arXiv subjects

P. Kosec

Publications and source records attributed to P. Kosec.

At least 19 recordsLinked to original sources

arXiv2026

Dust grain chemistry in the diffuse ISM towards the black hole transient GX 339-4

We present results on X-ray absorption and the dust grain chemistry in the diffuse interstellar medium (ISM), based on a new Cycle 25 Chandra High Energy Transmission Grating Spectrometer (HETGS) observational campaign targeting the black hole transient GX 339-4. The X-ray source offers an optimal combination of moderate hydrogen column density and high X-ray flux, enabling the first detailed simultaneous fitting of the photoabsorption edges of Fe, O, Si, and Mg which are key elemental constituents of interstellar dust. We performed a joint spectral analysis of Chandra/HETGS data and archival observations from the Reflection Grating Spectrometer (RGS) on board XMM-Newton. We found that the dust grain chemical composition along this diffuse Galactic line of sight is best described by the silicate Mg-rich amorphous pyroxene (Mg0.75Fe0.25SiO3) and metallic iron. We also discuss the elemental abundances and depletions of Fe, O, Si, and Mg, and the presence of absorption features in the X-ray spectrum of this source associated with highly ionised plasma.

astro-ph.HE↗

arXiv2026

Unveiling the biconical geometry of the outflow in the ultraluminous X-ray source NGC 5204 X-1

Ultraluminous X-ray sources (ULXs) are non-nuclear X-ray binary systems that exceed the Eddington luminosity for a 10 Msun black hole. The majority of these sources are thought to be stellar-mass compact objects accreting at super-Eddington rates, exhibiting powerful relativistic winds. These winds have been identified through the detection of absorption lines with a blueshift as high as 0.3c and emission lines typically found at their laboratory wavelengths. In this work, we analysed the XMM-Newton data of the ULX NGC 5204 X-1, which has been observed to exhibit emission lines with a blueshift of about 0.3c. The aim of this study is to examine the geometry and physical properties of the accretion disc and the relativistic outflows. In addition, we aim to explore the factors that influence the ULX spectral transitions. We undertook an observing campaign with XMM-Newton to explore the source behaviour at different luminosities. In this first paper of the series, we performed high-resolution X-ray spectroscopy, including archival data, with the RGS instrument which allowed us to resolve both emission and absorption lines. The outflows features were characterised using physical models of plasma in collisional-ionisation and photoionisation equilibrium. We identify collisionally-ionised blueshifted and redshifted components at about 0.3c. These findings have high statistical significance and suggest a biconical structure for the outflow. Additionally, the analysis of the O VII line triplet observed in the spectrum enables us to infer physical properties of the low-velocity line-emitting plasma, e.g. electron density (ne $\sim 10^{10}$ cm$^{-3}$) and temperature (Te $ \geq 1.5 \times 10^5$ K). A hybrid plasma whose ionisation balance is affected by both collisions and radiation is favoured.

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arXiv2025

XMM-Newton multi-year campaign on NGC 55 ULX-1: Resolving the wind and its variability with RGS

Winds are an important ingredient in the evolution of X-ray binary (XRB) systems, particularly those at high accretion rates such as ultra-luminous X-ray sources (ULXs), because they may regulate the accretion of matter onto the compact object. We aim at understanding the properties of ULX winds and their link with the source spectral and temporal behavior. We performed high-resolution X-ray spectroscopy of the variable source NGC 55 ULX-1 to resolve emission and absorption lines as observed with XMM-Newton at different epochs. Optically-thin plasma models are used to characterise the wind. We confirmed and thoroughly strengthened previous evidence of outflows in NGC 55 ULX-1. The presence of radiative recombination signatures and the ratios between the fluxes of the emission lines favours photoionisation balance and low-to-moderate densities, which confirm that the lines originate from classical XRB disc winds. An in-depth parameter space exploration shows line emission from a slowly moving, cool, and variable plasma perhaps associated with a thermal wind. Mildly-relativistic Doppler shifts (about -0.15c) associated with the absorption lines confirm, at higher confidence, the presence of powerful, radiatively-driven, winds. The comparison between results obtained at different epochs revealed that the wind responds to the variability of the underlying continuum and these variations may be used to understand the actual accretion regime and the nature of the source.

