arXiv subjects

Arash Bahramian

Publications and source records attributed to Arash Bahramian.

At least 19 recordsLinked to original sources

arXiv2026

A Real-Time Jet Laboratory in Swift J1727.8-1613

We present the results of our intensive VLBI campaign on the black-hole low-mass X-ray binary (LMXB) Swift J1727.8-1613 during its 2023-2024 outburst. We observed the repeated quenching and re establishment of the highly-extended continuous jet during several transitions between hard-intermediate and soft-intermediate states, and the repeated ejection of transient jets. Using time-dependent visibility model fitting, we tracked the motion of nine discrete jet knots, obtaining some of the most precise measurements of transient jet proper motions and ejection dates in an LMXB. These ejecta were only detectable for a short time with VLBI, and some showed rapid intra-observation flux density variability that was not captured in image reconstructions. For the first time, we use time-dependent visibility modelling to fit a piecewise model for the jet knot flux densities, allowing us to create complex, non-parametric light curves of their intra-observation variability. We observed the launching of multiple ejecta across several state transitions, however, we could not identify a consistent signature of jet ejection in the available X-ray intensity or hardness data. We constrained the intrinsic speeds and bulk Lorentz factors of the jet knots, finding that Swift J1727.8-1613 launched both mildly relativistic and highly relativistic ejecta throughout its outburst. We used their proper motions to constrain a posterior distribution for the maximum inclination angle of the jet axis, which had 50th, 84th, and 99th percentiles of 40°, 50°, and 66°, respectively. These unique observations of the repeated ejection of transient jets by a single LMXB reveal that fixed parameters such as black-hole mass, black-hole spin, and spin-orbit misalignment do not uniquely determine the varying properties of transient jets, particularly their speeds and Lorentz factors.

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arXiv2026

A new likely pulsar binary in 47~Tucanae from continuum searches

We present evidence that the X-ray source 47~Tuc~W41, long considered to be an X-ray source powered by coronal activity, is actually a redback pulsar binary. The source continually shows $L_X = 3 \times 10^{31}$ erg s$^{-1}$ (0.5--10 keV), which is well in excess of the coronal saturation limit for active binaries in quiescence, but is consistent with the intrabinary shock observed in redback pulsars, as is its photon index of $Γ= 1.4\pm0.1$. In addition, using deep data from the Australia Telescope Compact Array, we show that W41 is a faint ($9.8\pm1.4 μ$Jy at 5.5 GHz) steep spectrum ($α= -1.8\pm0.4$) radio continuum source, as expected for a pulsar. Light curve modelling of Hubble Space Telescope photometry shows evidence for ellipsoidal modulation with mild irradiation of a $\sim 0.5$--$0.55 M_{\odot}$ secondary around an invisible companion, also consistent with the redback interpretation. The precise position of W41, along with its well-measured 10.4-hr orbital period, should enable it to be matched to a newly-discovered radio pulsar in future data. Our result shows that close pulsar binaries are still hiding, misclassified among active binaries, even in well-studied clusters like 47 Tuc.

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arXiv2026

The Homogeneous MeerKAT and Swift/XRT X-ray Binary Radio:X-ray Plane

During the hard and quiescent spectral states in X-ray binaries, a non-linear correlation is observed between radio and X-ray luminosities, providing a valuable tool to probe the connection between accretion and jet production. This relation was originally thought to define a single 'standard' correlation spanning several orders of magnitude in X-ray luminosity, and was extended to active galactic nuclei by including a mass term. However, subsequent studies revealed a more complex picture, with some sources deviating from the standard correlation and instead populating distinct tracks. To date, all large studies of the radio:X-ray plane have combined data from multiple telescopes, introducing uncertainties due to differing instrument systematics and flux conversions between observing frequencies, thereby complicating comparisons and limiting constraints. ThunderKAT was a five-year programme on the MeerKAT radio telescope that monitored X-ray binaries in outburst, and ran alongside SwiftKAT which provided quasi-simultaneous Swift/XRT X-ray coverage. We present the full set of light curves from these programmes, comprising 948 radio and 1029 X-ray data points. An important finding is the frequent detection of unresolved radio emission during the soft state, likely dominated by previously launched jet ejecta. Using these data, we construct the largest, observationally homogeneous X-ray binary radio:X-ray plane to date. We relate these results to the physical mechanisms proposed to drive inter-source diversity, and outline directions for future observational and theoretical work. This paper is accompanied by a public data release of the ThunderKAT and SwiftKAT measurements and a compiled radio:X-ray plane, available through an interactive website.

