arXiv subjects

Giacomo Cannizzaro

Publications and source records attributed to Giacomo Cannizzaro.

4 recordsLinked to original sources

arXiv2021

Host Galaxy Line Diagnostics for the Candidate Tidal Disruption Events XMMSL1 J111527.3+180638 and PTF09axc

We present results of our analysis of spectra of the host galaxies of the candidate Tidal Disruption Events (TDEs) XMMSL1 J111527.3+180638 and PTF09axc to determine the nature of these transients. We subtract the starlight component from the host galaxy spectra to determine the origin of the nuclear emission lines. Using a Baldwin-Phillips-Terlevich (BPT) diagram we conclude that the host galaxy of XMMSL1 J111527.3+180638 is classified as a Seyfert galaxy, suggesting this transient is likely to be caused by (extreme) variability in the active galactic nucleus. We find that the host of PTF09axc falls in the 'star-forming' region of the BPT-diagram, implying that the transient is a strong TDE candidate. For both galaxies we find a WISE-colour difference of $W1-W2<0.8$, which means there is no indication of a dusty torus and therefore an active galactic nucleus, seemingly contradicting our BPT finding for the host of XMMSL1 J111527.3+180638. We discuss possible reasons for the discrepant results obtained through the two methods.

astro-ph.HE↗

arXiv2021

Spectroscopic monitoring of the candidate tidal disruption event in F01004-2237

We present results of spectroscopic monitoring observations of the Ultra-Luminous Infra Red Galaxy F01004-2237. This galaxy was observed to undergo changes in its optical spectrum, detected by comparing a spectrum from 2015 with one from 2000. These changes were coincident with photometric brightening. The main changes detected in the optical spectrum are enhanced He II $λ$4686 emission and the appearance of He I $λ$3898,$λ$5876 emission lines. The favoured interpretation of these changes was that of a tidal disruption event (TDE) happening in 2010. However, subsequent work suggested that these changes are caused by another hitherto unknown reason related to variations in the accretion rate in the active galactic nucleus (AGN). Our optical spectroscopic monitoring observations show that the evolution of the He lines is in line with the evolution seen in TDEs and opposite of what observed from reverberation mapping studies of AGNs, renewing the discussion on the interpretation of the flare as a TDE.

astro-ph.HE↗

arXiv2020

The spectral evolution of AT 2018dyb and the presence of metal lines in tidal disruption events

We present light curves and spectra of the tidal disruption event (TDE) ASASSN-18pg / AT 2018dyb spanning a period of one year. The event shows a plethora of strong emission lines, including the Balmer series, He II, He I and metal lines of O III $λ$3760 and N III $λλ$ 4100, 4640 (blended with He II). The latter lines are consistent with originating from the Bowen fluorescence mechanism. By analyzing literature spectra of past events, we conclude that these lines are common in TDEs. The spectral diversity of optical TDEs is thus larger than previously thought and includes N-rich events besides H- and He-rich events. We study how the spectral lines evolve with time, by means of their width, relative strength, and velocity offsets. The velocity width of the lines starts at $\sim$ 13000 km s$^{-1}$ and decreases with time. The ratio of He II to N III increases with time. The same is true for ASASSN-14li, which has a very similar spectrum to AT 2018dyb but its lines are narrower by a factor of $>$2. We estimate a black hole mass of $M_{\rm BH}$ = $3.3^{+5.0}_{-2.0}\times 10^6$ $M_{\odot}$ by using the $M$-$σ$ relation. This is consistent with the black hole mass derived using the MOSFiT transient fitting code. The detection of strong Bowen lines in the optical spectrum is an indirect proof for extreme ultraviolet and (reprocessed) X-ray radiation and favors an accretion origin for the TDE optical luminosity. A model where photons escape after multiple scatterings through a super-Eddington thick disk and its optically thick wind, viewed at an angle close to the disk plane, is consistent with the observations.

astro-ph.HE↗

arXiv2019

Initial characterization of interstellar comet 2I/Borisov

Interstellar comets penetrating through the Solar System had been anticipated for decades. The discovery of asteroidal-looking 'Oumuamua was thus a huge surprise and a puzzle. Furthermore, the physical properties of the 'first scout' turned out to be impossible to reconcile with Solar System objects, challenging our view of interstellar minor bodies. Here, we report the identification and early characterization of a new interstellar object, which has an evidently cometary appearance. The body was discovered by Gennady Borisov on 30 August 2019 UT and subsequently identified as hyperbolic by our data mining code in publicly available astrometric data. The initial orbital solution implies a very high hyperbolic excess speed of ~32 km/s, consistent with 'Oumuamua and theoretical predictions. Images taken on 10 and 13 September 2019 UT with the William Herschel Telescope and Gemini North Telescope show an extended coma and a faint, broad tail. We measure a slightly reddish colour with a g'-r' colour index of 0.66 +/- 0.01 mag, compatible with Solar System comets. The observed morphology is also unremarkable and best explained by dust with a power-law size-distribution index of -3.7 +/- 1.8 and a low ejection speed (44 +/- 14 m/s for $β$ = 1 particles, where $β$ is the ratio of the solar gravitational attraction to the solar radiation pressure). The nucleus is probably ~1 km in radius, again a common value among Solar System comets, and has a negligible chance of experiencing rotational disruption. Based on these early characteristics, and putting its hyperbolic orbit aside, 2I/Borisov appears indistinguishable from the native Solar System comets.

astro-ph.EP↗