arXiv subjects

Jeroen Homan

Publications and source records attributed to Jeroen Homan.

At least 19 recordsLinked to original sources

arXiv2026

X-ray and Radio campaign of the Z-source GX 340+0: discovery of X-ray polarization and its implications

We present the discovery of X-ray polarization from the neutron star low-mass X-ray binary and Z-source, GX~340$+$0, using an Imaging X-ray Polarimetry Explorer (IXPE) observation in March 2024. Along with the IXPE observation, we conducted an extensive X-ray and radio monitoring campaign to ascertain the source properties during and around the IXPE observation. The source was within the horizontal branch throughout the multiwavelength campaign. We measured a significant X-ray polarization in 2--8 keV with polarization degree (PD) = $4.02 \pm 0.35$% and polarization angle (PA) = $37.6 \pm 2.5^\circ$. The energy-dependent polarization indicates that in the 2-2.5 keV energy range, the PA is much lower, $\sim9\pm8^\circ$, while other energy bands are consistent with the PA found over 2.5--8 keV. The simultaneous AstroSat-IXPE spectro-polarimetric observations provide some evidence for independent polarization from various spectral components, hinting at a disparity in the PA from the accretion disk and the Comptonized emission, while suggesting an unpolarized emission from the blackbody component. Radio observations in the 0.7--9 GHz frequency range reveal a non-detection of radio emission in 0.7-1.5 GHz and a significant detection in 5.5--9 GHz, suggesting the presence of a spectral break in 1.5-5.5 GHz. Using ATCA observation we place upper limits on the radio polarization at $<$6% on the linear polarization and $<$4% on the circular polarization at 3$σ$ level. We discuss the origin of the X-ray polarization and its implications on the geometry of the spectral components.

astro-ph.HE↗

arXiv2025

Dual perspectives on GX 17+2: a simultaneous NICER and NuSTAR study

We performed the first simultaneous NICER & NuSTAR spectral and timing study of the Sco-like Z source GX 17+2. The source traced the full Z track during four observations. We detect signatures of relativistic reflection in the broadband spectra and report results using a reflection framework. The disk is relatively close to the innermost stable circular orbit ($\sim$ 1-4 R$_{ISCO}$), which agrees with previous studies of GX 17+2, but the location of the inner disk is farther out in the horizontal branch (HB) and moves inward toward the flaring branch (FB). We find the FB to be the point of closest approach of the disk to the neutron star. We qualitatively conclude that the evolution of the source along the HID is that of a relatively truncated disk in the HB ($\sim$ 4 R$_{ISCO}$) that approaches the neutron star as it goes along the HID towards the normal branch (NB), soft apex (SA), and finally the FB. We attribute the source evolution along the Z track to varying mass accretion rate and disk instabilities. Rms variability increases from the NB towards the SA and then drops to a constant along the FB indicating that the observed variability likely originates from the disk/boundary layer rather than the corona.

astro-ph.HE↗

arXiv2025

A candidate to the long sought optical counterpart to the Rapid Burster in the bulge fossil fragment Liller 1

We report on the possible identification of the optical counterpart of the Rapid Burster MXB 1730-335 in the stellar system Liller 1. The identification was performed by taking advantage of a set of images acquired with the Hubble Space Telescope/Advanced Camera for Surveys in the optical band, and with the Gemini South Telescope in the near-infrared. The analysis of these images revealed the presence of a star with a position possibly compatible with the X-ray and radio band coordinates of the Rapid Burster, and showing significant optical variability. According to its location in the color-magnitude diagram, the candidate companion appears to belong to the young (~ 1-2 Gyr old) super-solar metallicity ([M/H]= +0.3) sub-population recently discovered in Liller 1. We discuss the main characteristics of the candidate counterpart and the Rapid Burster binary system as derived from the available data, also highlighting the need for further coordinated observations to solidly confirm their association and better clarify their physical properties.

astro-ph.SR↗

arXiv2025

A Multi-wavelength Characterization of the 2023 Outburst of MAXI J1807+132: Manifestations of Disk Instability and Jet Emission

