arXiv subjects

M. Dadina

Publications and source records attributed to M. Dadina.

At least 37 records · Page 2Linked to original sources

arXiv2021

The X-ray emission of the Seyfert 2 galaxy MCG-01-24-12

We present a detailed X-ray spectral analysis of the nearby Seyfert 2 galaxy MCG-01-24-12 based on a multi-epoch data set. Data have been taken with different X-ray satellites, namely XMM-Newton, NuSTAR, Swift and Chandra and cover different time intervals, from years down to a few days. From 2006 to 2013 the source had a 2-10 keV flux of $\sim$1.5$\times$10$^{-11}$ erg cm$^{-2}$ s$^{-1}$, consistent with archival observations based on \textit{HEAO} and \textit{BeppoSAX} data, though a 2019 \textit{Chandra} snapshot caught the source in an extreme low flux state, a factor of $\sim$10 fainter than its historical one. Based on phenomenological and physically motivated models, we find the X-ray spectrum of MCG-01-24-12 to be best modelled by a power-law continuum emission with $Γ$=1.76$\pm$0.09 with a high energy cut-off at E$_{\rm c}=70^{+21}_{-14}$ keV that is absorbed by a fairly constant column density of N$_{\rm H}$=(6.3$\pm$0.5)$\times10^{22}$ cm$^{-2}$. These quantities allowed us to estimate the properties of the hot corona in MCG-01-24-12 for the cases of a spherical or slab-like hot Comptonising plasma to be kT$_{\rm e}$=27$^{+8}_{-4}$ keV, $τ_{\rm e}$=5.5$\pm$1.3 and kT$_{\rm e}$=28$^{+7}_{-5}$ keV, $τ$=3.2$\pm$0.8, respectively. Finally, despite the short duration of the exposures, possible evidence of the presence of outflows is discussed.

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arXiv2020

X-raying winds in distant quasars: the first high-redshift wind duty cycle

Theoretical models of wind-driven feedback from Active Galactic Nuclei (AGN) often identify Ultra-fast outflows (UFOs) as being the main cause for generating galaxy-size outflows, possibly the main actors in establishing the so-called AGN-galaxy co-evolution. UFOs are well characterized in local AGN but much less is known in quasars at the cosmic time when SF and AGN activity peaked ($z\simeq1-3$). It is therefore mandatory to search for evidences of UFOs in high-$z$ sources to test the wind-driven AGN feedback models. Here we present a study of Q2237+030, the Einstein Cross, a quadruply-imaged radio-quiet lensed quasar located at $z=1.695$. We performed a systematic and comprehensive temporally and spatially resolved X-ray spectral analysis of all the available Chandra and XMM-Newton data (as of September 2019). We find clear evidence for spectral variability, possibly due to absorption column density (or covering fraction) variability intrinsic to the source. We detect, for the first time in this quasar, a fast X-ray wind outflowing at $v_{\rm out}\simeq0.1c$ that would be powerful enough ($\dot{E}_{\rm kin}\simeq0.1 L_{\rm bol}$) to significantly affect the host galaxy evolution. We report also on the possible presence of an even faster component of the wind ($v_{\rm out}\sim0.5c$). Given the large sample and long time interval spanned by the analyzed X-ray data, we are able to roughly estimate, for the first time in a high-$z$ quasar, the wind duty cycle as $\approx0.46\,(0.31)$ at $90\%\,(95\%)$ confidence level. Finally, we also confirm the presence of a Fe K$α$ emission line with variable energy, which we discuss in the light of microlensing effects as well as considering our findings on the source.

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arXiv2020

Search for the optical counterpart of the GW170814 gravitationalwave event with the VLT Survey Telescope

