arXiv subjects

Irene Parenti

Publications and source records attributed to Irene Parenti.

7 recordsLinked to original sources

arXiv2026

AT 2016blu: Accretion-Powered Outbursts in a Luminous Blue Variable and Compact Object Binary

We present the first X-ray detection of the supernova (SN) impostor AT 2016blu in NGC 4559. AT 2016blu exhibited 27 detected quasiperiodic (~113 day) outbursts between 2012 and June 2026, presumably triggered by periastron passages in an eccentric binary system in which the primary is a $\gtrsim 33~M_{\odot}$ luminous blue variable (LBV). AT 2016blu was serendipitously observed by Chandra in 2001-2002, prior to its first documented outburst in 2012, but the stacked archival data show no X-ray detection. We monitored the source's visual-wavelength variability around its predicted outburst time and triggered a Chandra Target of Opportunity program in March 2026, obtaining observations with detections over five closely spaced epochs. These data indicate an X-ray luminosity of $\log L_{\rm X} \approx 38.64 \pm 0.11$~erg~s$^{-1}$ and an accretion rate of $\dot{M} \gtrsim 8\times10^{-8}~M_{\odot}~\mathrm{yr^{-1}}$, consistent with the presence of a compact companion. We therefore conclude that AT 2016blu is the first known case of an LBV SN impostor whose outbursts are driven by intermittent accretion onto a compact object. Given that the system consists of a massive star and a compact companion, AT 2016blu is a high-mass X-ray binary, similar to SN 2010da, although the donor star in SN 2010da is a red supergiant.

astro-ph.HE↗

arXiv2025

Low-Eddington ratio, changing-look active galactic nuclei: the case of NGC 4614

Active galactic nuclei (AGN) are known to be variable sources across the entire electromagnetic spectrum, in particular at optical/ultraviolet and X-ray energies. Over the past decades, a growing number of AGN have displayed type transitions: from type 1 to type 2 or viceversa within a few years or even several months. These galaxies have been commonly referred to as changing-look AGN (CLAGN). Here we report on a new CLAGN, NGC 4614, which transitioned from a type 1.9 to a type 2 state. NGC 4614 is a nearly face-on barred galaxy at redshift $z = 0.016$, classified as a low-luminosity AGN. Its central black hole has a mass of about $1.6\times 10^7 M_\odot$ and an Eddington ratio around 1 percent. We recently acquired optical spectra of NGC 4614 at the Telescopio Nazionale Galileo and the data clearly suggest that the broad H$α$ component has strongly dimmed, if not disappeared. A very recent Swift observation confirmed our current optical data, with the AGN weakened by almost a factor of 10 with respect to previous X-ray observations. Indeed, NGC 4614 had been also observed by Swift/XRT 6 times in 2011, when the source was clearly detected in all observations. By fitting the stack of the 2011 Swift observations we obtain a photon index of $Γ=1.3\pm0.3$ and an equivalent hydrogen column density of $N_{\rm H}$=$1.2\pm0.3$ $\times$10$^{22}$ cm$^{-2}$, indicating that NGC 4614 can be moderately absorbed in the X-rays. Although a significant change in the foreground gas absorption that may have obscured the broad line region cannot be entirely ruled out, the most likely explanation for our optical and X-ray data is that NGC 4614 is experiencing a change in the accretion state that reduces the radiative efficiency of the X-ray corona.

astro-ph.GA↗

arXiv2008

A compact star rotating at 1122 Hz and the r-mode instability

We show that r-mode instabilities severely constraint the composition of a compact star rotating at a submillisecond period. In particular, the only viable astrophysical scenario for such an object, present inside the Low Mass X-ray Binary associated with the x-ray transient XTE J1739-285, is that it has a strangeness content. Since previous analysis indicate that hyperonic stars or stars containing a kaon condensate are not good candidates, the only remaining possibility is that such an object is either a strange quark star or a hybrid quark-hadron star. We also discuss under which conditions sub-millisecond pulsars are rare.

astro-ph↗

arXiv2007

Transition to Quark Matter and long Gamma Ray Bursts

The energy released by the inner engine of GRBs can originate from structural readjustments inside a compact star. In particular, the formation of deconfined quark matter can liberate enough energy to power the burst. We show that the burning of a neutron star into a quark star likely proceeds as a deflagration and not as a detonation. In that way no strong baryon contamination is produced near the surface of the star. It is tempting to associate the temporal structures observed in the light curves with specific processes taking place inside the compact star. The so-called quiescent times, during which no signal is emitted in the highest energy band, correspond to pauses during the processes of readjustment. If the quark (or hybrid) star formed after these transformations is strongly magnetized and rotates rapidly, a prolonged gamma emission can be produced, as proposed by Usov years ago. This can explain the quasi-plateau observed by Swift in several GRBs.

astro-ph↗

arXiv2006

Burning of an hadronic star into a quark or a hybrid star

We study the hydrodynamical transition from an hadronic star into a quark or a hybrid star. We discuss the possible mode of burning, using a fully relativistic formalism and realistic Equations of State in which hyperons can be present. We take into account the possibility that quarks form a diquark condensate. We also discuss the formation of a mixed phase of hadrons and quarks, and we indicate which region of the star can rapidly convert in various possible scenarios. An estimate of the final temperature of the system is provided. We find that the conversion process always corresponds to a deflagration and never to a detonation. Hydrodynamical instabilities can develop on the front. We estimate the increase in the conversion's velocity due to the formation of wrinkles and we find that, although the increase is significant, it is not sufficient to transform the deflagration into a detonation in essentially all realistic scenarios. Concerning convection, it does not always develop. In particular the system does not develop convection if hyperons are not present in the initial phase and if the newly formed quark phase is made of ungapped (or weakly gapped) quarks. At the contrary, the process of conversion from ungapped quark matter to gapped quarks always allows the formation of a convective layer. Finally, we discuss possible astrophysical implications of our results.

astro-ph↗

arXiv2006

Quark Deconfinement inside Compact Stars and Gamma Ray Bursts Inner Engine

The temporal structure of Gamma Ray Bursts can be interpreted assuming as a inner engine a neutron star which undergoes a progressive compactification via production of strangeness (hyperons and kaons) and quarks. We will propose a tentative identification of various emission periods of the burst with specific structural changes of the star. Each of these modifications of the composition of the compact star takes place as a deflagration and not as a detonation, so the energy released in the transition goes mainly into heat and not into a mechanical wave. This is important in order to avoid an excessive baryonic contamination of the region surrounding the compact star. In this way a ultrarelativistic plasma of electron-positron pairs and of photons can be obtained, powering the Gamma Ray Burst.

astro-ph↗

arXiv2004

Quark deconfinement and implications for the radius and the limiting mass of compact stars

We study the consequences of the hadron-quark deconfinement phase transition in stellar compact objects when finite size effects between the deconfined quark phase and the hadronic phase are taken into account. We show that above a threshold value of the central pressure (gravitational mass) a neutron star is metastable to the decay (conversion) to a hybrid neutron star or to a strange star. The "mean-life time" of the metastable configuration dramatically depends on the value of the stellar central pressure. We explore the consequences of the metastability of ``massive'' neutron stars and of the existence of stable compact quark stars (hybrid neutron stars or strange stars) on the concept of limiting mass of compact stars. We discuss the implications of our scenario on the interpretation of the stellar mass and radius extracted from the spectra of several X-ray compact sources. Finally, we show that our scenario implies, as a natural consequence a two step-process which is able to explain the inferred ``delayed'' connection between supernova explosions and GRBs, giving also the correct energy to power GRBs.

astro-ph↗