arXiv subjects

G. Borisov

Publications and source records attributed to G. Borisov.

14 recordsLinked to original sources

arXiv2026

Shape model and dynamical state of the asteroid (300) Geraldina: Implications for its possible ancient origin

In this study, we present a comprehensive photometric and physical characterisation of the main-belt asteroid (300) Geraldina. Our analysis includes determining its sidereal rotational period, shape modelling, spin-axis orientation, as well as dynamical and spectral properties. The investigation is based on two decades of archival photometry from the Bulgarian National Astronomical Observatory (BNAO) Rozhen, complemented by dense CCD lightcurve observations obtained since 2017 at the Astronomical Station Vidojevica (ASV), dense data from ALCDEF, and augmented with sparse-in-time measurements from Gaia Data Release 3 (DR3). By combining these heterogeneous datasets within and using the lightcurve inversion method, we confirmed the asteroid's prograde sense of rotation and obtained two symmetrically mirrored pole solutions for the asteroid model. Our dynamical studies show that (300) Geraldina remains stable in 100 Myr. Although close to the 2:1 mean-motion resonance with Jupiter, it is not captured into it during the observed time. Instead, it exhibits interactions with an unidentified high-order mean-motion resonance, which appears to contribute to its long-term stability. Its spectral type (C) with a primitive carbonaceous composition, in combination with low albedo and long-term stability, is consistent with the assumption that (300) Geraldina is an ancient asteroid.

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arXiv2026

Polarimetry and albedo of the Near-Earth Asteroid 2025 FA22

We report spectropolarimetric and broadband polarimetric observations of the near-Earth asteroid 2025 FA22 during its close approach of 18 September 2025 (about two Moon distances). With a diameter estimated between 130 and 290 m, 2025 FA22 is among the largest NEAs observable at such proximity, prompting an International AsteroidWarning Network (IAWN) rapid-response campaign. Although early orbital solutions indicated a possible impact in 2089, further follow-up astrometric observations ruled out collision hazard. The favourable geometry of this close encounter enabled a dense coverage of the positive part of the phase-polarisation curve, from the high polarisation domain (high phase angles), nearly to the inversion angle where the linear polarisation fraction vanishes. The spectropolarimetric observations provided the wavelength dependence of the polarisation fraction. Using empirical relationships, an estimate of the geometric albedo could be drawn from the slope of the phase-polarisation curve at inversion angle : $\rho_v = 0.17\pm0.04$ in the V band. This value, together with the spectropolarimetric trend, provides constraints on the taxonomic class, with the results being most consistent with an M-type classification. These results demonstrate the interest of polarimetry and spectropolarimetry for rapid characterisation of newly discovered NEAs in planetary defence campaigns.

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arXiv2025

Cross validation of albedo determination for 1627 Ivar from three different techniques

Near Earth Asteroids are of great interest to the scientific community due to their proximity to Earth, making them both potential hazards and possible targets for future missions, as they are relatively easy to reach by spacecraft. A number of techniques and models can be used to constrain their physical parameters and build a comprehensive assessment of these objects. In this work, we compare physical property results obtained from improved $H_V$ absolute magnitude values, thermophysical modeling, and polarimetry data for the well-known Amor-class NEO 1627 Ivar. We show that our fits for albedo are consistent with each other, thus demonstrating the validity of this cross-referencing approach, and propose a value for Ivar's albedo of $0.24^{+0.04}_{-0.02}$ . Future observations will extend this work to a larger sample size, increasing the reliability of polarimetry for rapid asteroid property characterization, as a technique independent of previously established methods and requiring significantly fewer observations.

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arXiv2024

The possible dual nature of the asteroid (12499) 1998 FR47

We present the R-band lightcurves of the Flora family asteroid (12499) 1998 FR47, obtained in 2022 at two different astronomical sites: Bulgarian National Astronomical Observatory Rozhen (MPC Code 071) and Astronomical Station Vidojevica (MPC Code C89). The quadramodal lightcurves reveal a rotation period of 6.172+/-0.003 h and an amplitude of about 0.44 mag. Using the lightcurve inversion method, with the combination of our dense lightcurves and the sparse data from Gaia DR3, we found the sidereal period, an indication of a retrograde rotation of (12499) and its low-convex resolution shape. Nonetheless, the unusual shape of the quadramodal lightcurve and its additional analysis reveals two possible periods, 3.0834+/-0.0085 h and 4.1245+/-0.0151 h, making the suspect that the asteroid might be a non-synchronised wide binary system. Spectral analysis of the asteroid using data from the GAIA DR3 shows that it is either an M- or an L-type object and maybe a piece of the first planetesimals to form in the Solar System protoplanetary disk. On the other hand, (12499)'s dynamical properties indicate a significantly shorter lifetime. The asteroid lies exactly on the chaotic border of the 7:4 mean motion resonance with Mars (7M:4), alternating between being in and out of it for almost 190 Myrs. During 200 Myrs of integration, (12499) visited other resonances in the Flora family, but it never became a Near Earth Object (NEO). Additional integration of fictive objects from the 7M:4 resonance showed a possibility of transportation to the NEO region already at about 20 Myrs.

