Nikolaos Mandarakas

• PhD Student
• PhD Student at University of Crete

Context. The Serkowski relation is the cornerstone of studies of starlight polarizations as a function of wavelength. Although empirical, its extensive use since its inception in 1975 to describe polarization induced by interstellar dust has elevated the relation to the status of an indisputable “law”: this is the benchmark against which models of...

We present the first degree-scale tomography map of the dusty magnetized interstellar medium (ISM) from stellar polarimetry and distance measurements. We used the RoboPol polarimeter at Skinakas Observatory to conduct a survey of the polarization of starlight in a region of the sky of about four square degrees. We propose a Bayesian method to decom...

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-con...

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...

The Fermi γ -ray telescope has detected 6658 sources, with 1845 of them remaining unidentified. We show that polarimetry of γ -ray fields is a powerful asset in the hunt of active galactic nuclei (AGN) as potential optical counterparts for γ -ray sources. We have studied an unidentified Fermi field (3FGL J0221.2+2518) and found a previously-unknown...

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...

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 used pola...

A tidal disruption event (TDE) occurs when a supermassive black hole rips apart a passing star. Part of the stellar material falls toward the black hole, forming an accretion disk that in some cases launches a relativistic jet. We performed optical polarimetry observations of a TDE, AT 2020mot. We find a peak linear polarization degree of 25 ± 4%,...

Next-generation wide-field optical polarimeters like the Wide-Area Linear Optical Polarimeters (WALOPs) have a field of view (FoV) of tens of arcminutes. For efficient and accurate calibration of these instruments, wide-field polarimetric flat sources will be essential. Currently, no established wide-field polarimetric standard or flat sources exis...

The growth of supermassive black holes (SMBHs) through merging has long been predicted but its detection remains elusive. However, a promising target has been discovered in the Seyfert-1 galaxy J1430+2303. If a binary system truly lies at the center of J1430+2303, the usual symmetry expected from pole-on views in active galactic nuclei (AGNs) respo...

Context. The growth of supermassive black holes (SMBHs) through merging has long been predicted but its detection remains elusive. However, a promising target has been discovered in the Seyfert-1 galaxy J1430+2303, where two SMBHs may be about to merge. Aims. If a binary system truly lies at the center of J1430+2303, the usual symmetry expected fro...

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...

Aims. We report on the follow-up of the extremely bright long gamma-ray burst (GRB) GRB 210619B with optical polarimetry. Methods. We conducted optopolarimetric observations of the optical afterglow of GRB 210619B in the SDSS- r band in the time window ∼5967 − 8245 s after the burst, using the RoboPol instrument at the Skinakas observatory. Results...

Supermassive black holes have been known to disrupt passing stars producing outbursts called Tidal Disruption Events offering a unique view on the early stages of accretion disk and jet formation. The advent of large scale optical time-domain surveys has significantly increased the number of known events and challenged our understanding of their dy...

We report on the follow-up of the extremely bright gamma-ray burst GRB 210619B with optical polarimetry. We conducted optopolarimetric observations of the optical afterglow of GRB 210619B in the SDSS-r band in the time window ~ 5967 - 8245 seconds after the burst, using the RoboPol instrument at the Skinakas observatory. We find signs of variabilit...

The Wide-Area Linear Optical Polarimeter (WALOP)-South instrument is an upcoming wide-field and high-accuracy optical polarimeter to be used as a survey instrument for carrying out the Polar-Areas Stellar Imaging in Polarization High Accuracy Experiment (PASIPHAE) program. Designed to operate as a one-shot four-channel and four-camera imaging polar...

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 3D within dusty regions. We develop a...

Context. Atomic gas in the diffuse interstellar medium (ISM) is organized in filamentary structures. These structures usually host cold and dense molecular clumps. The Galactic magnetic field is considered to play an important role in the formation of these clumps. Aims. Our goal is to explore the role of the magnetic field in the H I -H 2 transiti...

Atomic gas in the diffuse interstellar medium (ISM) is organized in filamentary structures. These structures usually host cold and dense molecular clumps. The Galactic magnetic field is considered to play an important role in the formation of these clumps. Our goal is to explore the role of the magnetic field in the HI - H$_{2}$ transition process....

At optical wavelengths, blazar Electric Vector Position Angle (EVPA) rotations linked with gamma-ray activity have been the subject of intense interest and systematic investigation for over a decade. One difficulty in the interpretation of EVPA rotations is the inherent 180○ ambiguity in the measurements. It is therefore essential, when studying EV...