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arXiv2025

The Broadband View of the Bare Seyfert PG 1426+015: Relativistic Reflection, the Soft Excess and the Importance of Oxygen

We present results from a deep, coordinated $XMM$-$Newton$ + $NuSTAR$ observation of the type 1 Seyfert PG 1426+015, a source of particular interest as the most massive reverberation-mapped black hole to date ($\log [M_{\rm{BH}}/M_{\odot}]$ = $9.01^{+0.11}_{-0.16}$). The high-resolution RGS data confirm the 'bare' nature of the source, showing no evidence for absorption beyond the Galactic column, while the broadband spectrum unambiguously reveals the presence of relativistic reflection from the innermost accretion disc (in the form of a relativistically broadened iron emission and associated Compton reflection hump) as well as confirming the presence of the strong soft excess reported previously. We explore whether relativistic reflection can successfully account for the soft excess along with the higher-energy reflection features, utilizing the two most-commonly used reflection codes (REFLIONX, XILLVER). Ultimately we find that both models are able to successfully reproduce the soft excess, though in the case of the XILLVER model this is contingent on reducing the strength of the O VIII line included in the model, as otherwise this feature prevents the model from reproducing the data. The reflection models that successfully reproduce the broadband data imply a relatively high density for the accretion disc of $\log [n_{\rm{e}} / \rm{cm}^{-3}] \sim 18$, consistent with the loose anti-correlation seen from other AGN in the $\log [n_{\rm{e}} / \rm{cm}^{-3}]$ vs $\log[m_{\rm{BH}} \dot{m}^2]$ plane, as well as a moderate-to-high black hole spin of $a^* \gtrsim 0.7$. This preliminary spin constraint is strongly dependent on the assumption that the soft excess is dominated by relativistic reflection.

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arXiv2025

A new pulsating neutron star in the Ultraluminous X-ray source NGC 4559 X7?

Ultraluminous X-ray sources (ULX) are extragalactic objects with X-ray luminosities above the Eddington limit for a 10 Msun black hole (BH). ULXs may host super-Eddington accreting neutron stars or stellar mass BH, although the exact proportion of the two populations is not yet known. We investigate the properties of the ULX NGC 4559 X7, which shows flux variability up to a factor of 5 on months-to-years and hours-to-days timescales. A flaring activity was also observed during the source highest flux epochs. Flares are unpredictable, with different durations and all flat-topped in flux. The latter suggests that, at the flare peaks, there is likely a common switch-off mechanism for the accretion onto the compact object. We analysed all the available XMM-Newton and Swift/XRT observations to investigate the spectral and temporal evolution of X7, looking for short and long-term variability. We look for long-term periodicities and for coherent signals through accelerated searches that included orbital corrections. We described the X7 spectra with two thermal components plus a cut-off powerlaw model. We found three well defined spectral states, where the spectral variability is mainly driven by the two harder components. In addition, a pulsed signal at 2.6-2.7s was detected in two XMM-Newton observations. The significance of these coherent signals is relatively weak but they are found in two different observations with the same parameter space for the orbital properties. If confirmed, it would imply a high spin-down of 1e-9 s/s, which could be extreme amongst the known pulsating ULXs. X7 would become a new extragalactic ULX pulsar. We discuss the spectral and temporal results of X7 in the context of super-Eddington accretion onto a stellar-mass compact object, in particular suggesting that the source might likely host a neutron star.

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arXiv2025

QPEs as Lense-Thirring precession of super-Eddington flows

Quasi-periodic eruptions (QPEs) are a recently identified class of X-ray transient associated with tidal disruption events by supermassive black holes, and for which there are multiple possible explanations. In this paper we present a simple model which requires the black hole be spinning, be misaligned with the accretion flow (both conditions of which are almost certainly met) and that the accretion rate is a few times the Eddington limit. We speculate that the resulting Lense-Thirring torques force the disc and entrained outflows to precess, leading to increased X-ray flux when the wind-cone is oriented at lower inclinations to the observer. We test the range of parameters for which this model could explain the period and brightness of the QPE events discovered thus far, and make qualitative comparisons between the observed X-ray spectra and lightcurves to those extracted from GR-RMHD simulations. Overall, we find some areas of promising concordance, and identify challenges related to the details of current simulations.

