Article

Radio observations of the Black Hole X-ray Binary EXO 1846−031 re-awakening from a 34-year slumber

D. R. WilliamsDepartment of Physics, University of Oxford , Denys Wilkinson Building, Keble Road, Oxford, OX1 3RH, UKS. MottaDepartment of Physics, University of Oxford , Denys Wilkinson Building, Keble Road, Oxford, OX1 3RH, UKR. P. FenderDepartment of Astronomy, University of Cape Town , Private Bag X3, Rondebosch 7701, South AfricaJames C. A. Miller-JonesInternational Centre for Radio Astronomy Research - Curtin University , Perth, Western Australia 6845, AustraliaJoey NeilsenDepartment of Physics, Villanova University, Mendel Science Center , Villanova PA, 19085, USAJ. R. AllisonDepartment of Physics, University of Oxford , Denys Wilkinson Building, Keble Road, Oxford, OX1 3RH, UKJoe BrightCenter for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and Department of Physics and Astronomy, Northwestern University , Evanston, IL 60208, USAIan HeywoodDepartment of Physics and Electronics, Rhodes University , PO Box 94, Grahamstown 6140, South AfricaPedro F. JacobDepartment of Physics, University of Oxford , Denys Wilkinson Building, Keble Road, Oxford, OX1 3RH, UKLauren RhodesDepartment of Physics, University of Oxford , Denys Wilkinson Building, Keble Road, Oxford, OX1 3RH, UKEvangelia TremouNational Radio Astronomy Observatory , Socorro, NM 87801, USAP. A. WoudtDepartment of Astronomy, University of Cape Town , Private Bag X3, Rondebosch 7701, South AfricaJ. van den EijndenDepartment of Physics, University of Oxford , Denys Wilkinson Building, Keble Road, Oxford, OX1 3RH, UKFrancesco CarotenutoDepartment of Physics, University of Oxford , Denys Wilkinson Building, Keble Road, Oxford, OX1 3RH, UKDr Sarah GreenAstrophysics Group, Cavendish Laboratory , 19 J. J. Thomson Avenue, Cambridge CB3 0HE, UKDavid TitteringtonAstrophysics Group, Cavendish Laboratory , 19 J. J. Thomson Avenue, Cambridge CB3 0HE, UKA. J. van der HorstAstronomy, Physics, and Statistics Institute of Sciences (APSIS) , 725 21st Street NW, Washington, DC 20052, USAPayaswini Saikia

2022en

ABI

Abstract

ABSTRACT We present radio [1.3 GHz MeerKAT, 4–8 GHz Karl G. Jansky Very Large Array (VLA), and 15.5 GHz Arcminute Microkelvin Imager Large Array (AMI-LA)] and X-ray (Swift and MAXI) data from the 2019 outburst of the candidate Black Hole X-ray Binary (BHXB) EXO 1846−031. We compute a Hardness–Intensity diagram, which shows the characteristic q-shaped hysteresis of BHXBs in outburst. EXO 1846−031 was monitored weekly with MeerKAT and approximately daily with AMI-LA. The VLA observations provide sub-arcsecond-resolution images at key points in the outburst, showing moving radio components. The radio and X-ray light curves broadly follow each other, showing a peak on ∼MJD 58702, followed by a short decline before a second peak between ∼MJD 58731–58739. We estimate the minimum energy of these radio flares from equipartition, calculating values of Emin ∼ 4 × 1041 and 5 × 1042 erg, respectively. The exact date of the return to ‘quiescence’ is missed in the X-ray and radio observations, but we suggest that it likely occurred between MJD 58887 and 58905. From the Swift X-ray flux on MJD 58905 and assuming the soft-to-hard transition happened at 0.3–3 per cent Eddington, we calculate a distance range of 2.4–7.5 kpc. We computed the radio:X-ray plane for EXO 1846−031 in the ‘hard’ state, showing that it is most likely a ‘radio-quiet’ BH, preferentially at 4.5 kpc. Using this distance and a jet inclination angle of θ = 73°, the VLA data place limits on the intrinsic jet speed of βint = 0.29c, indicating subluminal jet motion.

Identifiers

DOI: 10.1093/mnras/stac2700

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