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arXiv2026

Exploring LSST's capabilities for early detection of outbursts in low-mass X-ray binaries

Following long periods of quiescence, low-mass X-ray binaries can exhibit intense X-ray outbursts triggered by instabilities within the accretion disk. These outbursts can sometimes be detected in optical wavelengths before being detected in X-rays, acting as an early onset warning and enabling a deep study of accretion disk properties informed by the lag between optical and X-ray rise. We explore the potential of Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) to detect these outbursts early through optical observations. We evaluate the capabilities of LSST based on currently planned survey cadence, filter-specific depth, and other observational factors that affect early detection. We develop and apply an extended metric to assess outburst detectability and recovery fraction. We find that despite inherent potential for early detection of XRB outbursts, the currently planned survey strategy makes it challenging to detect early onset of XRBs. Lastly, we demonstrate how this estimate can be used to infer the wider LMXB population in the Galaxy as the LSST progresses.

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arXiv2026

Reconciling the Systemic Kicks of Observed Millisecond Pulsars, Spider Pulsars, and Low-mass X-ray Binaries

Millisecond pulsars (MSPs) have been proposed as evolutionary products of low-mass X-ray binaries (LMXBs) through a stage in which they are spider pulsars (i.e., redbacks and black widows). However, recent work has found that the systemic kicks of observed MSPs are significantly lower than the kicks of LMXBs and spiders, which appears to be in tension with this evolutionary model. We argue that this tension can be relieved, at least to some degree, by considering the fact that the observed MSPs are located at relatively short distances, whereas spider pulsars are located at greater distances and LMXBs are situated even further away. We model the distance-dependent kinematic bias for dynamically old objects, which favors observing objects that have received low kicks at short distances and correct the observed systemic kicks for this bias. We find that this kinematic bias can be big enough to close the gap between the MSP and LMXB kicks, although the spider pulsars appear to come from a slightly different systemic kick distribution, but this difference is not necessarily physical. All corrected systemic kick distributions are consistent with predictions from binary population synthesis for progenitor systems with a post-supernova orbital period of $P_{\text{orb}}\leq10\,$d and a companion mass of $M_{c}\leq1\,M_{\odot}$, where the natal kicks are calibrated to the velocities of young isolated pulsars. We conclude that the difference in observed systemic kicks is not necessarily in tension with a common origin for MSPs, spider pulsars, and LMXBs.

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arXiv2026

MWA tied-array processing V: Super-resolved localisation via amplitude-only maximum likelihood direction finding

Interferometric localisation of transients and pulsars via tied-array beam processing is challenging and can be limited by the native spatial resolution achievable by the instrument, especially at low frequencies and for compact interferometers. Knowledge of the telescope primary and tied-array beam patterns allows the exploitation of the beam structures and the relationship to measured quantities, such as signal-to-noise ratio, through radio direction finding techniques. The additional information provides a "super-resolved" localisation (i.e., where the precision is much better than the native spatial resolution) of a source when there are multiple detections in adjacent tied-array beams. We demonstrate this approach using the Murchison Widefield Array (MWA) and its voltage capture and tied-array processing capabilities, with a specific focus on how it benefits the on-going Southern-sky MWA Rapid Two-metre pulsar survey as it starts producing more candidates requiring follow-up. Examples of localisations with previously discovered MWA pulsars which were subsequently localised via imaging with higher spatial resolution interferometers are used to validate the process, along with localisations of a sample of known pulsars to demonstrate the robustness of the method and its uncertainty estimation.