We report on the search for the optical counterpart of the gravitational event GW170814, which was carried out with the VLT Survey Telescope (VST) by the GRAvitational Wave Inaf TeAm (GRAWITA). Observations started 17.5 hours after the LIGO/Virgo alert and we covered an area of 99 deg$^2$ that encloses $\sim 77\%$ and $\sim 59\%$ of the initial and refined localization probability regions, respectively. A total of six epochs were secured over nearly two months. The survey reached an average limiting magnitude of 22 AB mag in the $r-$band. After assuming the model described in Perna et al. 2019 that derives as possible optical counterpart of a BBH event a transient source declining in about one day, we have computed a survey efficiency of about $5\%$. This paper describes the VST observational strategy and the results obtained by our analysis pipelines developed to search for optical transients in multi-epoch images. We report the catalogue of the candidates with possible identifications based on light-curve fitting. We have identified two dozens of SNe, nine AGNs, one QSO. Nineteen transients characterized by a single detection were not classified. We have restricted our analysis only to the candidates that fall into the refined localization map. None out of 39 left candidates could be positively associated with GW170814. This result implies that the possible emission of optical radiation from a BBH merger had to be fainter than r $\sim$ 22 ($L_{optical}$ $\sim$ $1.4 \times 10^{42}$ erg/s) on a time interval ranging from a few hours up to two months after the GW event.

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arXiv2020

A comparison between short GRB afterglows and AT2017gfo: shedding light on kilonovae properties

Multi-messenger astronomy received a great boost following the discovery of kilonova AT2017gfo, the optical counterpart of the gravitational wave source GW170817 associated with the short gamma-ray burst GRB 170817A. AT2017gfo was the first kilonova that could be extensively monitored in time both photometrically and spectroscopically. Previously, only few candidates have been observed against the glare of short GRB afterglows. In this work, we aim to search the fingerprints of AT2017gfo-like kilonova emissions in the optical/NIR light curves of 39 short GRBs with known redshift. For the first time, our results allow us to study separately the range of luminosity of the blue and red components of AT2017gfo-like kilonovae in short GRBs. In particular, the red component is similar in luminosity to AT2017gfo, while the blue kilonova can be more than 10 times brighter. Finally, we find further evidence to support all the claimed kilonova detections and we exclude an AT2017gfo-like kilonova in GRBs 050509B and 061201.

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arXiv2019

Nustar Measurement Of Coronal Temperature In Two Luminous, High Redshift Qsos

X-ray emission from AGN is believed to be produced via Comptonization of optical/UV seed photons emitted by the accretion disk, up-scattered by hot electrons in a corona surrounding the black hole. A critical compactness vs. temperature threshold is predicted above which any increase in the source luminosity, for a fixed size, would then generate positron-electron pairs rather than continue heating the coronal plasma. Current observations seem to confirm that all AGN populate the region below this critical line. These models, however, have never been probed by observations in the high-luminosity regime, where the critical line is expected to reach low temperatures. To fill this observational gap, we selected two luminous ($log(L_{bol})>47.5$ erg/s) quasars, 2MASSJ1614346+470420 ($z=1.86$) and B1422+231 ($z=3.62$), and obtained XMM-Newton and NuSTAR deep observations for them. We performed detailed spectral analysis of their quasi-simultaneous soft and hard X-ray data, in order to constrain the parameters of their coronae. Using a phenomenological cut-off power-law model, with the inclusion of a reflection component, we derived rest-frame values of the high energy cut-off of $E_{cut}=106^{+102}_{-37}$ keV and $E_{cut}=66^{+17}_{-12}$ keV, respectively. Comptonization models consistently give as best-fit parameters electron temperatures of $\sim45$ keV and $\sim28$ keV, respectively, and optically thick coronae ($τ>1$). These low coronal temperatures fall in the limited region allowed at these luminosities to avoid runaway pair production.

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arXiv2018

Unveiling the enigma of ATLAS17aeu

Aim. The unusual transient ATLAS17aeu was serendipitously detected within the sky localisation of the gravitational wave trigger GW170104. The importance of a possible association with gravitational waves coming from a binary black hole merger led to an extensive follow-up campaign, with the aim of assessing a possible connection with GW170104. Methods. With several telescopes, we carried out both photometric and spectroscopic observations of ATLAS17aeu, for several epochs, between $\sim 3$ and $\sim 230$ days after the first detection. Results. We studied in detail the temporal and spectroscopic properties of ATLAS17aeu and its host galaxy. We detected spectral features similar to those of a broad lined supernova superposed to an otherwise typical long-GRB afterglow. Based on analysis of the optical light curve, spectrum and host galaxy SED, we conclude that the redshift of the source is probably $z \simeq 0.5 \pm 0.2$. Conclusions. While the redshift range we have determined is marginally compatible with that of the gravitational wave event, the presence of a supernova component and the consistency of this transient with the E$_{\rm p}$-E$_{\rm iso}$ correlation support the conclusion that ATLAS17aeu was associated with the long gamma-ray burst GRB170105A. This rules out the association of the GRB170105A/ATLAS17aeu transient with the gravitational wave event GW170104, which was due to a binary black hole merger.

