arXiv subjects

Andreja Gomboc

Publications and source records attributed to Andreja Gomboc.

At least 19 recordsLinked to original sources

arXiv2026

The Rubin Observatory Target-of-Opportunity System in the First Year of Operations

The NSF/DOE Vera C. Rubin Observatory is a discovery machine, with unprecedented survey speed, which can be used to identify exotic astrophysical transients. In its prime mission, the ten year Legacy Survey of Space and Time will use 3% of its total time for Target of Opportunity observations, which includes response to gravitational wave events, high energy neutrinos, potentially-hazardous asteroids, and other astrophysical phenomena. Target of Opportunity observations exist outside of the usual LSST operational mode, requiring special attention to maximize performance. We review the Rubin Target of Opportunity system during its first year of Rubin Observatory operations, the Targets of Opportunity pursued since LSST first light, and the overall efficiency of the system.

astro-ph.IM↗

arXiv2026

Insights from GRBs for optical follow-up of gravitational wave counterparts

Identifying the electromagnetic counterparts to gravitational wave sources is vital to enabling the myriad of investigations possible with multimessenger astronomy. However, locating faint, fast-varying transients within large localisations remains challenging given the uncertainty in their detailed properties. In this work, we investigate how the nearby merger-induced GRBs would be localised by the LIGO-Virgo-KAGRA detector network during the fifth gravitational wave observing run (O5) and assess whether their optical counterparts could be detected using gravitational wave localisations alone, without additional localisation from gamma-ray instruments. Counterpart detectability is evaluated using the observed optical afterglow lightcurves of these GRBs and the distance-scaled lightcurve of the kilonova AT2017gfo as a fiducial template. We find that such events can be localised to comparatively small regions of the sky, often only a few to tens of square degrees. As a result, counterparts are detectable by at least one of the available optical telescopes during O5. However, detectability depends strongly on observational depth, as the counterparts are fainter than $22$ mag within a day. Facilities capable of reaching depths of $\gtrsim23$ mag therefore play a key role in recovering these faint counterparts. These results indicate that for such events during O5, the primary challenge for multimessenger discovery will be in achieving sufficient observational depth and reliably identifying the true counterpart among unrelated transients rather than gravitational wave localisation itself.

astro-ph.HE↗

arXiv2024

Rubin ToO 2024: Envisioning the Vera C. Rubin Observatory LSST Target of Opportunity program

The Legacy Survey of Space and Time (LSST) at Vera C. Rubin Observatory is planned to begin in the Fall of 2025. The LSST survey cadence has been designed via a community-driven process regulated by the Survey Cadence Optimization Committee (SCOC), which recommended up to 3% of the observing time to carry out Target of Opportunity (ToO) observations. Experts from the scientific community, Rubin Observatory personnel, and members of the SCOC were brought together to deliver a recommendation for the implementation of the ToO program during a workshop held in March 2024. Four main science cases were identified: gravitational wave multi-messenger astronomy, high energy neutrinos, Galactic supernovae, and small potentially hazardous asteroids possible impactors. Additional science cases were identified and briefly addressed in the documents, including lensed or poorly localized gamma-ray bursts and twilight discoveries. Trigger prioritization, automated response, and detailed strategies were discussed for each science case. This document represents the outcome of the Rubin ToO 2024 workshop, with additional contributions from members of the Rubin Science Collaborations. The implementation of the selection criteria and strategies presented in this document has been endorsed in the SCOC Phase 3 Recommendations document (PSTN-056). Although the ToO program is still to be finalized, this document serves as a baseline plan for ToO observations with the Rubin Observatory.

astro-ph.IM↗

arXiv2024

The fast X-ray transient EP240315a: a z ~ 5 gamma-ray burst in a Lyman continuum leaking galaxy

The nature of the minute-to-hour long Fast X-ray Transients (FXTs) localised by telescopes such as Chandra, Swift, and XMM-Newton remains mysterious, with numerous models suggested for the events. Here, we report multi-wavelength observations of EP240315a, a 1600 s long transient detected by the Einstein Probe, showing it to have a redshift of z=4.859. We measure a low column density of neutral hydrogen, indicating that the event is embedded in a low-density environment, further supported by direct detection of leaking ionising Lyman-continuum. The observed properties are consistent with EP240315a being a long-duration gamma-ray burst, and these observations support an interpretation in which a significant fraction of the FXT population are lower-luminosity examples of similar events. Such transients are detectable at high redshifts by the Einstein Probe and, in the (near) future, out to even larger distances by SVOM, THESEUS, and Athena, providing samples of events into the epoch of reionisation.