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arXiv2024

A New Broadband Spectral State in the Ultraluminous X-ray Source Holmberg IX X-1

We present a series of five new broadband X-ray observations of the ultraluminous X-ray source Holmberg IX X-1, performed by $XMM$-$Newton$ and $NuSTAR$ in coordination. The first three of these show high soft X-ray fluxes but a near total collapse of the high-energy ($\gtrsim$15 keV) emission, previously seen to be surprisingly stable across all prior broadband observations of the source. The latter two show a recovery in hard X-rays, remarkably once again respecting the same stable high-energy flux exhibited by all of the archival observations. We also present a joint analysis of all broadband observations of Holmberg IX X-1 to date (encompassing 11 epochs in total) in order to investigate whether it shows the same luminosity-temperature behaviour as NGC 1313 X-1 (which also shows a stable high-energy flux), whereby the hotter disc component in the spectrum exhibits two distinct, positively-correlated tracks in the luminosity-temperature plane. Holmberg IX X-1 may show similar behaviour, but the results depend on whether the highest energy emission is assumed to be an up-scattering corona or an accretion column. The strongest evidence for this behaviour is found in the former case, while in the latter the new 'soft' epochs appear distinct from the other high-flux epochs. We discuss possible explanations for these new 'soft' spectra in the context of the expected structure of super-Eddington accretion flows around black holes and neutron stars, and highlight a potentially interesting analogy with the recent destruction and re-creation of the corona seen in the AGN 1ES 1927+654.

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arXiv2024

Detection of a Highly Ionized Outflow in the Quasi-periodically Erupting Source GSN 069

Quasi-periodic eruptions (QPEs) are high-amplitude, soft X-ray bursts recurring every few hours, associated with supermassive black holes. Many interpretations for QPEs were proposed since their recent discovery in 2019, including extreme mass ratio inspirals and accretion disk instabilities. But, as of today, their nature still remains debated. We perform the first high-resolution X-ray spectral study of a QPE source using the RGS gratings onboard XMM-Newton, leveraging nearly 2 Ms of exposure on GSN 069, the first discovered source of this class. We resolve several absorption and emission lines including a strong line pair near the N VII rest-frame energy, resembling the P-Cygni profile. We apply photoionization spectral models and identify the absorption lines as an outflow blueshifted by $1700-2900$ km/s, with a column density of about $10^{22}$ cm$^{-2}$ and an ionization parameter $\log (\xi$/erg cm s$^{-1})$ of $3.9-4.6$. The emission lines are instead redshifted by up to 2900 km/s, and likely originate from the same outflow that imprints the absorption features, and covers the full $4\pi$ sky from the point of view of GSN 069. The column density and ionization are comparable to the outflows detected in some tidal disruption events, but this outflow is significantly faster and has a strong emission component. The outflow is more highly ionized when the system is in the phase during which QPEs are present, and from the limits we derive on its location, we conclude that the outflow is connected to the recent complex, transient activity of GSN 069 which began around 2010.

astro-ph.HE↗

arXiv2024

Constraining the Number Density of the Accretion Disk Wind in Hercules X-1 Using its Ionization Response to X-ray Pulsations

X-ray binaries are known to launch powerful accretion disk winds that can have significant impact on the binary systems and their surroundings. To quantify the impact and determine the launching mechanisms of these outflows, we need to measure the wind plasma number density, an important ingredient in the theoretical disk wind models. While X-ray spectroscopy is a crucial tool to understanding the wind properties, such as their velocity and ionization, in nearly all cases, we lack the signal-to-noise to constrain the plasma number density, weakening the constraints on outflow location and mass outflow rate. We present a new approach to determine this number density in the X-ray binary Hercules X-1 by measuring the speed of the wind ionization response to time-variable illuminating continuum. Hercules X-1 is powered by a highly magnetized neutron star, pulsating with a period of 1.24 s. We show that the wind number density in Hercules X-1 is sufficiently high to respond to these pulsations by modeling the ionization response with the time-dependent photoionization model TPHO. We then perform a pulse-resolved analysis of the best-quality XMM-Newton observation of Hercules X-1 and directly detect the wind response, confirming that the wind density is at least $10^{12}$ cm$^{-3}$. Finally, we simulate XRISM observations of Hercules X-1 and show that they will allow us to accurately measure the number density at different locations within the outflow. With XRISM we will rule out $\sim3$ orders of magnitude in density parameter space, constraining the wind mass outflow rate, energetics, and its launching mechanism.