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arXiv2023

Transient and asymmetric dust structures in the TeV-bright nova RS Oph revealed by spectropolarimetry

A long-standing question related to nova eruptions is how these eruptions can lead to the formation of dust despite the ostensibly inhospitable environment for dust within the hot, irradiated ejecta. Novae in systems such as the symbiotic binary RS Oph offers a articularly clear view of some nova shocks and any associated dust production. Here we use spectropolarimetric monitoring of the RS Oph starting two days after its eruption in 2021 Aug. to show that: dust was present in the RS Oph system as early as two days into the 2021 eruption; the spatial distribution of this early dust was asymmetric, with components both aligned with and perpendicular to the orbital plane of the binary; between two and nine days after the start of the eruption, this early dust was gradually destroyed; and dust was again created, aligned roughly with the orbital plane of the binary, more than 80 days after the start of the outburst, most likely as a result of shocks that arose as the ejecta interacted with circumbinary material concentrated in the orbital plane. Modelling of X-rays and very-high energy GeV and TeV emission from RS Oph days to months into the 2021 eruption suggests that collisions between the ejecta and the circumbinary material may have led to shock formation in two regions: the polar - perpendicular to the orbital plane where collimated outflows have been observed after prior eruptions, and a circumbinary torus in the orbital plane. The observations described here indicate that dust formed in approximately the same two regions, supporting the connection between shocks and dust in novae and revealing a very early onset of asymmetry. The spectropolarimetric signatures of RS Oph in the first week into the 2021 outburst indicate: polarized flux across the H{\alpha} emission line and position angle orientation relative to the radio axis are similar to the spectropolarimetric signatures of AGNs.

astro-ph.SR↗

arXiv2018

(121514) 1999 UJ7: A primitive, slow-rotating Martian Trojan

Aims. The goal of this investigation is to determine the origin and surface composition of the asteroid (121514) 1999 UJ7, the only currently known L4 Martian Trojan asteroid. Methods. We have obtained visible reflectance spectra and photometry of 1999 UJ7 and compared the spectroscopic results with the spectra of a number of taxonomic classes and subclasses. A light curve was obtained and analysed to determine the asteroid spin state. Results. The visible spectrum of 1999 UJ7 exhibits a negative slope in the blue region and the presence of a wide and deep absorption feature centred around ~0.65 microns. The overall morphology of the spectrum seems to suggest a C-complex taxonomy. The photometric behaviour is fairly complex. The light curve shows a primary period of 1.936 d, but this is derived using only a subset of the photometric data. The asteroid may be in a non-principal axis rotational state, but our observational coverage is insufficient to draw definitive conclusions. Conclusions. Although the observed spectral absorption is wider and deeper, this finding may be compatible with the 0.7 microns spectral feature exhibited by some Ch-type asteroids and could possibly be interpreted as diagnostic of the presence of hydrated minerals. The inferred composition of 1999 UJ7 as a primitive object can be consistent with a volatile-rich object originally accreted beyond the snow line of the solar system and subsequently evolved to reach the inner regions of the solar system.

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arXiv2018

Rotational variation of the linear polarisation of the asteroid (3200) Phaethon as evidence for inhomogeneity in its surface properties

Asteroid (3200) Phaethon is a Near-Earth Apollo asteroid with an unusual orbit that brings it closer to the Sun than any other known asteroid. Its last close approach to the Earth was in mid-December 2017 and the next one will be on October 2026. Previous rotationally time-resolved spectroscopy of Phaethon showed that its spectral slope is slightly bluish, in agreement with its B/F taxonomic classification, but at some rotational phases, it changes to slightly reddish. Motivated by this result we performed time-resolved imaging polarimetry of Phaethon during its recent close approach to the Earth. Phaethon has a spin period of 3.604 hours and we found a variation of the linear polarisation with rotation. This seems to be a rare case in which such variation is unambiguously found, also a consequence of its fairly large amplitude. Combining this new information with the brightness and colour variation, as well as previously reported results from Arecibo radar observations, we conclude that there is no variation of the mineralogy across the surface of Phaeton. However, the observed change in the linear polarisation may be related to differences in the thickness of the surface regolith in different areas or local topographic features.