Several phenomenological aspects of low-luminosity neutron star transients, such as atolls, remain poorly understood. One such source, MAXI J1807+132, entered its latest outburst in July 2023. To thoroughly characterize this outburst, we conducted an extensive observational campaign spanning radio to X-ray wavelengths. Here, we present the results of this campaign, which covered the period from before the outburst to the return to quiescence. We detected a delay between the X-ray and optical rise times, which is consistent with the predictions of the disk instability model with a truncated disk. The color evolution and optical/X-ray correlations, along with infrared and radio detections, support the presence of jet synchrotron emission during the gradual decay phase following the peak. We also report for the first time in an X-ray binary a near-orthogonal rotation of the optical polarization just before a small flare, after which the jet is thought to be quenched. The main outburst is followed by several high-amplitude, rapid reflares in the optical, ultraviolet, and X-ray bands, the origin of which remains difficult to constrain.

astro-ph.HE↗

arXiv2025

X-ray and Radio Campaign of the Z-source GX 340+0 II: the X-ray polarization in the normal branch

We present the first X-ray polarization measurement of the neutron star low-mass X-ray binary and Z-source, GX 340$+$0, in the normal branch (NB) using a 200 ks observation with the Imaging X-ray Polarimetric Explorer (IXPE). This observation was performed in 2024 August. Along with IXPE, we also conducted simultaneous observations with NICER, AstroSat, Insight-HXMT, ATCA, and GMRT to investigate the broadband spectral and timing properties in the X-ray and radio wavelengths. During the campaign, the source traced a complete Z-track during the IXPE observation but spent most of the time in the NB. We measure X-ray polarization degree (PD) of $1.22\pm0.25\%$ in the 2-8 keV energy band with a polarization angle (PA) of $38\pm6^\circ$. The PD in the NB is observed to be weaker than in the horizontal branch (HB) but aligned in the same direction. The PD of the source exhibits a marginal increase with energy while the PA shows no energy dependence. The joint spectro-polarimetric modeling is consistent with the observed X-ray polarization originating from a single spectral component from the blackbody or the Comptonized emission while the disk emission does not contribute towards the X-ray polarization. GMRT observations at 1.26 GHz during HB had a tentative detection at 4.5$\pm$0.7 mJy while ATCA observations a day later during the NB detected the source at 0.70$\pm$0.05 mJy and 0.59$\pm$0.05 mJy in the 5.5 & 9 GHz bands, respectively, suggesting an evolving jet structure depending on the Z-track position.

astro-ph.HE↗

arXiv2025

A persistent disk wind and variable jet outflow in the neutron-star low-mass X-ray binary GX 13+1

In low-mass X-ray binaries (LMXBs), accretion flows are often associated with either jet outflows or disk winds. Studies of LMXBs with luminosities up to roughly 20% of the Eddington limit indicate that these outflows generally do not co-occur, suggesting that disk winds might inhibit jets. However, previous observations of LMXBs accreting near or above the Eddington limit show that jets and winds can potentially coexist. To investigate this phenomenon, we carried out a comprehensive multi-wavelength campaign (using VLA, Chandra/HETG, and NICER) on the near-Eddington neutron-star Z source LMXB GX 13+1. NICER and Chandra/HETG observations tracked GX 13+1 across the entire Z-track during high Eddington rates, detecting substantial resonance absorption features originating from the accretion disk wind in all X-ray spectra, which implies a persistent wind presence. Simultaneous VLA observations captured a variable radio jet, with radio emission notably strong during all flaring branch observations-contrary to typical behavior in Z-sources-and weaker when the source was on the normal branch. Interestingly, no clear correlation was found between the radio emission and the wind features. Analysis of VLA radio light curves and simultaneous Chandra/HETG spectra demonstrates that an ionized disk wind and jet outflow can indeed coexist in GX 13+1, suggesting that their launching mechanisms are not necessarily linked in this system.

astro-ph.HE↗

arXiv2024

Evolution of the Accretion Disk and Corona During the Outburst of the Neutron Star Transient MAXI J1807+132