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arXiv2026

New Deep Radio Continuum Imaging Still Indicates a Large Reservoir of Undiscovered Millisecond Pulsars in Terzan 5

We present the deepest and highest-resolution radio continuum imaging of the Galactic globular cluster Terzan 5, one of the most crowded locations in the radio sky. In these new 2$-$4 GHz Karl G. Jansky Very Large Array images, we detect 38 of the 49 confirmed pulsars, including extensive multi-frequency eclipse mapping of the luminous redback Ter5A. Nonetheless, there is still a large amount of diffuse residual flux from pulsars that are fainter than our 2.5 GHz continuum detection limit of $\sim 11\,μ$Jy. Using a range of approaches including image-based simulations, we model the fluxes of the detected pulsars together with the residual flux. We find a minimum total population of $N\sim250$ detectable pulsars in Terzan 5 and perhaps substantially more, though the luminosity function remains very uncertain. Consideration of the $γ$-ray properties of the cluster, though also not unambiguous to interpret, leads to consistent conclusions. These pulsar population estimates are larger than inferred from previous work and highlight Terzan 5 as a keystone target for next-generation radio facilities.

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arXiv2026

What determines the $γ$-ray luminosities of classical novae?

Classical novae in the Milky Way have now been well-established as high-energy GeV $γ$-ray sources. In novae with main-sequence companions, this emission is believed to result from shocks internal to the nova ejecta, as a later fast wind collides with an earlier slow outflow. To test this model and constrain the $γ$-ray production mechanism, we present a systematic study of a sample of recent Galactic novae, comparing their $γ$-ray properties ($γ$-ray luminosity and duration) with their outflow velocities, peak $V$-band magnitudes, and the decline times of their optical light curves ($t_2$). We uniformly estimate distances in a luminosity-independent manner, using spectroscopic reddening estimates combined with three-dimensional Galactic dust maps. Across our sample, $γ$-ray luminosities ($>$100 MeV) vary by three orders of magnitude, spanning $10^{34}-10^{37}$ erg s$^{-1}$. Novae with larger velocity of the fast outflow (or larger differential between the fast and slow outflow) have larger $γ$-ray luminosities, but are detectable for a shorter duration. The optical and $γ$-ray fluxes are correlated, consistent with substantial thermal emission in the optical from shock-heated gas. Across six novae with $γ$-ray and infrared light curves, evidence for dust formation appears soon after the end of the detected $γ$-ray emission. Dusty and non-dusty novae appear to have similar $γ$-ray luminosities, though novae that have more material processed by the shocks may be more likely to form dust. We find that the properties of the $γ$-ray emission in novae depend heavily on the ejecta properties, and are consistent with expectations for internal shocks.

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arXiv2026

The Advanced X-ray Imaging Satellite (AXIS) Community Science Book

The AXIS Community Science Book represents the collective effort of 592 scientists worldwide to define the transformative science enabled by the Advanced X-ray Imaging Satellite (AXIS), a next-generation X-ray mission selected by NASA's Astrophysics Probe Program for Phase A study. AXIS will advance the legacy of high-angular-resolution X-ray astronomy with ~1.5'' imaging over a wide 24' field of view and an order of magnitude greater collecting area than Chandra in the 0.3-12 keV band. Combining sharp imaging, high throughput, and rapid response capabilities, AXIS will open new windows on virtually every aspect of modern astrophysics, exploring the birth and growth of supermassive black holes, the feedback processes that shape galaxies, the life cycles of stars and exoplanet environments, and the nature of compact stellar remnants, supernova remnants, and explosive transients. This book compiles 138 community-contributed science cases developed by five Science Working Groups focused on AGN and supermassive black holes, galaxy evolution and feedback, compact objects and supernova remnants, stellar physics and exoplanets, and time-domain and multi-messenger astrophysics. Together, these studies establish the scientific foundation for next-generation X-ray exploration in the 2030s and highlight strong synergies with facilities of the 2030s, such as JWST, Roman, Rubin/LSST, SKA, ALMA, ngVLA, and next-generation gravitational-wave and neutrino networks.