astro-ph.HE↗

arXiv2023

Accretion disk wind of Hercules X-1 during the Short High state

Hercules X-1 is a nearly edge-on X-ray binary with a warped, precessing accretion disk, which manifests through a 35-day cycle of alternating High and Low flux states. This disk precession introduces a changing line of sight towards the X-ray source, through an ionized accretion disk wind. The sightline variation allows us to uniquely determine how the wind properties vary with height above the disk. All the previous wind measurements were made in the brighter Main High state of Her X-1. Here, we analyze the only Chandra observation during the fainter `Short' High state, and significantly detect blueshifted ionized absorption. We find a column density of $2.0_{-0.6}^{+1.1}\times10^{22}$ cm$^{-2}$, an ionization parameter $\log (\xi$/erg cm s$^{-1})=3.41_{-0.12}^{+0.15}$ and an outflow velocity of $380 \pm 40$ km/s. The properties of the outflow measured during the Short High state are in good agreement with those measured at equivalent precession phases during Main High. We conclude that we are sampling the same wind structure, seen during both Main and Short High, which is precessing alongside with the warped accretion disk every 35 days. Finally, the high spectral resolution of Chandra gratings above 1 keV in this observation enabled us to measure the abundances of certain elements in the outflow. We find Mg/O$=1.5_{-0.4}^{+0.5}$, Si/O$=1.5 \pm 0.4$ and S/O$=3.0_{-1.1}^{+1.2}$, whereas in our previous study of Her X-1 with XMM-Newton, we found an over-abundance of N, Ne and Fe compared with O. These peculiar abundance ratios were likely introduced by pollution of the donor by the supernova which created Her X-1.

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arXiv2023

Discovery of a variable multi-phase outflow in the X-ray-emitting tidal disruption event ASASSN-20qc

Tidal disruption events (TDEs) are exotic transients that can lead to temporary super-Eddington accretion onto a supermassive black hole. Such accretion mode is naturally expected to result in powerful outflows of ionized matter. However, to date such an outflow has only been directly detected in the X-ray band in a single TDE, ASASSN-14li. This outflow has a low velocity of just a few 100 km/s, although there is also evidence for a second, ultra-fast phase. Here we present the detection of a low-velocity outflow in a second TDE, ASASSN-20qc. The high-resolution X-ray spectrum reveals an array of narrow absorption lines, each blueshifted by a few 100 km/s, which cannot be described by a single photo-ionization phase. For the first time, we confirm the multiphase nature of a TDE outflow, with at least two phases and two distinct velocity components. One highly ionized phase is outflowing at $910^{+90}_{-80}$ km/s, while a lower ionization component is blueshifted by $400_{-120}^{+100}$ km/s. We perform time-resolved analysis of the X-ray spectrum and detect that, surprisingly, the mildly ionized absorber strongly varies in ionization parameter over the course of a single 60 ks observation, indicating that its distance from the black hole may be as low as 400 gravitational radii. We discuss these findings in the context of TDEs and compare this newly detected outflow with that of ASASSN-14li.

astro-ph.HE↗

arXiv2023

Vertical wind structure in an X-ray binary revealed by a precessing accretion disk