Low-mass X-ray binaries with a neutron star as the primary object show a complex array of phenomenology during outbursts. The observed variability in X-ray emission primarily arises from changes in the innermost regions of the accretion disk, neutron star surface, and corona. In this work, we present the results of a comprehensive X-ray spectral and timing analysis of the neutron star transient MAXI J1807+132 during its 2023 outburst using data from the NICER observatory. The outburst is marked by a very rapid rise in the count rate by about a factor of 20 in a day. The source undergoes full state transitions and displays hysteresis effect in the hardness and rms intensity diagrams. Spectral analysis with a three-component model is consistent with disk truncation during the hard states and reaching the last stable orbit during the intermediate and soft states. We discuss the different values of the last stable radius in the context of possible distance of the source and magnetic field strength. The characteristic frequencies throughout the hard and intermediate states are found to be strongly correlated with the inner radius of the disk. Together with the spectral and fast variability properties, we attempt to trace the evolution of the size of the corona along the outburst. Following the main outburst, the source undergoes a high amplitude reflare wherein it shows a complex behavior with relatively high variability (10 %), but low hardness.

astro-ph.HE↗

arXiv2024

Evidence for a dynamic corona in the short-term time lags of black hole X-ray binary MAXI J1820+070

In X-ray observations of hard state black hole X-ray binaries, rapid variations in accretion disc and coronal power-law emission are correlated and show Fourier-frequency-dependent time lags. On short (~0.1 s) time-scales, these lags are thought to be due to reverberation and therefore may depend strongly on the geometry of the corona. Low-frequency quasi-periodic oscillations (QPOs) are variations in X-ray flux that have been suggested to arise because of geometric changes in the corona, possibly due to General Relativistic Lense-Thirring precession. Therefore one might expect the short-term time lags to vary on the QPO time-scale. We performed novel spectral-timing analyses on NICER observations of the black hole X-ray binary MAXI J1820+070 during the hard state of its outburst in 2018 to investigate how the short-term time lags between a disc-dominated and a coronal power-law-dominated energy band vary on different time-scales. Our method can distinguish between variability due to the QPO and broadband noise, and we find a linear correlation between the power-law flux and lag amplitude that is strongest at the QPO frequency. We also introduce a new method to resolve the QPO signal and determine the QPO-phase-dependence of the flux and lag variations, finding that both are very similar. Our results are consistent with a geometric origin of QPOs, but also provide evidence for a dynamic corona with a geometry varying in a similar way over a broad range of time-scales, not just the QPO time-scale.

astro-ph.HE↗

arXiv2024

Discovery of a second eclipsing, bursting neutron-star low-mass X-ray binary in the globular cluster Terzan 6

We have analyzed Chandra and Suzaku observations of the globular cluster Terzan 6, made when the recurrent transient GRS 1747-312 was in quiescence. Our analysis reveals the presence of a second eclipsing, bursting neutron-star low-mass X-ray binary in the central regions of the cluster, in addition to GRS 1747-312. The new source, which we name Terzan 6 X2, is located only ~0.7 arcsec away from GRS 1747-312 in the 2021 Chandra images. The detection of a 5.14 ks-long eclipse in the light curve of X2 at a time not predicted by the ephemeris of GRS 1747-312 confirms that it is an unrelated source. Using the Suzaku light curve from 2009, which in addition to a type-I X-ray burst also showed an eclipse-like feature, we constrain the orbital period to be longer than 16.27 h. The 0.5-10 keV luminosities of X2 vary in the range of ~0.24-5.9x10^34 erg/s on time scales of months to years. We have identified a plausible optical counterpart of X2 in HST F606W and F814W images. This star varied by 2.7 mag in V_606 between epochs separated by years. In the cluster color-magnitude diagram, the variable counterpart lies in the blue-straggler region when it was optically bright, about 1.1-1.7 mag above the main-sequence turn-off. From the orbital period-density relation of Roche-lobe filling stars we find the mass-donor radius to be >0.8 Rsun.

astro-ph.HE↗

arXiv2024

X-ray and Radio Monitoring of the Neutron Star Low Mass X-ray Binary 1A 1744-361: Quasi Periodic Oscillations, Transient Ejections, and a Disk Atmosphere