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arXiv2018

Observations of the MIssing Baryons in the warm-hot intergalactic medium

It has been known for decades that the observed number of baryons in the local universe falls about 30-40% short of the total number of baryons predicted by Big-Bang Nucleosynthesis, as inferred from density fluctuations of the Cosmic Microwave Background and seen during the first 2-3 billion years of the universe in the so called Lyman-alpha Forest. A theoretical solution to this paradox locates the missing baryons in the hot and tenuous filamentary gas between galaxies, known as the warm-hot intergalactic medium. However, it is difficult to detect them there because the largest by far constituent of this gas - hydrogen - is mostly ionized and therefore almost invisible in far-ultraviolet spectra with typical signal-to-noise ratios. Indeed, despite the large observational efforts, only a few marginal claims of detection have been made so far. Here we report observations of two absorbers of highly ionized oxygen (OVII) in the high signal-to-noise-ratio X-ray spectrum of a quasar at redshift >0.4. These absorbers show no variability over a 2-year timescale and have no associated cold absorption, making the assumption that they originate from the quasar's intrinsic outflow or the host galaxy's interstellar medium implausible. The OVII systems lie in regions characterized by large (x4 compared to average) galaxy over-densities and their number (down to the sensitivity threshold of our data), agrees well with numerical simulation predictions for the long-sought warm-hot intergalactic medium (WHIM). We conclude that the missing baryons have been found.

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arXiv2018

Radio/X-ray monitoring of the broad-line radio galaxy 3C 382. High-energy view with XMM-Newton and NuSTAR

We present the analysis of five joint XMM-Newton/NuSTAR observations, 20 ks each and separated by 12 days, of the broad-line radio galaxy 3C 382. The data were obtained as part of a campaign performed in September-October 2016 simultaneously with VLBA. The radio data and their relation with the X-ray ones will be discussed in a following paper. The source exhibits a moderate flux variability in the UV/X-ray bands, and a limited spectral variability especially in the soft X-ray band. In agreement with past observations, we find the presence of a warm absorber, an iron K$α$ line with no associated Compton reflection hump, and a variable soft excess well described by a thermal Comptonization component. The data are consistent with a "two-corona" scenario, in which the UV emission and soft excess are produced by a warm ($kT \simeq 0.6$ keV), optically thick ($τ\simeq 20$) corona consistent with being a slab fully covering a nearly passive accretion disc, while the hard X-ray emission is due to a hot corona intercepting roughly 10 per cent of the soft emission. These results are remarkably similar to those generally found in radio-quiet Seyferts, thus suggesting a common accretion mechanism.

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arXiv2018

Yet another UFO in the X-ray spectrum of a high-z lensed QSO

Ultra-fast outflows (UFO) appear to be common in local active galactic nuclei (AGN) and may be powerful enough ($\dot{E}_{kin}$$\geq$1\% of L$_{bol}$) to effectively quench the star formation in their host galaxies. To test feedback models based on AGN outflows, it is mandatory to investigate UFOs near the peak of AGN activity, that is, at high-z where only a few studies are available to date. UFOs produce Fe resonant absorption lines measured above $\approx$7 keV. The most critical problem in detecting such features in distant objects is the difficulty in obtaining X-ray data with sufficient signal-to-noise. We therefore selected a distant QSO that gravitational lensing made bright enough for these purposes, the z=2.64 QSO MG J0414+0534, and observed it with XMM-Newton for $\approx$78 ks.} The X-ray spectrum of MG J0414+0534 is complex and shows signatures of cold absorption (N$_{H}\approx$4$\times$10$^{22}$ cm$^{-2}$) and of the presence of an iron emission line (E$\approx$6.4 keV, EW$=$95$\pm$53 eV) consistent with it originating in the cold absorber. Our main result, however, is the robust detection (more than 5$σ$) of an absorption line at E$_{int}\approx$9.2 keV (E$_{obs}\approx$2.5 keV observer frame). If interpreted as due to FeXXVI, it implies gas outflowing at $v_{out}\approx$0.3c. To our knowledge, this is the first detection of an UFO in a radio-loud quasar at z$\geq$1.5. We estimated that the UFO mechanical output is $\dot{E}_{kin}$$\approx$2.5$L_{bol}$ with $\dot{p}_{out}/\dot{p}_{rad}\approx$17 indicating that it is capable of installing significant feedback between the super-massive black hole (SMBH) and the bulge of the host galaxy. We argue that this also suggests a magnetic driving origin of the UFO.

