arXiv subjects

Mislav Baloković

Publications and source records attributed to Mislav Baloković.

26 records · Page 2Linked to original sources

arXiv2021

A Highly Accreting Low-Mass Black Hole Hidden in the Dust: Suzaku and NuSTAR observations of the NLS1 Mrk 1239

We present torus modelling for the X-ray spectra of a nearby narrow-line Seyfert 1 galaxy Mrk 1239 ($z=0.0199$), based on archival Suzaku, NuSTAR and Swift observations. Our model suggests very soft intrinsic power-law continuum emission of $Γ\approx2.57$ in 2019 and $Γ\approx2.98$ in 2007. By applying a correction factor to the unabsorbed X-ray luminosity, we find that Mrk 1239 is accreting near or around the Eddington limit. Our best-fit spectral model also suggests a torus with a column density of $\log(N_{\rm H, ave}/$cm$^{-2})=25.0\pm0.2$ and a high covering factor of $0.90$ in Mrk 1239, indicating that this source is most likely to be viewed almost face-on with $i\approx26^{\circ}$. Our line of sight might cross the edge of the torus with $N_{\rm H, los}=2-5\times10^{23}$cm$^{-2}$. The high Eddington ratio and the high line-of-sight column density makes Mrk 1239 one of the AGNs that are close to the limit where wind may form near the edge of the torus due to high radiation pressure.

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arXiv2019

The Broadband X-ray Spectrum of the X-ray Obscured Type 1 AGN 2MASX J193013.80+341049.5

We present results from modeling the broadband X-ray spectrum of the Type 1 AGN 2MASX J193013.80+341049.5 using NuSTAR, Swift and archival XMM-Newton observations. We find this source to be highly X-ray obscured, with column densities exceeding 10$^{23}$ cm$^{-2}$ across all epochs of X-ray observations, spanning an 8 year period. However, the source exhibits prominent broad optical emission lines, consistent with an unobscured Type 1 AGN classification. We fit the X-ray spectra with both phenomenological reflection models and physically-motivated torus models to model the X-ray absorption. We examine the spectral energy distribution of this source and investigate some possible scenarios to explain the mismatch between X-ray and optical classifications. We compare the ratio of reddening to X-ray absorbing column density ($E_{B-V}/N_{H}$) and find that 2MASX J193013.80+341049.5 likely has a much lower dust-to-gas ratio relative to the Galactic ISM, suggesting that the Broad Line Region (BLR) itself could provide the source of extra X-ray obscuration, being composed of low-ionization, dust-free gas.

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arXiv2019

The relativistic jet of the $γ$-ray emitting narrow-line Seyfert 1 galaxy PKS J1222$+$0413

We present a multi-frequency study of PKS J1222$+$0413 (4C$+$04.42), currently the highest redshift $γ$-ray emitting narrow-line Seyfert 1 ($γ$-NLS1). We assemble a broad spectral energy distribution (SED) including previously unpublished datasets: X-ray data obtained with the NuSTAR and Neil Gehrels Swift observatories; near-infrared, optical and UV spectroscopy obtained with VLT X-shooter; and multiband radio data from the Effelsberg telescope. These new observations are supplemented by archival data from the literature. We apply physical models to the broadband SED, parameterising the accretion flow and jet emission to investigate the disc-jet connection. PKS J1222$+$0413 has a much greater black hole mass than most other NLS1s, $M_\mathrm{BH}\approx2\times10^{8}$ M$_\odot$, similar to those found in flat spectrum radio quasars (FSRQs). Therefore this source provides insight into how the jets of $γ$-NLS1s relate to those of FSRQs.

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arXiv2019

Investigating the Covering Fraction Distribution of Swift/BAT AGN with X-ray and IR Observations

We present an analysis of a sample of 69 local obscured Swift/Burst Alert Telescope active galactic nuclei (AGNs) with X-ray spectra from NuSTAR and infrared (IR) spectral energy distributions from Herschel and WISE. We combine this X-ray and IR phenomenological modeling and find a significant correlation between reflected hard X-ray emission and IR AGN emission, with suggestive indications that this correlation may be stronger than the one between intrinsic hard X-ray and IR emissions. This relation between the IR and reflected X-ray emission suggests that both are the result of the processing of intrinsic emission from the corona and accretion disk by the same structure. We explore the resulting implications on the underlying distribution of covering fraction for all AGNs, by generating mock observables for the reflection parameter and IR luminosity ratio using empirical relations found for the covering fraction with each quantity. We find that the observed distributions of the reflection parameter and IR-to-X-ray ratio are reproduced with broad distributions centered around covering fractions of at least ~40%-50%, whereas narrower distributions match our observations only when centered around covering fractions of ~70%-80%. Our results are consistent with both independent estimates of the covering fractions of individual objects and the typical covering fraction obtained on the basis of obscured fractions for samples of AGNs. These results suggest that the level of reprocessing in AGNs, including X-ray reflection, is related in a relatively straightforward way to the geometry of the obscuring material.