Context. Coherence in the characteristics of neighboring sources in the 2D and 3D space may suggest the existence of large-scale cosmic structures that are useful for cosmological studies. Numerous works have been conducted to detect such features in global scales as well as in confined areas of the sky. However, their results are often contradicto...

Dark Matter (DM) halos with masses below ∼108M⊙, 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. Wi...

Dark Matter (DM) halos with masses below $\sim10^{8}$ $M_{\odot}$, 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 o...

Context.Coherence in the characteristics of neighboring sources in 2D and 3D space may suggest the existence of large-scale cosmic structures, which are useful for cosmological studies. Numerous works have been conducted to detect such features in global scalesas well as in confined areas of the sky. However, results are often contradictory and the...

At optical wavelengths, blazar Electric Vector Position Angle (EVPA) rotations linked with gamma-ray activity have been the subject of intense interest and systematic investigation for over a decade. One difficulty in the interpretation of EVPA rotations is the inherent 180{\deg} ambiguity in the measurements. It is therefore essential, when studyi...

At optical wavelengths, blazar Electric Vector Position Angle (EVPA) rotations linked with gamma-ray activity have been the subject of intense interest and systematic investigation for over a decade. One difficulty in the interpretation of EVPA rotations is the inherent 180° ambiguity in the measurements. It is therefore essential, when studying EV...

The Wide-Area Linear Optical Polarimeter (WALOP)-South instrument will be mounted on the 1-m South African Astronomical Observatory telescope in South Africa as part of the Polar-Areas Stellar Imaging Polarization High Accuracy Experiment (PASIPHAE) program to carry out a linear imaging polarization survey of the Galactic polar regions in the optic...

The WALOP-South instrument will be mounted on the 1 m SAAO telescope in South Africa as part of the PASIPHAE program to carry out a linear imaging polarization survey of the Galactic polar regions in the optical band. Designed to achieve polarimetric sensitivity of $0.05~\%$ across a $35\times35$ arcminute field of view, it will be capable of measu...

WALOP (Wide-Area Linear Optical Polarimeter)-South, to be mounted on the 1m SAAO telescope in South Africa, is first of the two WALOP instruments currently under development for carrying out the PASIPHAE survey. Scheduled for commissioning in the year 2021, the WALOP instruments will be used to measure the linear polarization of around $10^{6}$ sta...

We present uniformly reprocessed and re-calibrated data from the RoboPol programme of optopolarimetric monitoring of active galactic nuclei (AGN), covering observations between 2013, when the instrument was commissioned, and 2017. In total, the dataset presented in this paper includes 5068 observations of 222 AGN with Dec > −25○. We describe the cu...

WALOP (Wide-Area Linear Optical Polarimeter)-South, to be mounted on the 1m SAAO telescope in South Africa, is first of the two WALOP instruments currently under development for carrying out the PASIPHAE survey. Scheduled for commissioning in the year 2021, the WALOP instruments will be used to measure the linear polarization of around 10⁶ stars in...

We present uniformly reprocessed and re-calibrated data from the RoboPol programme of optopolarimetric monitoring of active galactic nuclei (AGN), covering observations between 2013, when the instrument was commissioned, and 2017. In total, the dataset presented in this paper includes 5068 observations of 222 AGN with Dec > -25 deg. We describe the...

Context. A number of works have reported that the polarization plane of extragalactic sources as well as the structural axes of radio sources are aligned on the large scale. However, both the claims and their interpretation remain controversial. Aims. For the first time, we explore the alignment of parsec-scale jets. Additionally, we use archival p...

A number of works reported on the existence of a large scale alignment of the polarization plane of extragalactic sources as well as the alignment of radio-sources structural axes. However, both claims and their interpretation remain controversial. For the first time we explore the parsec-scale jets alignments. Additionally, we use archival polarim...

We present the optical analysis of a sample of quasars selected for their significant variation in optical emission. Photometric analysis was carried for 22 sources and it was found that the colour change for the majority of the sample follows a typical behavior of Changing Look Quasars (CLQs) as suggested in the literature. Spectroscopic analysis...

Context . The third Fermi -LAT catalog (3FGL) presented the data of the first four years of observations from the Fermi Gamma-ray Space Telescope mission. There are 3034 sources, 1010 of which still remain unidentified. Identifying and classifying γ -ray emitters is of high significance with regard to studying high-energy astrophysics. Aims . We de...

The third Fermi-LAT catalog (3FGL) presented the data of the first four years of observations from the Fermi Gamma-ray Space Telescope mission. There are 3034 sources, 1010 of which still remain unidentified. Identifying and classifying gamma-ray emitters is of high significance with regard to studying high energy astrophysics. We aim to demonstrat...