Uncovering the invisible: A study of Gaia18ajz, a candidate black hole revealed by microlensing

¹ Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
² Faculty of Mathematics and Computer Science, Jagiellonian University, Łojasiewicza 6, 30-348 Kraków, Poland
³ Las Cumbres Observatory, 6740 Cortona Drive, Suite 102, Goleta, CA 93117, USA
⁴ Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University in Toruń, Grudziądzka 5, 87-100 Toruń, Poland
⁵ IPAC, Mail Code 100-22, Caltech, 1200 E. California Blvd., Pasadena, CA 91125, USA
⁶ Research School of Astronomy and Astrophysics, Australian National University, Mount Stromlo Observatory, Cotter Road Weston Creek, ACT 2611, Australia
⁷ Zentrum für Astronomie der Universität Heidelberg, Astronomisches Rechen-Institut, Mönchhofstr. 12-14, 69120 Heidelberg, Germany
⁸ Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot 76100, Israel
⁹ Astronomical Institute, University of Wrocław, ul. Mikołaja Kopernika 11, 51-622 Wrocław, Poland
¹⁰ Sydney Institute for Astronomy (SIfA), The University of Sydney, Physics Road, Sydney 2050, Australia
¹¹ Donald Bren School of Information and Computer Sciences, University of California, Irvine, CA 92697, USA
¹² Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, United Kingdom
¹³ University of Bonn, Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
¹⁴ School of Physics, Trinity College Dublin, College Green, Dublin 2, Ireland
¹⁵ Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (UB), Martí i Franquès 1, E-08028 Barcelona, Spain
¹⁶ Departament de Física Quàntica i Astrofísica (FQA), Universitat de Barcelona (UB), Martí i Franquès 1, E-08028 Barcelona, Spain
¹⁷ Institut d’Estudis Espacials de Catalunya (IEEC), Esteve Terradas, 1, Edifici RDIT, Campus PMT-UPC, 08860 Castelldefels, Barcelona, Spain
¹⁸ ICAMER Observatory of National Academy of Sciences of Ukraine, 27 Acad. Zabolotnoho str., Kyiv 03143, Ukraine
¹⁹ Astronomy and Space Physics Department, Taras Shevchenko National University of Kyiv, 4 Glushkova ave., Kyiv 03022, Ukraine
²⁰ National Center “Junior Academy of Sciences of Ukraine”, 38-44 Dehtiarivska St., Kyiv 04119, Ukraine
²¹ INAF-Osservatorio di Astrofisica e Scienza dello Spazio, Via Gobetti 93/3, I-40129 Bologna, Italy
²² Astronomical Institute of the Academy of Sciences of the Czech Republic (ASU CAS), 25165 Ondrejov, Czech Republic
²³ Czech Technical University in Prague, Faculty of Electrical Engineering, Prague, Czech Republic
²⁴ Astronomical Institute, Faculty of Mathematics and Physics, Charles University, V Holešovičkách 2, 180 00 Prague, Czech Republic
²⁵ Research Centre for Theoretical Physics and Astrophysics, Institute of Physics, Silesian University, Bezručovo nám. 13, 746 01 Opava, Czech Republic
²⁶ Department of Space Sciences and Technologies, Akdeniz University, Campus, Antalya 07058, Türkiye
²⁷ TÜBİTAK National Observatory, Akdeniz University Campus, Antalya 07058, Türkiye
²⁸ UKIRT Observatory, Institute for Astronomy, 640 N. A’ohoku Place, University Park, Hilo, Hawai’i 96720, USA
²⁹ School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK

^⋆ Corresponding author; kornel.howil@student.uj.edu.pl

Received: 9 June 2024
Accepted: 15 November 2024

Abstract

Context. Identifying black holes is essential for our understanding of the development of stars and can reveal novel principles of physics. Gravitational microlensing provides an exceptional opportunity to examine an undetectable population of black holes in the Milky Way. In particular, long-lasting events are likely to be associated with massive lenses, including black holes.

Aims. We present an analysis of the Gaia18ajz microlensing event reported by the Gaia Science Alerts system. Gaia18ajz is a long-timescale event exhibiting features indicative of the annual microlensing parallax effect. Our objective is to estimate its lens parameters based on the best-fitting model.

Methods. We used photometric data obtained from the Gaia satellite and terrestrial observatories to investigate a variety of microlensing models and calculate the most probable mass and distance to the lens, taking into consideration a Galactic model as a prior. Subsequently, we applied a mass–brightness relation to evaluate the likelihood that the lens is a main sequence star. We also describe the DarkLensCode (DLC), an open-source routine that computes the distribution of probable lens mass, distance, and luminosity employing the Galaxy priors on stellar density and velocity for microlensing events with detected microlensing parallax.

Results. We modelled the Gaia18ajz event and found its two possible models, the most probable Einstein timescales for which are 316₋₃₀⁺³⁶ days and 299₋₂₂⁺²⁵ days. Applying Galaxy priors for stellar density and motion, we calculated a most probable lens mass of 4.9_−2.3^+5.4 M_⊙ located at 1.14_−0.57^+0.75 kpc, and a less probably mass of 11.1_−4.7^+10.3 M_⊙ located at 1.31_−0.60^+0.80 kpc. Our analysis of the blended light suggests that the lens is likely a dark remnant of stellar evolution rather than a main sequence star.