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arXiv2019

Probing the Physical Properties of the Corona in Accreting Black Holes

The corona is a key component of most luminous accreting black holes, carrying 5 - 30 % of the power and in non-jetted Active Galactic Nuclei (AGN), creating all the X-ray emission above $\simeq 1-2$ keV. Its emission illuminates the inner accretion disc, creating the atomic line-rich reflection spectrum used to diagnose and map the accretion flow and measure black hole spin. The corona is likely powered magnetically by the strong differential rotation of the accretion disc and it may be intimately related to relativistic jets. Recent work shows that many black hole coronae may be dominated by electron-positron pairs produced by photon-photon collisions in the compact coronal environment. Despite the corona being an integral component of AGN and black hole binary systems, much is still unknown about the nature of the corona, such as its geometry, location, and the physical mechanisms powering the emission. In this white paper we explore our current understanding of coronal properties, such as its temperature, obtained from measurements with existing hard X-ray telescopes such as NuSTAR, and discuss important questions to be addressed in the coming decade surrounding the nature of the corona. Hard X-ray observations will continue to dispel the mystery of coronae and open up this part of the quasar engine to full understanding.

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arXiv2019

A new class of flares from accreting supermassive black holes

Accreting supermassive black holes (SMBHs) can exhibit variable emission across the electromagnetic spectrum and over a broad range of timescales. The variability of active galactic nuclei (AGNs) in the ultraviolet and optical is usually at the few tens of per cent level over timescales of hours to weeks. Recently, rare, more dramatic changes to the emission from accreting SMBHs have been observed, including tidal disruption events, 'changing look' AGNs and other extreme variability objects. The physics behind the 're-ignition', enhancement and 'shut-down' of accretion onto SMBHs is not entirely understood. Here we present a rapid increase in ultraviolet-optical emission in the centre of a nearby galaxy, marking the onset of sudden increased accretion onto a SMBH. The optical spectrum of this flare, dubbed AT 2017bgt, exhibits a mix of emission features. Some are typical of luminous, unobscured AGNs, but others are likely driven by Bowen fluorescence - robustly linked here with high-velocity gas in the vicinity of the accreting SMBH. The spectral features and increased ultraviolet flux show little evolution over a period of at least 14 months. This disfavours the tidal disruption of a star as their origin, and instead suggests a longer-term event of intensified accretion. Together with two other recently reported events with similar properties, we define a new class of SMBH-related flares. This has important implications for the classification of different types of enhanced accretion onto SMBHs.

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arXiv2018

Coronal properties of Swift/BAT-selected Seyfert 1 AGN observed with NuSTAR

The NuSTAR observatory, with its high sensitivity in hard X-rays, has enabled detailed broadband modeling of the X-ray spectra of Active Galactic Nuclei (AGN), thereby allowing constraints to be placed on the high-energy cutoff of the X-ray coronal continuum. We investigate the spectral properties of a sample of 46 NuSTAR-observed Seyfert 1 AGN selected from the Swift/BAT 70-month hard X-ray survey. Our measurements of the high-energy cutoff of the continuum from modeling the NuSTAR X-ray spectra are used to map out the temperature - compactness ($θ-l$) plane for AGN coronae. We find that most of the coronae lie clustered near the boundary for runaway pair production, suggesting that annihilation and pair production act to regulate the temperature of the corona. We discuss the implications of coronae whose high-energy cutoff may indicate a low coronal temperature on the heating and thermalization mechanisms in the corona.

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arXiv2018

A Mid-IR Selected Changing-Look Quasar and Physical Scenarios for Abrupt AGN Fading

We report a new changing-look quasar, WISE~J105203.55+151929.5 at $z=0.303$, found by identifying highly mid-IR variable quasars in the WISE/NEOWISE data stream. Compared to multi-epoch mid-IR photometry of a large sample of SDSS-confirmed quasars, WISE J1052+1519 is an extreme photometric outlier, fading by more than a factor of two at $3.4$ and $4.6 μ$m since 2009. Swift target-of-opportunity observations in 2017 show even stronger fading in the soft X-rays compared to the ROSAT detection of this source in 1995, with at least a factor of fifteen decrease. We obtained second-epoch spectroscopy with the Palomar telescope in 2017 which, when compared with the 2006 archival SDSS spectrum, reveals that the broad H$β$ emission has vanished and that the quasar has become significantly redder. The two most likely interpretations for this dramatic change are source fading or obscuration, where the latter is strongly disfavored by the mid-IR data. We discuss various physical scenarios that could cause such changes in the quasar luminosity over this timescale, and favor changes in the innermost regions of the accretion disk that occur on the thermal and heating/cooling front timescales. We discuss possible physical triggers that could cause these changes, and predict the multiwavelength signatures that could distinguish these physical scenarios.

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