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arXiv2025

AT2022zod: An Unusual Tidal Disruption Event in an Elliptical Galaxy at Redshift 0.11

Tidal Disruption Events (TDEs) have long been hypothesized as valuable indicators of black holes, offering insight into their demographics and behaviour out to high redshift. TDEs have also enabled the discovery of a few Massive Black Holes (MBHs) with inferred masses of $10^4$--$10^6\,M_\odot$, often associated with the nuclei of dwarf galaxies or ultra-compact dwarf galaxies (UCDs). Here we present AT2022zod, an extreme and short-lived optical flare in an elliptical galaxy at z=0.11, residing within 3kpc of the galaxy's centre. Its luminosity and ~30-day duration make it unlikely to have originated from the host galaxy's central supermassive black hole (SMBH), which estimate to have a mass around $10^8\,M_\odot$. Assuming that the emission mechanism is consistent with known observed TDEs, we find that such a rapidly evolving transient could either be produced by a MBH in the intermediate-mass range or, alternatively, result from the tidal disruption of a star on a non-parabolic orbit around the central SMBH. We suggest that the most plausible origin for AT2022zod is the tidal disruption of a star by a MBH embedded in a UCD. As the Vera C. Rubin Observatory's Legacy Survey of Space and Time comes online, we propose that AT2022zod serves as an important event for refining search strategies and characterization techniques for intermediate-mass black holes.

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arXiv2025

A jet bent by a stellar wind in the black hole X-ray binary Cygnus X-1

Jets provide an important channel for kinetic feedback from accreting black holes into their environment, without which models of the formation of large-scale structure in the universe fail to reproduce the observed properties of galaxies. Hence, an accurate measurement of jet power is critical for understanding black hole growth through accretion and also for quantifying the impact of kinetic feedback. However, the absence of instantaneous jet power measurements has precluded direct comparisons with the accretion luminosity, forcing kinetic feedback models to rely on ad hoc assumptions about how much jet power is released per accreted amount of mass. Here we report the detection of stellar wind-induced bending of the jets in the black hole X-ray binary Cygnus X-1, using 18 years of high-resolution radio imaging. By modeling jet-wind interactions, we determine the current kinetic instantaneous power of the jet to be log$_{10}(L_{\rm jet}/{\rm erg\,s}^{-1}) = 37.3_{-0.2}^{+0.1}$, comparable to the accretion energy determined from its bolometric X-ray luminosity. This result critically places prevailing assumptions about the energetics of black hole powered jets in both galaxy formation simulations, and in scaling models of black hole accretion, on a firm empirical footing.

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arXiv2025

No evidence for accretion around the intermediate-mass black hole in Omega Centauri

For over a decade, both theoretical predictions and observational studies have suggested that $ω$ Centauri ($ω$ Cen), the most massive Milky Way globular cluster, might harbor an intermediate-mass black hole (IMBH). Recently, identification of fast-moving stars in the core of $ω$ Cen provided the strongest evidence to date for the presence of such an IMBH. One of the key questions in the study of IMBHs is their accretion efficiency, which determines their radio and X-ray signatures. We investigate the accretion signature of the IMBH in $ω$ Cen with ultra-deep radio continuum observations of the central region of the cluster. Using approximately 170 hours of Australia Telescope Compact Array observations, we achieve a root mean square noise of 1.1 $μ$Jy at 7.25 GHz, making this the most sensitive radio image of the cluster to date. We detect no radio emission at any of the proposed centers of the cluster, imposing stringent constraints on the presence of an accreting IMBH in $ω$ Cen. Considering the fundamental plane of black hole activity, our findings indicate that the accretion efficiency around the black hole is exceptionally low (with a conservative 3-$σ$ upper limit of $ε\lesssim 4\times10^{-3}$).

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arXiv2025

An Alternative Explanation for the Helium Star Pulsar Binary J1928$+$1815: The Most Heavyweight Black Widow System to Date