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arXiv2018

The phase-polarization curve of asteroid (3200) Phaethon

A multi-colour phase-polarization curve of asteroid (3200)~Phaethon has been obtained during the December 2017 apparition by merging measurements taken at the observing station of Calern (France) and at the Rhozen observatory (Bulgaria). All the observations were obtained in the positive polarization branch, the phase angle ranging from 36$^\circ$ to 116$^\circ$. The measured values of linear polarization are among the highest ever observed for a Solar system body. The covered interval of phase angle was not sufficiently extended to derive a firm determination of the $P_{\rm max}$ parameter, but this appears to occur at a phase angle around 130$^\circ$ and reaches more than 45\% of linear polarization. Phaethon is the parent body of the Geminid meteor shower, and the real physical nature of this object (asteroid or comet) has been a long-debated subject. Our polarimetric measurements seem to support the asteroid hypothesis with a phase-polarization curve similar to the asteroid (2)~Pallas, but further observations at smaller phase angles are needed to draw definitive conclusions.

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arXiv2018

New polarimetric and spectroscopic evidence of anomalous enrichment in spinel-bearing Calcium-Aluminium-rich Inclusions among L-type asteroids

Asteroids can be classified into several groups based on their spectral reflectance. Among these groups, the one belonging to the L-class in the taxonomic classification based on visible and near-infrared spectra exhibit several peculiar properties. First, their near-infrared spectrum is characterized by a strong absorption band interpreted as the diagnostic of a high content of the FeO bearing spinel mineral. This mineral is one of the main constituents of Calcium-Aluminum-rich Inclusions (CAI) the oldest mineral compounds found in the solar system. In polarimetry, they possess an uncommonly large value of the inversion angle incompatible with all known asteroid belonging to other taxonomical classes. Asteroids found to possess such a high inversion angle are commonly called Barbarians based on the first asteroid on which this property was first identified, (234)~Barbara. In this paper we present the results of an extensive campaign of polarimetric and spectroscopic observations of L-class objects. We have derived phase-polarization curves for a sample of 7 Barbarians, finding a variety of inversion angles ranging between 25 and 30$^{\circ}$. Spectral reflectance data exhibit variations in terms of spectral slope and absorption features in the near-infrared. We analyzed these data using a Hapke model to obtain some inferences about the relative abundance of CAI and other mineral compounds. By combining spectroscopic and polarimetric results, we find evidence that the polarimetric inversion angle is directly correlated with the presence of CAI, and the peculiar polarimetric properties of Barbarians are primarily a consequence of their anomalous composition.

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arXiv2017

The 67P/Churyumov-Gerasimenko observation campaign in support of the Rosetta mission

We present a summary of the campaign of remote observations that supported the European Space Agency's Rosetta mission. Telescopes across the globe (and in space) followed comet 67P/Churyumov-Gerasimenko from before Rosetta's arrival until nearly the end of mission in September 2016. These provided essential data for mission planning, large-scale context information for the coma and tails beyond the spacecraft, and a way to directly compare 67P with other comets. The observations revealed 67P to be a relatively `well behaved' comet, typical of Jupiter family comets and with activity patterns that repeat from orbit-to-orbit. Comparison between this large collection of telescopic observations and the in situ results from Rosetta will allow us to better understand comet coma chemistry and structure. This work is just beginning as the mission ends -- in this paper we present a summary of the ground-based observations and early results, and point to many questions that will be addressed in future studies.

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arXiv2017

The olivine-dominated composition of the Eureka family of Mars Trojan asteroids

We have used the XSHOOTER echelle spectrograph on the European Southern Obseratory (ESO) Very Large Telescope (VLT) to obtain UVB-VIS-NIR (ultraviolet-blue (UVB), visible (VIS) and near-infrared (NIR)) reflectance spectra of two members of the Eureka family of L5 Mars Trojans, in order to test a genetic relationship to Eureka. In addition to obtaining spectra, we also carried out VRI photometry of one of the VLT targets using the 2-m telescope at the Bulgarian National Astronomical Observatory - Rozhen and the two-channel focal reducer. We found that these asteroids belong to the olivine-dominated A, or Sa, taxonomic class. As Eureka itself is also an olivine-dominated asteroid, it is likely that all family asteroids share a common origin and composition. We discuss the significance of these results in terms of the origin of the martian Trojan population.

