SMILE: Search for MIlli-LEnses Casadio, C; Blinov, D; Readhead, A C S ...
Monthly notices of the Royal Astronomical Society. Letters,
10/2021, Letnik:
507, Številka:
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Journal Article
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ABSTRACT
Dark matter (DM) haloes with masses below ∼108 M⊙, which would help to discriminate between DM models, may be detected through their gravitational effect on distant sources. The same applies ...to primordial black holes, considered as an alternative scenario to DM particle models. However, there is still no evidence for the existence of such objects. With the aim of finding compact objects in the mass range of ∼106–109 M⊙, we search for strong gravitational lenses on milliarcsec scales (<150 mas). For our search, we used the Astrogeo very long baseline interferometry (VLBI) fits image data base – the largest publicly available data base, containing multifrequency VLBI data of 13 828 individual sources. We used the citizen science approach to visually inspect all sources in all available frequencies in search for images with multiple compact components on mas scales. At the final stage, sources were excluded based on the surface brightness preservation criterion. We obtained a sample of 40 sources that passed all steps and therefore are judged to be mas lens candidates. These sources are currently followed up with ongoing European VLBI network observations at 5 and 22 GHz. Based on spectral index measurements, we suggest that two of our candidates have a higher probability to be associated with gravitational lenses.
We report 87 previously unpublished optical linear polarization measurements of 86 quasars obtained in May and October 2008, and from April to July 2015 with the FOcal Reducer and low-dispersion ...Spectrographs FORS1 and FORS2 attached to the Very Large Telescope at the Paranal Observatory. Of the 86 quasars, 37 have p ≥ 0.6%, 9 have p ≥ 2%, and 1 has p ≥ 10%.
We present the first Bayesian method for tomographic decomposition of the plane-of-sky orientation of the magnetic field with the use of stellar polarimetry and distance. This standalone tomographic ...inversion method presents an important step forward in reconstructing the magnetized interstellar medium (ISM) in three dimensions within dusty regions. We develop a model in which the polarization signal from the magnetized and dusty ISM is described by thin layers at various distances, a working assumption which should be satisfied in small-angular circular apertures. Our modeling makes it possible to infer the mean polarization (amplitude and orientation) induced by individual dusty clouds and to account for the turbulence-induced scatter in a generic way. We present a likelihood function that explicitly accounts for uncertainties in polarization and parallax. We develop a framework for reconstructing the magnetized ISM through the maximization of the log-likelihood using a nested sampling method. We test our Bayesian inversion method on mock data, representative of the high Galactic latitude sky, taking into account realistic uncertainties from
Gaia
and as expected for the optical polarization survey P
ASIPHAE
according to the currently planned observing strategy. We demonstrate that our method is effective at recovering the cloud properties as soon as the polarization induced by a cloud to its background stars is higher than ~0.1% for the adopted survey exposure time and level of systematic uncertainty. The larger the induced polarization is, the better the method’s performance, and the lower the number of required stars. Our method makes it possible to recover not only the mean polarization properties but also to characterize the intrinsic scatter, thus creating new ways to characterize ISM turbulence and the magnetic field strength. Finally, we apply our method to an existing data set of starlight polarization with known line-of-sight decomposition, demonstrating agreement with previous results and an improved quantification of uncertainties in cloud properties.
We propose a new method to analyse the alignment of optical polarization vectors from quasars. This method leads to a definition of intrinsic preferred axes and to a determination of the probability ...p
σ that the distribution of polarization directions is random. This probability is found to be as low as 3.0 × 10−5 for one of the regions of redshift.
Context . The calibration of optical polarimeters relies on the use of stars with negligible polarization (i.e., unpolarized standard stars) for determining the instrumental polarization zero point. ...For wide-field polarimeters, calibration is often done by imaging the same star over multiple positions in the field of view (FoV), which is a time-consuming process. A more effective technique is to target fields containing multiple standard stars. While this method has been used for fields with highly polarized stars, there are no such sky regions with well measured unpolarized standard stars. Aims . We aim to identify sky regions with tens of stars exhibiting negligible polarization that are suitable for a zero-point calibration of wide-field polarimeters. Methods . We selected stars in regions with extremely low reddening, located at high Galactic latitudes. We targeted four ~40′ × 40′ fields in the northern and eight in the southern equatorial hemispheres. Observations were carried out at the Skinakas Observatory and the South African Astronomical Observatory. Results . We found two fields in the north and seven in the south characterized by a mean polarization lower than p < 0.1%. Conclusions . At least 9 out of the 12 fields can be used for a zero-point calibration of wide-field polarimeters.