We report on X-ray (NICER/NuSTAR/MAXI/Swift) and radio (MeerKAT) timing and spectroscopic analysis from a three-month monitoring campaign in 2022 of a high-intensity outburst of the dipping neutron star low-mass X-ray binary 1A 1744-361. The 0.5-6.8 keV NICER X-ray hardness-intensity and color-color diagrams of the observations throughout the outburst suggests that 1A 1744-361 spent most of its outburst in an atoll-state, but we show that the source exhibited Z-state-like properties at the peak of the outburst, similar to a small sample of other atoll-state sources. A timing analysis with NICER data revealed several instances of an $\approx8$ Hz quasi-periodic oscillation (QPO; fractional rms amplitudes of ~5%) around the peak of the outburst, the first from this source, which we connect to the normal branch QPOs (NBOs) seen in the Z-state. Our observations of 1A 1744-361 are fully consistent with the idea of the mass accretion rate being the main distinguishing parameter between atoll- and Z-states. Radio monitoring data by MeerKAT suggests that the source was at its radio-brightest during the outburst peak, and that the source transitioned from the 'island' spectral state to the 'banana' state within ~3 days of the outburst onset, launching transient jet ejecta. The observations present the strongest evidence for radio flaring, including jet ejecta, during the island-to-banana spectral state transition at low accretion rates (atoll-state). The source also exhibited Fe XXV, Fe XXVI K$α$, and K$β$ X-ray absorption lines, whose origins likely lie in an accretion disk atmosphere.

astro-ph.HE↗

arXiv2024

Probing the accretion geometry of the atoll source 4U 1702-429 in different spectral states with NICER, NuSTAR, and AstroSat

We perform a comprehensive spectral study of a carefully selected sample (total exposure $\sim 50.5$ ks) of NICER observations of the atoll neutron star low-mass X-ray binary 4U 1702-429. Our sample encompasses nearly all classical spectral states found within the NICER dataset. We require two thermal emission components, originating from the accretion disc and the boundary layer, to describe the soft state spectra in the energy band 0.3-10.0 keV. In contrast, in our model, only the disc component directly contributes to the intermediate/hard state. Additionally, we use a thermally Comptonized component (or a power-law with pegged normalization) to represent the hard coronal emission in the soft and intermediate/hard state spectra. The boundary layer emerges as the principal source providing soft seed photons for Comptonization across all spectral states. In contrast to a previously held assertion regarding this source, our analyses reveal a decrease in the inner disc temperature coupled with the retreat of the inner disc from the neutron star surface as the source evolves from the soft to the intermediate/hard state. The reflection features are either absent or weak ($\sim 3σ-4σ$) in all these observations. Further investigation using broad-band NuSTAR (3.0-50.0 keV) and AstroSat spectra (1.3-25.0 keV) shows a slightly stronger iron emission line ($\sim 5.8σ$) in the NuSTAR spectra. However, this feature is not significantly detected in the AstroSat observation. The AstroSat data suggests a highly ionized disc, explaining the absence of reflection features. In the case of NuSTAR, a truncated disc is likely responsible for the weak reflection features.

astro-ph.HE↗

arXiv2024

Chasing the break: Tracing the full evolution of a black hole X-ray binary jet with multi-wavelength spectral modeling

Black hole X-ray binaries (BH XRBs) are ideal targets to study the connection between accretion inflow and jet outflow. Here we present quasi-simultaneous, multi-wavelength observations of the Galactic black hole system MAXI J1820+070, throughout its 2018-2019 outburst. Our data set includes coverage from the radio through X-ray bands from 17 different instruments/telescopes, and encompasses 19 epochs over a 7 month time period, resulting in one of the most well-sampled multi-wavelength data sets of a BH XRB outburst to date. With our data, we compile and model the broad-band spectra of this source using a phenomenological model that includes emission from the jet, companion star, and accretion flow. This modeling allows us to track the evolution of the spectral break in the jet spectrum, a key observable that samples the jet launching region. We find that the spectral break location changes over at least $\approx3$ orders of magnitude in electromagnetic frequency over this period. Using these spectral break measurements, we link the full cycle of jet behavior, including the rising, quenching, and re-ignition, to the changing accretion flow properties as the source evolves through its different accretion states. Our analyses show a consistent jet behavior with other sources in similar phases of their outbursts, reinforcing that the jet quenching and recovery may be a global feature of BH XRB systems in outburst. Our results also provide valuable evidence supporting a close connection between the geometry of the inner accretion flow and the base of the jet.

