IGR J19552+0044: A new asynchronous short period polar

Filling the gap between intermediate and ordinary polars^⋆

¹ Universidad Nacional Autónoma de México, Instituto de Astronomia, Unidad Academica en Ensenada, 22860 Baja California, Mexico
e-mail: gag@astro.unam.mx
² Department of Physics and Astronomy, University of California, Irvine, California 92697, USA
³ Department of Physics and Astronomy, Dartmouth College, Hanover, NH 03755, USA
⁴ South African Astronomical Observatory, PO Box 9, Observatory 7935, Cape Town, South Africa
⁵ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁶ New York University Abu Dhabi, PO Box 129188, Abu Dhabi, United Arab Emirates
⁷ Instituto de Física y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Av. Gran Bretaña 1111 Valparaíso, Chile
⁸ Center for Backyard Astrophysics (CBA), Backyard, UK
⁹ Departamento de Ciencias Integradas, Facultad de Ciencias Experimentales, Universidad de Huelva, 21071 Huelva, Spain
¹⁰ Department of Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA
¹¹ Department of Physics and Astronomy, University of North Carolina at Greensboro, Greensboro, NC 27402-6170, USA

Received: 7 June 2017
Accepted: 14 August 2017

Abstract

Context. Based on XMM-Newton X-ray observations IGR J19552+0044 appears to be either a pre-polar or an asynchronous polar.

Aims. We conducted follow-up optical observations to identify the sources and periods of variability precisely and to classify this X-ray source correctly.

Methods. Extensive multicolor photometric and medium- to high-resolution spectroscopy observations were performed and period search codes were applied to sort out the complex variability of the object.

Results. We found firm evidence of discording spectroscopic (81.29 ± 0.01 m) and photometric (83.599 ± 0.002 m) periods that we ascribe to the white dwarf (WD) spin period and binary orbital period, respectively. This confirms that IGR J19552+0044 is an asynchronous polar. Wavelength dependent variability and its continuously changing shape point at a cyclotron emission from a magnetic WD with a relatively low magnetic field below 20 MG.

Conclusions. The difference between the WD spin period and the binary orbital period proves that IGR J19552+0044 is a polar with the largest known degree of asynchronism (0.97 or 3%).