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arXiv2017

Spectroscopic identification of r-process nucleosynthesis in a double neutron star merger

The merger of two neutron stars is predicted to give rise to three major detectable phenomena: a short burst of gamma-rays, a gravitational wave signal, and a transient optical/near-infrared source powered by the synthesis of large amounts of very heavy elements via rapid neutron capture (the r-process). Such transients, named "macronovae" or "kilonovae", are believed to be centres of production of rare elements such as gold and platinum. The most compelling evidence so far for a kilonova was a very faint near-infrared rebrightening in the afterglow of a short gamma-ray burst at z = 0.356, although findings indicating bluer events have been reported. Here we report the spectral identification and describe the physical properties of a bright kilonova associated with the gravitational wave source GW 170817 and gamma-ray burst GRB 170817A associated with a galaxy at a distance of 40 Mpc from Earth. Using a series of spectra from ground-based observatories covering the wavelength range from the ultraviolet to the near-infrared, we find that the kilonova is characterized by rapidly expanding ejecta with spectral features similar to those predicted by current models. The ejecta is optically thick early on, with a velocity of about 0.2 times light speed, and reaches a radius of about 50 astronomical units in only 1.5 days. As the ejecta expands, broad absorption-like lines appear on the spectral continuum indicating atomic species produced by nucleosynthesis that occurs in the post-merger fast-moving dynamical ejecta and in two slower (0.05 times light speed) wind regions. Comparison with spectral models suggests that the merger ejected 0.03-0.05 solar masses of material, including high-opacity lanthanides.

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arXiv2017

GRAWITA: VLT Survey Telescope observations of the gravitational wave sources GW150914 and GW151226

We report the results of deep optical follow-up surveys of the first two gravitational-wave sources, GW150914 and GW151226, done by the GRAvitational Wave Inaf TeAm Collaboration (GRAWITA). The VLT Survey Telescope (VST) responded promptly to the gravitational-wave alerts sent by the LIGO and Virgo Collaborations, monitoring a region of $90$ deg$^2$ and $72$ deg$^2$ for GW150914 and GW151226, respectively, and repeated the observations over nearly two months. Both surveys reached an average limiting magnitude of about 21 in the $r-$band. The paper describes the VST observational strategy and two independent procedures developed to search for transient counterpart candidates in multi-epoch VST images. Several transients have been discovered but no candidates are recognized to be related to the gravitational-wave events. Interestingly, among many contaminant supernovae, we find a possible correlation between the supernova VSTJ57.77559-59.13990 and GRB150827A detected by {\it Fermi}-GBM. The detection efficiency of VST observations for different types of electromagnetic counterparts of gravitational-wave events are evaluated for the present and future follow-up surveys.

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arXiv2017

Testing warm Comptonization models for the origin of the soft X-ray excess in AGN

The X-ray spectra of many active galactic nuclei (AGN) show a soft X-ray excess below 1-2 keV on top of the extrapolated high- energy power law. The origin of this component is uncertain. It could be a signature of relativistically blurred, ionized reflection, or the high-energy tail of thermal Comptonization in a warm (kT $\sim$ 1 keV), optically thick ($τ\simeq$ 10-20) corona producing the optical/UV to soft X-ray emission. The purpose of the present paper is to test the warm corona model on a statistically significant sample of unabsorbed, radio-quiet AGN with XMM-newton archival data, providing simultaneous optical/UV and X-ray coverage. The sample has 22 objects and 100 observations. We use two thermal comptonization components to fit the broad-band spectra, one for the warm corona emission and one for the high-energy continuum. In the optical-UV, we also include the reddening, the small blue bump and the Galactic extinction. In the X-rays, we include a WA and a neutral reflection. The model gives a good fit (reduced $χ^2 <1.5$) to more than 90% of the sample. We find the temperature of the warm corona to be uniformly distributed in the 0.1-1 keV range, while the optical depth is in the range $\sim$10-40. These values are consistent with a warm corona covering a large fraction of a quasi-passive accretion disc, i.e. that mostly reprocesses the warm corona emission. The disk intrinsic emission represents no more than 20% of the disk total emission. According to this interpretation, most of the accretion power would be released in the upper layers of the accretion flow.