We present the results of deep near-infrared imaging of the recently discovered helium star pulsar binary J1928$+$1815 situated in the Galactic plane. Our observations did not achieve significant detections, providing limiting magnitudes of J=23.7 and H=22.2, which are both 2.4\,magnitudes deeper than the expected J and H magnitudes for a modeled stripped helium star with a mass of 1\,$\rm M_{\odot}$ after extinction. Although we cannot completely rule out the possibility of more significant extinction and the exact evolutionary status of the supposed helium star is uncertain, by comparing J1928$+$1815 with other pulsar binaries, we propose a natural alternative solution: that J1928$+$1815 is a heavyweight black widow system with a massive ablated white dwarf. Due to the pulsar's relatively high spin-down power and short orbital separation, the irradiation heating timescale is uniquely shorter than the cooling timescale for the WD companion. As a result, the WD effectively boils, with its outer layers expanding, overfilling the Roche lobe and producing low-density binary-scale haze opaque in the radio band. If this interpretation is correct, J1928$+$1815 would represent a new category distinct from canonical lightweight black widow systems. Radio eclipses can occur in pulsar binaries across a wider range of WD companion masses than previously thought. Therefore, they do not serve as a definitive indicator of a helium star without its direct detection. We contend that a spectroscopic identification remains the smoking gun for its existence. Given the crowding in this field, an \textit{HST} imaging in the near-infrared band would provide even better constraints.

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arXiv2025

Catalog of outbursts of neutron star LMXBs

Many X-ray binaries are transiently accreting. Having statistics on their recurrence times is helpful to address questions related to binary evolution and populations, as well as the physics of binary systems. We compile a catalog of known outbursts of 87 transient neutron star (identified through bursts or pulsations) low-mass X-ray binaries, until mid-2025. Most outbursts are taken from the literature, but we also identify some outbursts from public X-ray monitoring lightcurves. We find 109 outbursts not previously identified in the literature; most are from the frequent transients GRS 1747-312 and the Rapid Burster MXB 1730-335, though we suspect that two outbursts from Liller 1 may be from another transient, besides the Rapid Burster. We also find new outbursts for 10 other systems, and verify substantial quiescent intervals for XMM J174457-2850.3, XMMU J174716.1-281048, and AX J1754.2-2754. Outburst detection has been relatively efficient since 1996 for outbursts above $F_X$(2-10)$=3\times10^{-10}$ ergs/s/cm$^2$. While several systems have many known outbursts, 40 of the 87 systems we track have zero or one recorded outburst between 1996 and 2023. This suggests that many faint Galactic Center X-ray binaries may be neutron star X-ray binaries, though we cannot completely rule out the proposition that most neutron star X-ray binaries undergo frequent outbursts below all-sky monitor detection limits.

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arXiv2025

On the distance to the black hole X-ray binary Swift J1727.8$-$1613

We review the existing distance estimates to the black hole X-ray binary Swift J1727.8$-$1613, present new radio and near-UV spectra to update the distance constraints, and discuss the accuracies and caveats of the associated methodologies. We use line-of-sight HI absorption spectra captured using the MeerKAT radio telescope to estimate a maximum radial velocity with respect to the local standard of rest of $24.8 \pm 2.8 \, {\rm km\,s^{-1}}$ for Swift J1727.8$-$1613, which is significantly lower than that of a nearby extragalactic reference source. From this we derive a near kinematic distance of $d_{\rm near} = 3.6 \pm 0.3 \, ({stat}) \pm 2.3 \, ({sys}) \, {\rm kpc}$ as a lower bound after accounting for additional uncertainties given its Galactic longitude and latitude, $(l, b) \approx (8.6^{\circ}, 10.3^{\circ})$. Near-UV spectra from the Hubble Space Telescope's Space Telescope Imaging Spectrograph allows us to constrain the line-of-sight colour excess to $E(B\!-\!V) = 0.37 \pm 0.01 \, ({stat}) \pm 0.025 \, ({sys})$. We then implement this in Monte Carlo simulations and present a distance to Swift J1727.8$-$1613 of $5.5^{+1.4}_{-1.1} \, {\rm kpc}$, under the assumption that the donor star is an unevolved, main sequence K3-5V star. This distance implies a natal kick velocity of $190 \pm 30 \, {\rm km\,s^{-1}}$ and therefore an asymmetrical supernova explosion within the Galactic disk as the expected birth mechanism. A lower distance is implied if the donor star has instead lost significant mass during the binary evolution. Hence, more accurate measurements of the binary inclination angle or donor star rotational broadening from future observations would help to better constrain the distance.