Gigaparsec scale alignments of the quasar optical polarization vectors have been proven to be robust against a scenario of contamination by the Galactic interstellar medium (ISM). This claim has been ...established by means of optical polarization measurements of the starlight surrounding the lines of sight of the 355 quasars for which reliable optical polarization measurements are available. In this paper, we take advantage of the full-sky and high quality polarization data released by the
Planck
satellite to provide an independent, complementary, and up-to-date estimation of the contamination level of the quasar optical polarization data by the Galactic dust. Our analysis reveals signatures of Galactic dust contamination at the two sigma level for about 30 percent of the quasar optical polarization data sample. The remaining 70 percent of the lines of sight do not show Galactic dust contamination above the two sigma level, suggesting low to negligible contamination of the quasar optical polarization signal. We further found arguments suggesting that Galactic thermal dust cannot fully account for the reported quasar optical polarization alignments. Based on the measurements of the ratio of the polarized intensity of the dust in the submillimeter to the degree of linear polarization of the quasar in the optical, we provide a new and independent quality criteria to apply to the quasar optical polarization sample. We argue that, unless correction is applied, such a criterion should be imposed on the data for future investigations in the framework of the cosmological-scale correlations of quasar optical polarization vector orientations that still could compete with the isotropic principle of the cosmological paradigm.
Context.
Next-generation wide-field optical polarimeters such as Wide-Area Linear Optical Polarimeters (WALOPs) have a field of view (FoV) of tens of arcminutes. Wide-field polarimetric flat sources ...are essential to the efficient and accurate calibration of these instruments. However, no established wide-field polarimetric standard or flat sources exist at present.
Aims.
This study tests the feasibility of using the polarized sky patches of the size of around 10 × 10 arcminutes
2
, at a distance of up to 20° from the Moon, on bright-Moon nights as a wide-field linear polarimetric flat source.
Methods.
We observed 19 patches of the sky adjacent to the bright-Moon with the RoboPol instrument in the SDSS-
r
broadband filter. These patches were observed on five nights within two days of the full-Moon across two RoboPol observing seasons.
Results.
We find that for 18 of the 19 patches, the uniformity in the measured normalized Stokes parameters
q
and
u
is within 0.2%, with 12 patches exhibiting uniformity within 0.07% or better for both
q
and
u
simultaneously, making them reliable and stable wide-field linear polarization flats.
Conclusions.
We demonstrate that the sky on bright-Moon nights is an excellent wide-field linear polarization flat source. Various combinations of the normalized Stokes parameters
q
and
u
can be obtained by choosing suitable locations of the sky patch with respect to the Moon.
Context.
Optical polarimeters are typically calibrated using measurements of stars with known and stable polarization parameters. However, there is a lack of such stars available across the sky. Many ...of the currently available standards are not suitable for medium and large telescopes due to their high brightness. Moreover, as we find, some of the polarimetric standards used are in fact variable or have polarization parameters that differ from their cataloged values.
Aims.
Our goal is to establish a sample of stable standards suitable for calibrating linear optical polarimeters with an accuracy down to 10
−3
in fractional polarization.
Methods.
For 4 yr, we have been running a monitoring campaign of a sample of standard candidates comprised of 107 stars distributed across the northern sky. We analyzed the variability of the linear polarization of these stars, taking into account the non-Gaussian nature of fractional polarization measurements. For a subsample of nine stars, we also performed multiband polarization measurements.
Results.
We created a new catalog of 65 stars (see Table 2) that are stable, have small uncertainties of measured polarimetric parameters, and can be used as calibrators of polarimeters at medium and large telescopes.
