze the strength of the password generated by this scheme and present a prototype implementation in order to illustrate the feasibility of our proposal. Finally, we discuss NAVI’s security features and compare it with existing graphical password schemes as well as text-based passwords in terms of key security features, such aspassword keyspace, dictionary attacks and guessing attacks. The proposed scheme appears to have the same or better performance in the majority of the security features examined
Keck Imaging of the Globular Cluster Systems in the Early--type Galaxies NGC 1052 and NGC 7332
The presence of two globular cluster subpopulations in early-type galaxies is
now the norm rather than the exception. Here we present two more examples for
which the host galaxy appears to have undergone a recent merger. Using
multi-colour Keck imaging of NGC 1052 and NGC 7332 we find evidence for a
bimodal globular cluster colour distribution in both galaxies, with roughly
equal numbers of blue and red globular clusters. The blue ones have similar
colours to those in the Milky Way halo and are thus probably very old and
metal-poor. If the red GC subpopulations are at least solar metallicity, then
stellar population models indicate young ages. We discuss the origin of
globular clusters within the framework of formation models. We conclude that
recent merger events in these two galaxies have had little effect on their
overall GC systems. We also derive globular cluster density profiles, global
specific frequencies and in the case of NGC 1052, radial colour gradients and
azimuthal distribution. In general these globular cluster properties are normal
for early-type galaxies.Comment: 11 pages, Latex, 15 figures, 2 tables, accepted by MNRA
XMM and Chandra measurements of the AGN intrinsic absorption: dependence on luminosity and redshift
We combine bright XMM data with the Chandra Deep Field South observations in
order to explore the behavior of the intrinsic AGN absorption, as a function of
redshift and luminosity.Our sample consists of 359 sources selected in the hard
2-8 keV band, spanning the flux range 6\times10^{-16}-$3\times10^{-13} erg s^-1
cm^-2 with a high rate of spectroscopic or photometric redshift completeness
(100 and 85 per cent respectively for the Chandra and XMM data. We derive the
column density values using X-ray spectral fits. We find that the fraction of
obscured AGN falls with increasing luminosity in agreement with previous
findings. The fraction of obscured AGN shows an apparent increase at high
redshifts (z>2). Simulations show that this effect can be most probably
attributed to the fact that at high redshifts the column densities are
overestimated.Comment: 14 pages, 9 figures, A&A accepte
The XMM-Newton Needles in the Haystack Survey: the local X-ray luminosity function of 'normal' galaxies
In this paper we estimate the local (z<0.22) X-ray luminosity function of
`normal' galaxies derived from the XMM-Newton Needles in the Haystack Survey.
This is an on-going project that aims to identify X-ray selected `normal'
galaxies (i.e. non-AGN dominated) in the local Universe. We are using a total
of 70 XMM-Newton fields covering an area of 11 sq. degrees which overlap with
the Sloan Digital Sky Survey Data Release-2. `Normal' galaxies are selected on
the basis of their resolved optical light profile, their low X-ray--to--optical
flux ratio (log(f_x/f_o)<-2) and soft X-ray colours. We find a total of 28
candidate `normal' galaxies to the 0.5-8 keV band flux limit of ~2x10^{-15}
cgs. Optical spectra are available for most sources in our sample (82 per
cent). These provide additional evidence that our sources are bona-fide
'normal' galaxies with X-ray emission coming from diffuse hot gas emission
and/or X-ray binaries rather than a supermassive black hole. Sixteen of our
galaxies have narrow emission lines or a late-type Spectral Energy Distribution
(SED) while the remaining 12 present only absorption lines or an early-type
SED. Combining our XMM-Newton sample with 18 local (z<0.22) galaxies from the
Chandra Deep Field North and South surveys, we construct the local X-ray
luminosity function of `normal' galaxies. This can be represented with a
Schechter form with a break at L* ~ 3x10^{41} cgs and a slope of ~1.78+/-0.12.
Using this luminosity function and assuming pure luminosity evolution of the
form
(1+z)^{3.3} we estimate a contribution to the X-ray background from `normal'
galaxies of 10-20 per cent (0.5-8 keV). Finally, we derive, for the first time,
the luminosity functions for early and late type systems separately.Comment: To appear to MNRAS, 9 page
Radio observations of the CDF-South: a possible link between radio emission and star formation in X-ray selected AGN
We explore the nature of the radio emission of X-ray selected AGN by
combining deep radio (1.4GHz; 60micro-Jy) and X-ray data with multiwavelength
(optical, mid-infrared) observations in the Extended Chandra Deep Field South
(E-CDFS). The fraction of radio detected X-ray sources increases from 9% in the
E-CDFS to 14% in the central region of this field, which has deeper X-ray
coverage from the 1Ms CDFS. We find evidence that the radio emission of up to
60% of the hard X-ray/radio matched AGN is likely associated with
star-formation in the host galaxy. Firstly, the mid-IR (24micron) properties of
these sources are consistent with the infrared/radio correlation of starbursts.