astro-ph.HE↗

arXiv2024

The 2022 Outburst of IGR J17091-3624: Connecting the exotic GRS 1915+105 to standard black hole X-ray binaries

While the standard X-ray variability of black hole X-ray binaries (BHXBs) is stochastic and noisy, there are two known BHXBs that exhibit exotic `heartbeat'-like variability in their light curves: GRS 1915+105 and IGR J17091-3624. In 2022, IGR J17091-3624 went into outburst for the first time in the NICER/NuSTAR era. These exquisite data allow us to simultaneously track the exotic variability and the corresponding spectral features with unprecedented detail. We find that as in typical BHXBs, the outburst began in the hard state, then the intermediate state, but then transitioned to an exotic soft state where we identify two types of heartbeat-like variability (Class V and a new Class X). The flux-energy spectra show a broad iron emission line due to relativistic reflection when there is no exotic variability, and absorption features from highly ionized iron when the source exhibits exotic variability. Whether absorption lines from highly ionized iron are detected in IGR J17091-3624 is not determined by the spectral state alone, but rather is determined by the presence of exotic variability; in a soft spectral state, absorption lines are only detected along with exotic variability. Our finding indicates that IGR J17091-3624 can be seen as a bridge between the most peculiar BHXB GRS 1915+105 and `normal' BHXBs because it alternates between the conventional and exotic behavior of BHXBs. We discuss the physical nature of the absorbing material and exotic variability in light of this new legacy dataset.

astro-ph.HE↗

arXiv2024

Highly-coherent quasi-periodic oscillations in the 'heartbeat' black hole X-ray binary IGR J17091-3624

IGR J17091-3624 is a black hole X-ray binary (BHXB), often referred to as the 'twin' of GRS 1915+105 because it is the only other known BHXB that can show exotic 'heartbeat'-like variability that is highly structured and repeated. Here we report on observations of IGR J17091-3624 from its 2022 outburst, where we detect an unusually coherent quasi-periodic oscillation (QPO) when the broadband variability is low (total fractional rms $\lesssim$ 6%) and the spectrum is dominated by the accretion disk. Such spectral and variability behavior is characteristic of the soft state of typical BHXBs (i.e., those that do not show heartbeats), but we also find that this QPO is strongest when there is some exotic heartbeat-like variability (so-called Class V variability). This QPO is detected at frequencies between 5 and 8 Hz and has Q-factors (defined as the QPO frequency divided by the width) $\gtrsim$ 50, making it one of the most highly coherent low-frequency QPO ever seen in a BHXB. The extremely high Q factor makes this QPO distinct from typical low-frequency QPOs that are conventionally classified into Type-A/B/C QPOs. Instead, we find evidence that archival observations of GRS 1915+105 also showed a similarly high-coherence QPO in the same frequency range, suggesting that this unusually coherent and strong QPO may be unique to BHXBs that can exhibit 'heartbeat'-like variability.

astro-ph.HE↗

arXiv2023

Variability as a predictor for the hard-to-soft state transition in GX 339-4

During the outbursts of black hole X-ray binaries (BHXRBs), their accretion flows transition through several states. The source luminosity rises in the hard state, dominated by non-thermal emission, before transitioning to the blackbody-dominated soft state. As the luminosity decreases, the source transitions back into the hard state and fades to quiescence. This picture does not always hold, as $\approx$ 40$\%$ of the outbursts never leave the hard state. Identifying the physics that govern state transitions remains one of the outstanding open questions in black hole astrophysics. In this paper we present an analysis of archival RXTE data of multiple outbursts of GX 339-4. We compare the properties of the X-ray variability and time-averaged energy spectrum and demonstrate that the variability (quantified by the power spectral hue) systematically evolves $\approx$ 10-40 days ahead of the canonical state transition (quantified by a change in spectral hardness); no such evolution is found in hard state only outbursts. This indicates that the X-ray variability can be used to predict if and when the hard-to-soft state transition will occur. Finally, we find a similar behavior in ten outbursts of four additional BHXRBs with more sparse observational coverage. Based on these findings, we suggest that state transitions in BHXRBs might be driven by a change in the turbulence in the outer regions of the disk, leading to a dramatic change in variability. This change is only seen in the spectrum days to weeks later, as the fluctuations propagate inwards towards the corona.