Abstract
Gravitational lensing provides a powerful tool to determine the Hubble parameter H
0 from the measurement of the time delay Δt between two lensed images of a background variable source. ...Nevertheless, knowledge of the deflector mass distribution constitutes a hurdle. We propose in the present work interesting solutions for the case of nearly symmetric gravitational lens systems. For the case of a small misalignment between the source, the deflector and the observer, we first consider power-law (ɛ) axially symmetric models for which we derive an analytical relation between the amplification ratio and source position which is independent of the power-law slope ɛ. According to this relation, we deduce an expression for H
0 also irrespective of the value ɛ. Secondly, we consider the power-law axially symmetric lens models with an external large-scale gravitational field, the shear γ, resulting in the so-called ɛ−γ models, for which we deduce simple first-order equations linking the model parameters and the lensed image positions, the latter being observable quantities. We also deduce simple relations between H
0 and observables quantities only. From these equations, we may estimate the value of the Hubble parameter in a robust way. Nevertheless, comparison between the ɛ−γ and singular isothermal ellipsoid (SIE) models leads to the conclusion that these models remain most often distinct. Therefore, even for the case of a small misalignment, use of the first-order equations and precise astrometric measurements of the positions of the lensed images with respect to the centre of the deflector enables one to discriminate between these two families of models. Finally, we confront the models with numerical simulations to evaluate the intrinsic error of the first-order expressions used when deriving the model parameters under the assumption of a quasi-alignment between the source, the deflector and the observer. From these same simulations, we estimate for the case of the ɛ−γ family of models that the standard deviation affecting H
0 is which merely reflects the adopted astrometric uncertainties on the relative image positions, typically . In conclusions, we stress the importance of getting very accurate measurements of the relative positions of the multiple lensed images and of the time delays for the case of nearly symmetric gravitational lens systems, in order to derive robust and precise values of the Hubble parameter.
Context
In the framework of studying cosmic microwave background polarization and characterizing its Galactic foregrounds, the angular power spectrum analysis of the thermal dust polarization map has ...led to intriguing evidence of an
E/B
asymmetry and a positive
TE
correlation. The interpretation of these observations is the subject of theoretical and simulation-driven studies in which the correlation between the density structure of the interstellar medium (ISM) and the magnetic field appears to be a key aspect. In this context, and when the magnetized ISM structures are modeled in three dimensions, dust clouds are generally considered to be filamentary structures only, but both filamentary and sheet-like shapes are supported by observational and theoretical evidence.
Aims
. We aim to study the influence of the cloud shape and its connection to the local magnetic field, as well as the influence from the viewing angle, on the angular power spectra measured on thermal dust polarization maps; we specifically focus on the dependence of the
E/B
power asymmetry and
TE
correlation.
Methods
. To this end, we simulated realistic interstellar clouds with both filament-like and sheet-like shapes using the software A
sterion
, which also allowed us to generate synthetic maps of thermal dust polarized emission with an area of 400 square degrees. Then, we computed their polarization power spectra in the multipole range
ℓ
ϵ 100, 500 and focused on the
E/B
power asymmetry, quantified through the
ℛ
EB
ratio, and the correlation coefficient
r
TE
between
Τ
and
Ε
modes. We quantified the dependence of
ℛ
EB
and
r
TE
values on the offset angle (between the longest cloud axis and local magnetic field lines) and inclination angle (between the line of sight and the magnetic field) for both types of cloud shapes, either embedded in a regular magnetic field or coupled to a nonregular field to mimic turbulence.
Results
. We find that both types of cloud shapes cover the same regions of the (ℛ
EB
,
r
TE
) parameter space. The dependence on the inclination and offset angles is similar for both shapes, although sheet-like structures generally show larger scatter than filamentary structures. In addition to the known dependence on the offset angle, we find a strong dependence of ℛ
EB
and
r
TE
on the inclination angle.
Conclusions
. The very fact that filament-like and sheet-like structures may lead to polarization power spectra with similar (ℛ
EB
,r
TE
) values complicates their interpretation. We argue that interpreting them solely in terms of filament characteristics is risky, and in future analyses, this degeneracy should be accounted for, as should the connection to the magnetic field geometry. Our results based on maps of 400 square degrees clarify that the overall geometrical arrangement of the magnetized ISM surrounding the observer leaves its marks on polarization power spectra.
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