Secondly, most of them are found in galaxies with blue rest-frame optical
colours (U-V), suggesting a young stellar population. On the contrary,
X-ray/radio matched AGN which are not detected in the mid-infrared have red U-V
colours suggesting their radio emission is associated with AGN activity. We
also find no evidence for a population of heavily obscured radio-selected AGN
that are not detected in X-rays. Finally, we do no confirm previous claims for
a correlation between radio emission and X-ray obscuration. Assuming that the
radio continuum measures star-formation, this finding is against models where
the dust and gas clouds associated with circumnuclear starbursts are
spherically blocking our view to the central engine.Comment: Accepted by A&
On the use of photometric redshifts for X-ray selected AGNs
(Abridged) In this paper we present photometric redshift estimates for a
sample of X-ray selected sources detected in the wide field (~2 deg^2), bright
[f_{X} (0.5-8 keV)~10^{-14} cgs] XMM-Newton/2dF survey. Unlike deeper X-ray
samples comprising a large fraction of sources with colours dominated by the
host galaxy, our bright survey primarily probes the QSO X-ray population.
Therefore photometric redshift methods employing both galaxy and QSO templates
need to be used. We employ the photometric redshift technique of
Hatziminaoglou, Mathez & Pello (2000) using 5-band photometry from the SDSS. We
separate our X-ray sources according to their optical profile to point-like and
extended. We apply QSO and galaxy templates to the point-like and extended
sources respectively. X-ray sources associated with Galactic stars are
identified and discarded from our point-like sample on the basis of their low
X-ray--to--optical flux ratio and their broad band colours that are best fit by
stellar templates. Comparison of our results with spectroscopic redshifts
available, allows calibration of our method and estimation of the photometric
redshift accuracy. For ~70 per cent of the point-like sources photometric
redshifts are correct within dz <= 0.3 (or ~75 per cent have dz/(1+z) <= 0.2),
and the rms scatter is estimated to be sigma_z = 0.30. For the optically
extended objects the photometric redshifts work only in the case of red (g - r
> 0.5 mag) sources yielding dz <= 0.15 and dz/(1+z) <= 0.2 for 73 and 93 per
cent respectively. However, we find that the above photometric redshift
technique does not work in the case of extended sources with blue colours (g -
r < 0.5): such sources cannot be fit successfully by QSO or galaxy templates,
or any linear combination of the two.Comment: Replaced due to extended revision; 11 pages, 4 figures; Accepted in
A&
The Phoenix Deep Survey: X-ray properties of faint radio sources
In this paper we use a 50ks XMM-Newton pointing overlapping with the Phoenix
Deep Survey, a homogeneous radio survey reaching muJy sensitivities, to explore
the X-ray properties and the evolution of star-forming galaxies. UV, optical
and NIR photometry is available and is used to estimate photometric redshifts
and spectral types for radio sources brighter than R=21.5mag (total of 82).
Sources with R<21.5mag and spiral galaxy SEDs (34) are grouped into two
redshift bins with a median of z=0.240 and 0.455 respectively. Stacking
analysis for both the 0.5-2 and 2-8keV bands is performed on these subsamples.
A high confidence level signal (>3.5sigma) is detected in the 0.5-2keV band
corresponding to a mean flux of ~3e-16cgs for both subsamples. This flux
translates to mean luminosities of ~5e40 and 1.5e41cgs for the z=0.240 and
0.455 subsamples respectively. Only a marginally significant signal (2.6sigma)
is detected in the 2-8keV band for the z=0.455 subsample. We argue that the
stacked signal above is dominated by star-formation. The mean L_X/L_B ratio and
the mean L_X of the two subsamples are found to be higher than optically
selected spirals and similar to starbursts. We also find that the mean L_X and
L_1.4 of the faint radio sources studied here are consistent with the L_X-L_1.4
correlation of local star-forming galaxies. Moreover, the X-ray emissivity of
sub-mJy sources to z~0.3 is found to be elevated compared to local HII
galaxies. The observed increase is consistent with L_X evolution of the form
(1+z)^3. Assuming that our sample is indeed dominated by starbursts this is
direct evidence for evolution of such systems at X-ray wavelengths. Using an
empirical L_X to SFR conversion we estimate a global SFR density at z~0.3 of
\~0.029M_o/yr/Mpc in agreement with previous studies.Comment: 12 pages, 6 figures, accepted for publication in MNRA
The Phoenix Deep Survey: spectroscopic catalog
The Phoenix Deep Survey is a multi-wavelength survey based on deep 1.4 GHz
radio imaging, reaching well into the sub-100 microJy level. One of the aims of
this survey is to characterize the sub-mJy radio population, exploring its
nature and evolution. In this paper we present the catalog and results of the
spectroscopic observations aimed at characterizing the optically ``bright''
(R<~ 21.5 mag) counterparts of faint radio sources. Out of 371 sources with
redshift determination, 21% have absorption lines only, 11% show AGN
signatures, 32% are star-forming galaxies, 34% show narrow emission lines that
do not allow detailed spectral classification (due to poor signal-to-noise
ratio and/or lack of diagnostic emission lines) and the remaining 2% are
identified with stars. For the star-forming galaxies with a Balmer decrement
measurement we find a median extinction of A(Ha)=1.9 mag, higher than that of
optically selected samples. This is a result of the radio selection, which is
not biased against dusty systems. Using the available spectroscopic
information, we estimate the radio luminosity function of star-forming galaxies
in two independent redshift bins at z~0.1 and 0.3 respectively. We find direct
evidence for strong luminosity evolution of these systems consistent with L(1.4
GHz) ~ (1+z)^(2.7).Comment: 39 pages, 12 figures. References added, and minor changes to reflect
published versio
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