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arXiv2017

NuSTAR spectral analysis of two bright Seyfert 1 galaxies: MCG +8-11-11 and NGC 6814

We report on the NuSTAR observations of two bright Seyfert 1 galaxies, namely MCG +8-11-11 (100 ks) and NGC 6814 (150 ks). The main goal of these observations was to investigate the Comptonization mechanisms acting in the innermost regions of AGN which are believed to be responsible for the UV/X-ray emission. The spectroscopic analysis of the NuSTAR spectra of these two sources revealed that although they had different properties overall (black hole masses, luminosity and Eddington ratios) they had very similar coronal properties. Both presented a power law spectrum with a high-energy cutoff at $\sim 150-200$ keV, a relativistically broadened Fe K$α$ line and the associated disk reflection component, plus a narrow iron line likely emitted in Compton thin and distant matter. The intrinsic continuum was well described by Comptonization models that show for MCG +8-11-11 a temperature of the coronal plasma of kT$_{\rm e} \sim$ 60 keV and an extrapolated optical depth $τ$=1.8; for NGC 6814 the coronal temperature was kT$_{\rm e} \sim$ 45 keV with an extrapolated optical depth of $τ$=2.5. We compare and discuss these values to some most common Comptonization models which aim at explaining the energy production and stability of coronae in active galactic nuclei.

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arXiv2016

XMM-Newton reveals a Seyfert-like X-ray spectrum in the z=3.6 QSO B1422+231

Matter flows in the central regions of quasars during their active phases are probably responsible for the properties of the super-massive black holes and that of the bulges of host galaxies. To understand how this mechanism works, we need to characterize the geometry and the physical state of the accreting matter at cosmological redshifts. The few high quality X-ray spectra of distant QSO have been collected by adding sparse pointings of single objects obtained during X-ray monitoring campaigns. This could have introduced spurious spectral features due to source variability. Here we present a single epoch, high-quality X-ray spectrum of the z=3.62 quasar B1422+231 whose flux is enhanced by gravitationally lensing (F$_{2-10 keV}\sim$10$^{-12}$erg s$^{-1}$ cm$^{-2}$). The X-ray spectrum of B1422+231 is found to be very similar to the one of a typical nearby Seyfert galaxy. Neutral absorption is detected (N$_{H}\sim$5$\times$10$^{21}$ cm$^{-2}$ at the redshift of the source) while a strong absorption edge is measured at E$\sim$7.5 keV with an optical depth of $τ\sim$0.14. We also find hints of the FeK$α$ line in emission at E$\sim$6.4 keV line (EW$\lesssim$70 eV) and a hump is detected in the E$\sim$15-20 keV energy band (rest-frame) suggesting the presence of a reflection component. In this scenario, the primary emission of B1422+231 is most probably dominated by the thermal Comptonization of UV seed photons in a corona with kT$\sim$40 keV and the reflection component has a relative direct-to-reflect normalization r$\sim$1. These findings confirm that gravitational lensing is effective to obtain good quality X-ray spectral information of quasar at high-z, moreover they support the idea that the same general picture characterizing active galactic nuclei in the nearby Universe is valid also at high redshift.

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arXiv2016

NuSTAR reveals the extreme properties of the super-Eddington accreting SMBH in PG 1247+267