The accretion of matter onto black holes and neutron stars often leads to the launching of outflows that can greatly affect the environments surrounding the compact object. In supermassive black holes, these outflows can even be powerful enough to dictate the evolution of the entire host galaxy, and yet, to date, we do not understand how these so-called accretion disk winds are launched - whether by radiation pressure, magnetic forces, thermal irradiation, or a combination thereof. An important means of studying disk winds produced near the central compact object is through X-ray absorption line spectroscopy, which allows us to probe outflow properties along a single line of sight, but usually provides little information about the global 3D disk wind structure that is vital for understanding the launching mechanism and total wind energy budget. Here, we study Hercules X-1, a unique, nearly edge-on X-ray binary with a warped accretion disk precessing with a period of about 35 days. This disk precession results in changing sightlines towards the neutron star, through the ionized outflow. We perform time-resolved X-ray spectroscopy over the precession phase and detect a strong decrease in the wind column density by three orders of magnitude as our sightline progressively samples the wind at greater heights above the accretion disk. The wind becomes clumpier as it rises upwards and expands away from the neutron star. Modelling the warped disk shape, we create a 2D map of wind properties. This unique measurement of the vertical structure of an accretion disk wind allows direct comparisons to 3D global simulations to reveal the outflow launching mechanism.

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arXiv2022

The long stare at Hercules X-1 -- I. Emission lines from the outer disk, the magnetosphere boundary and the accretion curtain

Hercules X-1 is a nearly edge-on accreting X-ray pulsar with a warped accretion disk, precessing with a period of about 35 days. The disk precession allows for unique and changing sightlines towards the X-ray source. To investigate the accretion flow at a variety of sightlines, we obtained a large observational campaign on Her X-1 with XMM-Newton (380 ks exposure) and Chandra (50 ks exposure) for a significant fraction of a single disk precession cycle, resulting in one of the best datasets taken to date on a neutron star X-ray binary. Here we present the spectral analysis of the High State high-resolution grating and CCD datasets, including the extensive archival data available for this famous system. The observations reveal a complex Fe K region structure, with three emission line components of different velocity widths. Similarly, the high-resolution soft X-ray spectra reveal a number of emission lines of various widths. We correct for the uncertain gain of the EPIC-pn Timing mode spectra, and track the evolution of these spectral components with Her X-1 precession phase and observed luminosity. We find evidence for three groups of emission lines: one originates in the outer accretion disk (10^5 RG from the neutron star). The second line group plausibly originates at the boundary between the inner disk and the pulsar magnetosphere (10^3 RG). The last group is too broad to arise in the magnetically-truncated disk and instead must originate very close to the neutron star surface, likely from X-ray reflection from the accretion curtain (~10^2 RG).

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arXiv2022

Unveiling the disc structure in ultraluminous X-ray source NGC 55 ULX-1

Despite two decades of studies, it is still not clear whether ULX spectral transitions are due to stochastic variability in the wind or variations in the accretion rate or in the source geometry. The compact object is also unknown for most ULXs. In order to place constraints onto such scenarios and on the structure of the accretion disc, we studied the temporal evolution of the spectral components of the variable source NGC 55 ULX-1. Using recent and archival data obtained with the XMM-Newton satellite, we modelled the spectra with two blackbody components which we interpret as thermal emission from the inner accretion flow and the regions around or beyond the spherization radius. The luminosity-temperature (L-T) relation of each spectral component agrees with the L proportional T^4 relationship expected from a thin disc model, which suggests that the accretion rate is close to the Eddington limit. However, there are some small deviations at the highest luminosities, possibly due to an expansion of the disc and a contribution from the wind at higher accretion rates. Assuming that such deviations are due to the crossing of the Eddington or supercritical accretion rate, we estimate a compact object mass of 6-14 Msun, favouring a stellar-mass black hole as the accretor.

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arXiv2022

A transient ultraluminous X-ray source in NGC 55

Ultraluminous X-ray sources (ULXs) are a class of accreting compact objects with X-ray luminosities above 10$^{39}$ erg s$^{-1}$. The average number of ULXs per galaxy is still not well constrained, especially given the uncertainty on the fraction of ULX transients. Here, we report the identification of a new transient ULX in the galaxy NGC 55 (which we label as ULX-2), thanks to recent XMM-Newton and the Neil Gehrels Swift Observatory observations. This object was previously classified as a transient X-ray source with a luminosity around a few 10$^{38}$ erg s$^{-1}$ in a 2010 XMM-Newton observation. Thanks to new and deeper observations ($\sim$ 130 ks each), we show that the source reaches a luminosity peak $>1.6 \times 10^{39}$ erg s$^{-1}$. The X-ray spectrum of ULX-2 is much softer than in previous observations and fits in the class of soft ULXs. It can be well described using a model with two thermal components, as often found in ULXs. The time scales of the X-ray variability are of the order of a month and are likely driven by small changes in the accretion rate or due to super-orbital modulations, attributed to precession of the accretion disc, which is similar to other ULXs.

