Related papers: Extreme variability in an active galactic nucleus:…

We report on the discovery and observations of the extremely luminous optical transient CSS100217:102913+404220 (CSS100217 hereafter). Spectroscopic observations show this transient was coincident with a galaxy at redshift z=0.147, and…

Recently, Chornock and co-workers announced the Pan-STARRS discovery of a transient source reaching an apparent peak luminosity of ~4x10^44 erg s^-1. We show that the spectra of this transient source are well fit by normal Type Ia supernova…

In 2015, a radio transient named Cygnus A-2 was discovered in Cygnus A with the Very Large Array. Because of its radio brightness ($\nu F_{\nu} \approx 6 \times 10^{39}$ erg s$^{-1}$), this transient likely represents a secondary black hole…

The nearby active galaxy IC 310, located in the outskirts of the Perseus cluster of galaxies is a bright and variable multi-wavelength emitter from the radio regime up to very high gamma-ray energies above 100 GeV. Originally, the nucleus…

Gaia16aye was a binary microlensing event discovered in the direction towards the northern Galactic disc and was one of the first microlensing events detected and alerted to by the Gaia space mission. Its light curve exhibited five distinct…

Multiwavelength flares from tidal disruption and accretion of stars can be used to find and study otherwise dormant massive black holes in galactic nuclei. Previous well-monitored candidate flares are short-lived, with most emission…

The supersoft X-ray and optical transient ASASSN-16oh has been interpreted by Maccarone et al. (2019) as having being induced by an accretion event on a massive white dwarf, resembling a dwarf nova super-outburst. These authors argued that…

The black hole candidate MAXI J1535-571 is a recently discovered X-ray transient. We report on the monitoring observations of Swift Gamma-Ray Burst Mission during outburst. The source transits from the hard state to the intermediate state,…

The progenitors of astronomical transients are linked to a specific stellar population and galactic environment, and observing their host galaxies hence constrains the physical nature of the transient itself. Here, we use imaging from the…

The ESA Gaia satellite scans the whole sky with a temporal sampling ranging from seconds and hours to months. Each time a source passes within the Gaia field of view, it moves over 10 CCDs in 45 s and a lightcurve with 4.5 s sampling (the…

We have observed four low-luminosity active galactic nuclei classified as Type 1 LINERs with the X-ray Telescope (XRT) and the UltraViolet-Optical Telescope (UVOT) onboard Swift, in an attempt to clarify the main powering mechanism of this…

We report on the results of X-ray observations of 4XMM J111816.0-324910, a transient ultra-luminous X-ray source located in the galaxy NGC 3621. This system is characterised by a transient nature and marked variability with characteristic…

We have serendipitously discovered that Leoncino Dwarf, an ultra-faint, low-metallicity record-holder dwarf galaxy, may have hosted a transient source, and possibly exhibited a change in morphology, a shift in the center of brightness, and…

X-ray transience is the most extreme form of variability observed in AGN or normal non-active galaxies. While factors of 2-3 on timescales of days to years are quite common among AGN, X-ray transients appear only once and vanish from the…

We present the discovery and characterisation of PTF10iya, a short-lived (dt ~ 10 d, with an optical decay rate of ~ 0.3 mag per d), luminous (M_g ~ -21 mag) transient source found by the Palomar Transient Factory. The ultraviolet/optical…

The Lobster Eye Imager for Astronomy (LEIA) detected a new X-ray transient on 2022 November 7, identified as a superflare event occurring on a nearby K-type giant star HD 251108. The flux increase was also detected in follow-up observations…

We present the detection of persistent soft X-ray radiation with L_x ~ 10^41-10^42 erg/s at the location of the extremely luminous, double-humped transient ASASSN-15lh as revealed by Chandra and Swift. We interpret this finding in the…

The black hole X-ray binary V4641 Sgr experienced an outburst in 2002 May which was detected at X-ray, optical, and radio wavelengths. The outburst lasted for only 6 days, but the object remained active for the next several months. Here we…

The galaxy SDSS1335+0728, which had exhibited no prior optical variations during the preceding two decades, began showing significant nuclear variability in the Zwicky Transient Facility (ZTF) alert stream from December 2019 (as…

We report on the source Gaia 17bpi and identify it as a new, ongoing FU Ori type outburst, associated with a young stellar object. The optical lightcurve from Gaia exhibited a 3.5 mag rise with the source appearing to plateau in mid/late…