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arXiv2025

Investigating four new candidate redback pulsars discovered in the image plane

This paper reports the discovery and follow-up of four candidate redback spider pulsars: GPM J1723-33, GPM J1734-28, GPM J1752-30 and GPM J1815-14, discovered with the Murchison Widefield Array (MWA) from an imaging survey of the Galactic Plane. These sources are considered to be redback candidates based on their eclipsing variability, steep negative spectral indices, and potential Fermi $γ$-ray associations, with GPM J1723-33 and GPM J1815-14 lying within a Fermi 95% error ellipse. Follow-up pulsation searches with MeerKAT confirmed pulsations from GPM J1723-33, while the non-detections of the other three are likely due to scattering by material ablated from their companion stars. We identify possible orbital periods by applying folding algorithms to the light curves and determine that all sources have short orbital periods (<24 hours), consistent with redback spider systems. Following up on the sources at multiple radio frequencies revealed that the sources exhibit frequency-dependent eclipses, with longer eclipses observed at lower frequencies. We place broad constraints on the eclipse medium, ruling out induced Compton scattering and cyclotron absorption. Three sources are spatially consistent with optical sources in the Dark Energy Camera Plane Survey imaging, which may contain the optical counterparts. Each field is affected by strong dust extinction, and follow-up with large telescopes is needed to identify the true counterparts. Identifying potential radio counterparts to four previously unassociated Fermi sources brings us closer to understanding the origin of the unexplained $γ$-ray excess in the Galactic Centre.

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arXiv2025

Long term variability of Cygnus X-1. IX. A spectral-timing comparison of Cygnus X-1 and MAXI J1820+070 in the hard state

Cygnus X-1 is a persistent, high-mass black hole X-ray binary (BHXRB) which in the hard state shows many similar properties to transient BHXRBs, along with intriguing differences, such as the lack of quasi-periodic oscillations. Here, we compare for the first time the detailed spectral-timing properties of Cyg X-1 with a transient BHXRB, MAXI J1820+070, combining data from XMM-Newton and NICER with contemporaneous INTEGRAL data to study the power spectra, rms spectra and time-lags over a broad 0.5 - 200 keV range. We select bright hard state MAXI J1820+070 data with similar power-spectral shapes to the Cyg X-1 data, to compare the source behaviours while accounting for the evolution of spectral-timing properties, notably the lags, through the hard state. Cyg X-1 shows no evidence for soft lags in the 1 - 10 Hz frequency range where they are clearly detected for MAXI J1820+070. Furthermore the low-frequency hard lags and rms-spectra evolve much more strongly during the hard state of Cyg X-1 than for MAXI J1820+070. We argue that these differences cannot be explained by the different black hole masses of these systems, but may be related to their different accretion rates and corresponding locations on the hardness-intensity diagram. We conjecture that there is a significant luminosity-dependence of coronal geometry in the hard state of BHXRBs, rather than an intrinsic difference between Cyg X-1 and transient BHXRBs. This possibility has also been suggested to explain a common time-lag feature that appears in the hard intermediate states of Cyg X-1 and transient BHXRBs.

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arXiv2025

New Metrics for Identifying Variables and Transients in Large Astronomical Surveys

A key science goal of large sky surveys such as those conducted by the Vera C. Rubin Observatory and precursors to the Square Kilometre Array is the identification of variable and transient objects. One approach is the statistical analysis of the time series of the changing brightness of sources, that is, their light curves. However, finding adequate statistical representations of light curves is challenging because of data quality issues such as sparsity of observations, irregular sampling, and other nuisance factors inherent in astronomical data collection. The wide diversity of objects that a large-scale survey will observe also means that making parametric assumptions about the shape of light curves is problematic. We present a Gaussian process (GP) regression approach for characterising light curve variability that addresses these challenges. Our approach makes no assumptions about the shape of a light curve and, therefore, is general enough to detect a range of variable source types. In particular, we propose using the joint distribution of GP amplitude hyperparameters to distinguish variable and transient candidates from nominally stable ones and apply this approach to 6394 radio light curves from the ThunderKAT survey. We compare our results with two variability metrics commonly used in radio astronomy, namely $η_ν$ and $V_ν$, and show that our approach has better discriminatory power and interpretability. Finally, we conduct a rudimentary search for transient sources in the ThunderKAT dataset to demonstrate how our approach might be used as an initial screening tool. Computational notebooks in Python and R are available to help facilitate the deployment of this framework to other surveys.

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