CI Camelopardalis: The first sgB[e]-high mass X-ray binary twenty years on: A supernova imposter in our own Galaxy?

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.

A Census of B[e] Supergiants

“…These objects have been proposed to be binary systems, in which the high energy emission is caused by either accretion onto a compact object, or by shocks in a colliding wind binary with a second massive star. Members of this group are the Galactic objects Cl* Westerlund 1 W 9 (= Wd1-9), which is considered a colliding wind system ( [85]), CI Cam (= MWC 84), which might be interpreted as supernova imposter ( [187]), and the high-mass X-ray binary (HMXB) IGR J16318-4848 in which the compact object was proposed to be a neutron star ( [188]). Two ultra-luminous X-ray sources (ULXs), Holmberg II X-1 and NGC 300 ULX1, the latter being also named as supernova imposter SN2010da, have also been proposed to be HMXBs including a B[e]SG ( [189][190][191]).…”

Luminous Stars in Nearby Galaxies

“…These objects have been proposed to be binary systems, in which the high energy emission is caused by either accretion onto a compact object, or by shocks in a colliding wind binary with a second massive star. Members of this group are the Galactic objects Cl* Westerlund 1 W 9 (=Wd1-9), which is considered a colliding wind system [85]; CI Cam (=MWC 84), which might be interpreted as supernova imposter [187]; and the high-mass X-ray binary (HMXB) IGR J16318-4848, in which the compact object was proposed to be a neutron star [188]. Two ultra-luminous X-ray sources (ULXs), Holmberg II X-1 and NGC 300 ULX1, the latter being also named as supernova imposter SN2010da, have also been proposed to be HMXBs including a B[e]SG [189][190][191].…”

XMM-Newton discovery of very high obscuration in the candidate Supergiant Fast X-ray Transient AX J1714.1-3912

“…Chaty et al (2019) suggest that sgB[e] HMXBs are at the short evolutionary stage when a binary system is entering a common envelope phase of binary evolution. At present, this is a small class of rare HMXBs that, besides the Galactic sources CI Cam, IGR J16318−4848, and Wd1-9, includes a couple of candidates in the Magellanic Clouds and, remarkably, two ultra luminous X-ray sources, Holmberg II X-1 and NGC300 ULX-1/supernova imposter SN2010da (Bartlett et al 2019).…”

“…CI Cam and IGR J16318−4848 show variable absorbing column densities in the range 10 23 -10 24 cm −2 , intense FeKα line emission and X-ray flux variability (Bartlett et al 2019). But while IGR J16318-4848 is bright above 20 keV (Bird et al 2016) with some level of flaring activity (Sidoli & Paizis 2018), CI Cam has never been detected by INT EGRAL (Bird et al 2016).…”