A Possible 250 s X-Ray Quasi-periodicity in the Fast Blue Optical Transient AT2018cow

Abstract

The fast blue optical transients (FBOTs) are a new population of extragalactic transients of unclear physical origin. A variety of mechanisms have been proposed including failed supernova explosion, shock interaction with a dense medium, young magnetar, accretion onto a compact object, and stellar tidal disruption event, but none is conclusive. Here we report the discovery of a possible X-ray quasi-periodicity signal with a period of ~250 second (at a significance level of 99.76%) in the brightest FBOT AT2018cow through the analysis of XMM data. The signal is independently detected at the same frequency in the average power density spectrum from data taken from the Swift telescope, with observations covering from 6 to 37 days after the optical discovery, though the significance level is lower (94.26%). This suggests that the QPO frequency may be stable over at least 1.1×10^4 cycles. Assuming the ~250 second QPO to be a scaled-down analogue of that typically seen in stellar mass black holes, a black hole mass of ~10^3-10^5 solar masses could be inferred. The overall X-ray luminosity evolution could be modeled with the stellar tidal disruption by a black hole of ~10^4 solar masses, providing a viable mechanism to produce AT2018cow. Our findings suggest that other bright FBOTs may also harbor intermediate-mass black holes.

Late-time Hubble Space Telescope Observations of AT 2018cow. II. Evolution of a UV-bright Underlying Source 2–4 Yr Post-discovery

“…In 2018, a particularly intriguing FBOT-AT 2018cowwas discovered by the Asteroid Terrestrial-impact Last Alert System (Prentice et al 2018;Smartt et al 2018) in the starforming dwarf galaxy CGCG 137-068 (z = 0.0141) that rose to a peak brightness M ∼ −22, similar to superluminous SNe, in 2 days. Multiwavelength follow-up observations (e.g., Rivera Sandoval et al 2018;Ho et al 2019;Huang et al 2019;Kuin et al 2019;Margutti et al 2019;Perley et al 2019;Bietenholz et al 2020;Pasham et al 2021;Xiang et al 2021) revealed an array of peculiar properties, including (i) bright millimeter radio emission associated with mildly relativistic outflow (v ∼ 0.1c) in CSM (Ho et al 2019); (ii) bright X-ray emission with signs of quasiperiodic oscillation (QPO) indicative of a central engine (Rivera Sandoval et al 2018;Kuin et al 2019;Margutti et al 2019;Pasham et al 2021;Zhang et al 2022); (iii) rapidly fading optical emission with a persistent blue color and a receding photosphere (Margutti et al 2019;Perley et al 2019;Xiang et al 2021); (iv) a separate IR component likely associated with dust (Perley et al 2019;Metzger & Perley 2023); (v) a sudden transition from broad to intermediate-width spectral features ∼15-20 days after peak, suggesting multiple (possibly asymmetric) thermal emission components (Margutti et al 2019;Perley et al 2019;Xiang et al 2021); and (vi) flashes of high optical polarization that further support an asymmetric configuration (Maund et al 2023).…”

“…One may speculate that there could be a chance that a straggling IMBH or SMBH was left behind from this dynamical interaction. Another example is the low-frequency X-ray QPO discovered by Zhang et al (2022) from XMM-Newton and Swift observations. Zhang et al (2022) suggested that the low-frequency QPO is more consistent with IMBHs and SMBHs, and the NICER QPO limit could be relaxed by introducing the IMBH or SMBH in a compact binary.…”

“…Another example is the low-frequency X-ray QPO discovered by Zhang et al (2022) from XMM-Newton and Swift observations. Zhang et al (2022) suggested that the low-frequency QPO is more consistent with IMBHs and SMBHs, and the NICER QPO limit could be relaxed by introducing the IMBH or SMBH in a compact binary. Therefore, the TDE scenario for AT 2018cow may be worth revisiting, along with the hypothesis that the underlying source was an accreting IMBH or SMBH.…”

Magnetar as the Central Engine of AT2018cow: Optical, Soft X-Ray, and Hard X-Ray Emission

“…The result is that the magnetization in the nebula may be very low, which is necessary to explain the the observed late-time steepening of superluminous SN optical light curves (Vurm & Metzger 2021). It should also be emphasized that AT2018cow has many other peculiar observational properties that are not discussed in the present work, such as high-amplitude quasiperiodic oscillation of AT2018cowʼs soft X-rays with a frequency of 224 Hz (Pasham et al 2021); possible 250 s X-ray quasiperiodicity (Zhang et al 2022); luminous UV emission detected 2-4 yr after the explosion (Sun et al 2022;Chen et al 2023); and persistent X-ray emission at the location of AT2018cow ∼3.7 yr after discovery (Migliori et al 2023). It is a great challenge to explain all these peculiar observational properties of AT2018cow based on the millisecond magnetar as the central engine and there is still a lot of work to be done.…”

Roaring to Softly Whispering: X-Ray Emission after ∼3.7 yr at the Location of the Transient AT2018cow and Implications for Accretion-powered Scenarios*

“…Slowly decaying, luminous (L > 10 7 L ☉ ) UV emission years after discovery has been recently reported by Sun et al (2022Sun et al ( , 2023, Chen et al (2023aChen et al ( , 2023b, Inkenhaag et al (2023), andMummery et al (2024), and similarities have been noted with radiation powered by accretion processes on compact objects. Along the same lines, a high-frequency (224 Hz) quasi-periodic oscillation (QPO) feature in the X-ray timing properties of AT 2018cow supports the presence of a compact object, either an NS or a BH with mass M < 850 M e (Pasham et al 2021; but see also Zhang et al 2022). While no evidence was found for a long-lived relativistic jet such as those of GRBs (Bietenholz et al 2020;Mohan et al 2020), panchromatic observations of AT 2018cow further indicated a complex geometry that strongly departs from spherical symmetry, as was directly confirmed by the very large optical polarization (∼7%) at early times (Maund et al 2023).…”