PG1247+267 is one of the most luminous known quasars at $z\sim2$ and is a strongly super-Eddington accreting SMBH candidate. We obtained NuSTAR data of this intriguing source in December 2014 with the aim of studying its high-energy emission, leveraging the broad band covered by the new NuSTAR and the archival XMM-Newton data. Several measurements are in agreement with the super-Eddington scenario for PG1247+267: the soft power law ($Γ=2.3\pm0.1$); the weak ionized Fe emission line and a hint of the presence of outflowing ionized gas surrounding the SMBH. The presence of an extreme reflection component is instead at odds with the high accretion rate proposed for this quasar. This can be explained with three different scenarios; all of them are in good agreement with the existing data, but imply very different conclusions: i) a variable primary power law observed in a low state, superimposed on a reflection component echoing a past, higher flux state; ii) a power law continuum obscured by an ionized, Compton thick, partial covering absorber; and iii) a relativistic disk reflector in a lamp-post geometry, with low coronal height and high BH spin. The first model is able to explain the high reflection component in terms of variability. The second does not require any reflection to reproduce the hard emission, while a rather low high-energy cutoff of $\sim100$ keV is detected for the first time in such a high redshift source. The third model require a face-on geometry, which may affect the SMBH mass and Eddington ratio measurements. Deeper X-ray broad-band data are required in order to distinguish between these possibilities.

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arXiv2015

A multi-epoch spectroscopic study of the BAL quasar APM 08279+5255 II. Emission- and absorption-line variability time lags

The study of high-redshift bright quasars is crucial to gather information about the history of galaxy assembly and evolution. Variability analyses can provide useful data on the physics of the quasar processes and their relation with the host galaxy. In this study, we aim at measuring the black hole mass of the bright lensed BAL QSO APM 08279+5255 at $z=3.911$ through reverberation mapping, and at updating and extending the monitoring of its C IV absorption line variability. Thanks to 138 R-band photometric data and 30 spectra available over 16 years of observations, we perform the first reverberation mapping of the Si IV and C IV emission lines for a high-luminosity quasar at high redshift. We also cross-correlate the C IV absorption equivalent width variations with the continuum light curve, in order to estimate the recombination time lags of the various absorbers and infer the physical conditions of the ionised gas. We find a reverberation-mapping time lag of $\sim 900$ rest-frame days for both Si IV and C IV emission lines. This is consistent with an extension of the BLR size-to-luminosity relation for active galactic nuclei up to a luminosity of $\sim 10^{48}$ erg/s, and implies a black hole mass of $10^{10}$ $M_\odot$. Additionally, we measure a recombination time lag of $\sim 160$ days in the rest frame for the C IV narrow absorption system, which implies an electron density of the absorbing gas of $\sim 2.5 \cdot 10^4$ cm$^{-3}$. The measured black hole mass of APM 08279+5255 indicates that the quasar resides in an under-massive host-galaxy bulge with $M_{bulge} \sim 7.5 M_{BH}$, and that the lens magnification is lower than $\sim 8$. Finally, the inferred electron density of the narrow-line absorber implies a distance of the order of 10 kpc of the absorbing gas from the quasar, placing it within the host galaxy.

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arXiv2015

Accretion, ejection and reprocessing in supermassive black holes

This is a White Paper in support of the mission concept of the Large Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We discuss the potential of LOFT for the study of active galactic nuclei. For a summary, we refer to the paper.

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arXiv2014

CIV and CIII] reverberation mapping of the luminous quasar PG 1247+267

So far the masses of about 50 active galactic nuclei have been measured through the reverberation mapping technique (RM). Most measurements have been performed for objects of moderate luminosity and redshift, based on H$β$, which is also used to calibrate the scaling relation which allows single-epoch (SE) mass determination based on AGN luminosity and the width of different emission lines. The SE mass obtained from CIV$(1549 {\rmÅ})$ line shows a large spread around mean values, due to complex structure and gas dynamics of the relevant emission region. Direct RM measures of CIV exist for only 6 AGNs of low luminosity and redshift, and only one luminous quasar (Kaspi et al 2007). We have collected since 2003 photometric and spectroscopic observations of PG1247+267, the most luminous quasar ever analyzed for RM. We provide light curves for the continuum and for CIV$(1549 {\rmÅ})$ and CIII]$(1909{\rmÅ})$, and measures of the reverberation time lags based on the SPEAR method (Zu et al. 2011). The sizes of the line emission regions are in a ratio $R_{CIII]}/R_{CIV}\sim 2$, similar to the case of Seyfert galaxies, indicating for the first time a similar ionization stratification in a luminous quasar and low luminosity nuclei. Due to relatively small broad line region size and relatively narrow line widths, we estimate a small mass and an anomalously high Eddington ratio. We discuss the possibility that either the shape of the emission region or an amplification of the luminosity caused by gravitational lensing may be in part responsible of the result.

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