astro-ph.HE↗

arXiv2023

AstroSat view of the neutron star low-mass X-ray binary GX 340+0

Understanding the spectral evolution along the `Z'-shaped track in the hardness-intensity diagram of Z-sources, which are a class of luminous neutron star low-mass X-ray binaries, is crucial to probe accretion processes close to the neutron star. Here, we study the horizontal branch (HB) and the normal branch (NB) of the Z source GX 340+0 using $AstroSat$ data. We find that the HB and the NB appear as two different types of X-ray intensity dips, which can appear in any sequence and with various depths. Our $0.8-25$ ~keV spectra of dips and the hard apex can be modeled by the emissions from an accretion disk, a Comptonizing corona covering the inner disk, and the neutron star surface. We find, as the source moves onto the HB the corona is replenished and energized by the disk and a reduced amount of disk matter reaches the neutron star surface. We also conclude that quasi-periodic oscillations during HB/NB are strongly associated with the corona, and explain the evolution of strength and hard-lag of this timing feature using the estimated coronal optical depth evolution.

astro-ph.HE↗

arXiv2023

Simultaneous NICER and NuSTAR Observations of the Ultra-compact X-ray Binary 4U 0614+091

We present the first joint NuSTAR and NICER observations of the ultra-compact X-ray binary (UCXB) 4U 0614+091. This source shows quasi-periodic flux variations on the timescale of ~days. We use reflection modeling techniques to study various components of the accretion system as the flux varies. We find that the flux of the reflected emission and the thermal components representing the disk and the compact object trend closely with the overall flux. However, the flux of the power-law component representing the illuminating X-ray corona scales in the opposite direction, increasing as the total flux decreases. During the lowest flux observation, we see evidence of accretion disk truncation from roughly 6 gravitational radii to 11.5 gravitational radii. This is potentially analogous to the truncation seen in black hole low-mass X-ray binaries, which tends to occur during the low/hard state at sufficiently low Eddington ratios.

astro-ph.HE↗

arXiv2022

A multi-wavelength study of GRS 1716-249 in outburst : constraints on its system parameters

We present a detailed study of the evolution of the Galactic black hole transient GRS 1716-249 during its 2016-2017 outburst at optical (Las Cumbres Observatory), mid-infrared (Very Large Telescope), near-infrared (Rapid Eye Mount telescope), and ultraviolet (the Neil Gehrels Swift Observatory Ultraviolet/Optical Telescope) wavelengths, along with archival radio and X-ray data. We show that the optical/near-infrared and UV emission of the source mainly originates from a multi-temperature accretion disk, while the mid-infrared and radio emission are dominated by synchrotron emission from a compact jet. The optical/UV flux density is correlated with the X-ray emission when the source is in the hard state, consistent with an X-ray irradiated accretion disk with an additional contribution from the viscous disk during the outburst fade. We also report the long-term optical light curve of the source and find that the quiescent i-band magnitude is 21.39$\pm$0.15 mag. Furthermore, we discuss how previous estimates of the system parameters of the source are based on various incorrect assumptions, and so are likely to be inaccurate. By comparing our GRS 1716-249 dataset to those of other outbursting black hole X-ray binaries, we find that while GRS 1716-249 shows similar X-ray behaviour, it is noticeably optically fainter, if the literature distance of 2.4 kpc is adopted. Using several lines of reasoning, we argue that the source distance is further than previously assumed in the literature, likely within 4-17 kpc, with a most likely range of $\sim$4-8 kpc.

astro-ph.HE↗