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arXiv2021

Ionised Emission and Absorption in a Large Sample of Ultraluminous X-ray Sources

Most Ultraluminous X-ray sources (ULXs) are thought to be powered by super-Eddington accretion onto stellar-mass compact objects. Accretors in this extreme regime are naturally expected to ionise copious amounts of plasma in their vicinity and launch powerful radiation-driven outflows from their discs. High spectral resolution X-ray observations (with RGS gratings onboard XMM-Newton) of a few ULXs with the best datasets indeed found complex line spectra and confirmed such extreme (0.1-0.3c) winds. However, a search for plasma signatures in a large ULX sample with a rigorous technique has never been performed, thereby preventing us from understanding their statistical properties such as the rate of occurrence, to constrain the outflow geometry and its duty cycle. We developed a fast method for automated line detection in X-ray spectra and applied it to the full RGS ULX archive, rigorously quantifying the statistical significance of any candidate lines. Collecting the 135 most significant features detected in 89 observations of 19 objects, we created the first catalogue of spectral lines detected in soft X-ray ULX spectra. We found that the detected emission lines are concentrated around known rest-frame elemental transitions and thus originate from low-velocity material. The absorption lines instead avoid these transitions, suggesting they were imprinted by blueshifted outflows. Such winds therefore appear common among the ULX population. Additionally, we found that spectrally hard ULXs show fewer line detections than soft ULXs, indicating some difference in their accretion geometry and orientation, possibly causing over-ionisation of plasma by the harder spectral energy distributions of harder ULXs.

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arXiv2021

The nature of the extreme X-ray variability in the NLS1 1H 0707-495

We examine archival XMM-Newton data on the extremely variable narrow-line Seyfert 1 (NLS1) active galactic nucleus (AGN) 1H 0707-495. We construct fractional excess variance (Fvar) spectra for each epoch, including the recent 2019 observation taken simultaneously with eROSITA. We explore both intrinsic and environmental absorption origins for the variability in different epochs, and examine the effect of the photoionised emission lines from outflowing gas. In particular, we show that the unusual soft variability first detected by eROSITA in 2019 is due to a combination of an obscuration event and strong suppression of the variance at 1 keV by photoionised emission, which makes the variance below 1 keV appear more extreme. We also examine the variability on long timescales, between observations, and find that it is well described by a combination of intrinsic variability and absorption variability. We suggest that the typical extreme high frequency variability which 1H 0707-495 is known for is intrinsic to the source, but the large amplitude, low frequency variability that causes prolonged low-flux intervals is likely dominated by variable low-ionisation, low velocity absorption.

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arXiv2021

The chameleon on the branches: spectral state transition and dips in NGC 247 ULX-1

Soft Ultra-Luminous X-ray (ULXs) sources are a subclass of the ULXs that can switch from a supersoft spectral state, where most of the luminosity is emitted below 1 keV, to a soft spectral state with significant emission above 1 keV. In a few systems, dips have been observed. The mechanism behind this state transition and the dips nature are still debated. To investigate these issues, we obtained a long XMM-Newton monitoring campaign of a member of this class, NGC 247 ULX-1. We computed the hardness-intensity diagram for the whole dataset and identified two different branches: the normal branch and the dipping branch, which we study with four and three hardness-intensity resolved spectra, respectively. All seven spectra are well described by two thermal components: a colder ($kT_{\rm bb}$ $\sim$ 0.1-0.2 keV) black-body, interpreted as emission from the photo-sphere of a radiatively-driven wind, and a hotter ($kT_{\rm disk}$ $\sim$ 0.6 keV) multicolour disk black-body, likely due to reprocessing of radiation emitted from the innermost regions. In addition, a complex pattern of emission and absorption lines has been taken into account based on previous high-resolution spectroscopic results. We studied the evolution of spectral parameters and the flux of the two thermal components along the two branches and discuss two scenarios possibly connecting the state transition and the dipping phenomenon. One is based on geometrical occultation of the emitting regions, the other invokes the onset of a propeller effect.

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