Astronomy & Astrophysics manuscript no. cicam_accepted c ESO 2018 December 21, 2018 CI Camelopardalis: The first sgB[e]-High Mass X-ray Binary Twenty Years on A Supernova Imposter in our own Galaxy? E. S. Bartlett 1 , J. S. Clark 2 , and I. Negueruela 3 1 ESO - European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago de Chile, Chile e-mail: ebartlet@eso.org 2 Department of Physics and Astronomy, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK 3 Departamento de Física Aplicada, Facultad de Ciencias, Universidad de Alicante, Carretera San Vicente del Raspeig s/n, E03690, San Vicente del Raspeig, Spain Received September 25, 2018; accepted December 14, 2018 ABSTRACT Context. The Galactic supergiant B[e] star CI Camelopardalis (CI Cam) was the first sgB[e] star detected during an X-ray outburst. The star brightened to ∼2 Crab in the X-ray regime (∼ 5 × 10 −8 ergs cm −2 s −1 in the 2-25 keV range) within hours before decaying to a quiescent level in less than 2 weeks, clearly indicative of binarity. Since the outburst of CI Cam, a number of sgB[e] stars have been identified as X-ray overluminous for a single star (i.e., L X > 10 −7 L bol ). This small population has recently expanded to include two Ultra Luminous X-ray sources (ULX), Holmberg II X-1 and NGC300 ULX-1/supernova imposter SN2010da. Aims. Since the discovery of X-ray emission from CI Cam, there have been many developments in the field of massive binary evolution. In light of the recent inclusion of two ULXs in the population of X-ray bright sgB[e] stars, we revisit CI Cam to investigate its behaviour over several timescales and shed further light on the nature of the compact object in the system, its X-ray outburst in 1998 and the binary system parameters. Methods. We analyse archival XMM-Newton EPIC-pn spectra and light curves along with new data from Swift and NuSTAR. We also present high-resolution (R∼85,000) MERCATOR/HERMES optical spectra, including a spectrum taken 1.02 days after our NuSTAR observation. Results. Despite being in quiescence, CI Cam is highly X-ray variable on timescales of days, both in terms of total integrated flux and spectral shape. We interpret these variations by invoking the presence of an accreting compact companion immersed in a dense, highly structured, aspherical circumstellar envelope. The differences in the accretion flux and circumstellar extinction represent either changes in this environment, triggered by variable mass loss from the star, or the local conditions to the accretor due to its orbital motion. We find no evidence for pulsations in the X-ray light curve. Conclusions. CI Cam has many similarities with SN2010da across mid-IR, optical and X-ray wavelengths suggesting that, subject to distance determination for CI Cam, if CI Cam was located in an external galaxy its 1998 outburst would have led to a classification as a supernova imposter. Key words. Stars: emission-line, Stars: massive, Stars: individual: CI Camelopardalis, Stars: binaries, X-rays: binaries 1. Introduction The optical spectra of the evolved B[e] supergiants (sgB[e]s) are characterised by broad, high excitation emission lines; narrow, low excitation emission lines; and a strong infrared excess due to hot dust (Lamers et al. 1998). The standard explanation for this hybrid spectrum is a low density, fast polar wind and a much denser, slowly expanding equatorial outflow or torus in which dust forms (Zickgraf et al. 1985, 1986). Recent observations suggest that the equatorial material of many sgB[e]s is in Ke- plerian rotation rather than radially expanding (e.g. Kraus et al. 2010) with Maravelias et al. (2018) demonstrating that each of their studied sgB[e] stars showed a unique pattern of concentric rings of material, from which the forbidden emission arises. The formative agent of the equatorial material is a topic of active research with stellar rotation suggested as one potential cause. The rotational velocities of a handful of sgB[e] star’s have been determined, however, and only 1 of the 3 are rotating at an ap- preciable fraction of the critical velocity (Gummersbach et al. 1995; Zickgraf 2000, 2006a; Kraus et al. 2008, 2010). There is mounting support for the idea that sgB[e] stars represent either interacting or newly formed, post- interaction binary systems. In this scenario, the tori are ejected during non-conservative mass transfer in an evolutionary stage of close binary evolution (e.g. the colliding wind binary Wd1-9, Clark et al. 2013a), or are ma- terial from the stellar wind captured into circum-binary orbits during periastron passages of the secondary (considered the most plausible scenario for the material surrounding GG Car, Kraus et al. 2013). At least one sgB[e] star, R4 in the Small Magel- lanic Cloud, is though to be the result of a binary merger (Pod- siadlowski et al. 2006). Identifying bona fide sgB[e] binaries is a priority as the asymmetries seen in the circumstellar envi- ronment around the progenitor of SN1987A (Morris & Podsi- adlowski 2009) and SN2009ip (Mauerhan et al. 2014) may be Article number, page 1 of 13 arXiv:1812.08170v1 [astro-ph.HE] 19 Dec 2018