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arXiv2016

Exceptional outburst of the blazar CTA 102 in 2012: The GASP-WEBT campaign and its extension

After several years of quiescence, the blazar CTA 102 underwent an exceptional outburst in 2012 September-October. The flare was tracked from gamma-ray to near-infrared frequencies, including Fermi and Swift data as well as photometric and polarimetric data from several observatories. An intensive GASP-WEBT collaboration campaign in optical and NIR bands, with an addition of previously unpublished archival data and extension through fall 2015, allows comparison of this outburst with the previous activity period of this blazar in 2004-2005. We find remarkable similarity between the optical and gamma-ray behaviour of CTA 102 during the outburst, with a time lag between the two light curves of ~1 hour, indicative of co-spatiality of the optical and gamma-ray emission regions. The relation between the gamma-ray and optical fluxes is consistent with the SSC mechanism, with a quadratic dependence of the SSC gamma-ray flux on the synchrotron optical flux evident in the post-outburst stage. However, the gamma-ray/optical relationship is linear during the outburst; we attribute this to changes in the Doppler factor. A strong harder-when-brighter spectral dependence is seen both the in gamma-ray and optical non-thermal emission. This hardening can be explained by convexity of the UV-NIR spectrum that moves to higher frequencies owing to an increased Doppler shift as the viewing angle decreases during the outburst stage. The overall pattern of Stokes parameter variations agrees with a model of a radiating blob or shock wave that moves along a helical path down the jet.

astro-ph.HE↗

arXiv2012

Rotation-stimulated structures in the CN and C3 comae of comet 103P/Hartley 2 around the EPOXI encounter

In late 2010 a Jupiter Family comet 103P/Hartley 2 was a subject of an intensive world-wide investigation. On UT October 20.7 the comet approached the Earth within only 0.12 AU, and on UT November 4.6 it was visited by NASA's EPOXI spacecraft. We joined this international effort and organized an observing campaign. The images of the comet were obtained through narrowband filters using the 2-m telescope of the Rozhen National Astronomical Observatory. They were taken during 4 nights around the moment of the EPOXI encounter. Image processing methods and periodicity analysis techniques were used to reveal transient coma structures and investigate their repeatability and kinematics. We observe shells, arc-, jet- and spiral-like patterns, very similar for the CN and C3 comae. The CN features expanded outwards with the sky-plane projected velocities between 0.1 to 0.3 km/s. A corkscrew structure, observed on November 6, evolved with a much higher velocity of 0.66 km/s. Photometry of the inner coma of CN shows variability with a period of 18.32+/-0.30 h (valid for the middle moment of our run, UT 2010 Nov. 5.0835), which we attribute to the nucleus rotation. This result is fully consistent with independent determinations around the same time by other teams. The pattern of repeatability is, however, not perfect, which is understendable given the suggested excitation of the rotation state, and the variability detected in CN correlates well with the cyclic changes in HCN, but only in the active phases. The revealed coma structures, along with the snapshot of the nucleus orientation obtained by EPOXI, let us estimate the spin axis orientation. We obtained RA=122 deg, Dec=+16 deg (epoch J2000.0), neglecting at this point the rotational excitation.

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arXiv1999

Precise measurement of Higgs decay rate into WW* at future e^+e^- Linear Colliders

Assuming a SM or MSSM scenario, one expects a light Higgs boson which could be studied in great detail with a LC operating at \sqrt s>m_h+m_Z. In the TESLA scenario, with 500 fb^-1 accumulated at \sqrt s = 350 GeV, about 10^5 hZ events could be produced through the Higgstrahlung process. At a future LC with a \sim 1 cm beam-pipe radius and a thin Si detector there will be excellent separation between the various flavours. With the high statistics available it will thus become possible to measure the various branching ratios with a few % error. Typically one expects 8 % precision on BR(h\to\bar cc), 6 % on BR(h\to gg) and \sim 1 % on σ(hZ) x BR(h\to\bar bb) access to BR(h\to WW^*) and, as explained in section 3, this measurement can give access to the Higgs total decay width and therefore to all partial widths. In particular one can precisely measure Γ(h\to\bar bb) and Γ(h\toτ^+τ^-) which have a high sensitivity to MSSM effects\cite{gunion} and therefore allow an essential test of the Higgs sector. If m_A < 1 TeV, it becomes possible to measure a significant deviation and, within MSSM, to give an indirect estimate of m_A thus extending the effective sensitivity above the discovery domain of LHC. In the following section we describe a detailed analysis on the measurement of BR(h\to WW^*).

hep-ph↗