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arXiv2024

Spin-induced offset stream self-crossing shocks in tidal disruption events

Tidal disruption events occur when a star is disrupted by a supermassive black hole, resulting in an elongated stream of gas that partly falls back to the pericenter. Due to apsidal precession, the returning stream may collide with itself, leading to a self-crossing shock that launches an outflow. If the black hole spins, this collision may additionally be affected by Lense-Thirring precession that can cause an offset between the two stream components. We study the impact of this effect on the outflow properties by carrying out local simulations of collisions between offset streams. As the offset increases, we find that the geometry of the outflow becomes less spherical and more collimated along the directions of the incoming streams, with less gas getting unbound by the interaction. However, even the most grazing collisions we consider significantly affect the trajectories of the colliding gas, likely promoting subsequent strong interactions near the black hole and rapid disc formation. We analytically compute the dependence of the offset to stream width ratio, finding that even slowly spinning black holes can cause both strong and grazing collisions. We estimate that the self-crossing shock luminosity is lower for an offset collision than an aligned one since radiation energy injected by the shock is significantly lower for more offset collisions. We find that the deviation from outflow sphericity may cause significant variations in the efficiency at which X-ray radiation from the disc is reprocessed to the optical band, depending on the viewing angle, and increase the degree of the observed polarization. These potentially observable features hold the promise of constraining the black hole spin from tidal disruption events.

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arXiv2023

The Radio to GeV Afterglow of GRB 221009A

GRB 221009A ($z=0.151$) is one of the closest known long $γ$-ray bursts (GRBs). Its extreme brightness across all electromagnetic wavelengths provides an unprecedented opportunity to study a member of this still-mysterious class of transients in exquisite detail. We present multi-wavelength observations of this extraordinary event, spanning 15 orders of magnitude in photon energy from radio to $γ$-rays. We find that the data can be partially explained by a forward shock (FS) from a highly-collimated relativistic jet interacting with a low-density wind-like medium. Under this model, the jet's beaming-corrected kinetic energy ($E_K \sim 4\times10^{50}$ erg) is typical for the GRB population. The radio and mm data provide strong limiting constraints on the FS model, but require the presence of an additional emission component. From equipartition arguments, we find that the radio emission is likely produced by a small amount of mass ($\lesssim6\times10^{-7} M_\odot$) moving relativistically ($Γ\gtrsim9$) with a large kinetic energy ($\gtrsim10^{49}$ erg). However, the temporal evolution of this component does not follow prescriptions for synchrotron radiation from a single power-law distribution of electrons (e.g. in a reverse shock or two-component jet), or a thermal electron population, perhaps suggesting that one of the standard assumptions of afterglow theory is violated. GRB 221009A will likely remain detectable with radio telescopes for years to come, providing a valuable opportunity to track the full lifecycle of a powerful relativistic jet.

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arXiv2023

The mass fallback rate of the debris in relativistic stellar tidal disruption events

Highly energetic stellar tidal disruption events (TDEs) provide a way to study black hole characteristics and their environment. We simulate TDEs with the PHANTOM code in a general relativistic and Newtonian description of a supermassive black hole's gravity. Stars, which are placed on parabolic orbits with different parameters $β$, are constructed with the stellar evolution code MESA and therefore have realistic stellar density profiles. We study the mass fallback rate of the debris $\dot{M}$ and its dependence on the $β$, stellar mass and age as well as the black hole's spin and the choice of the gravity's description. We calculate peak value $\dot{M}_\mathrm{peak}$, time to the peak $t_\mathrm{peak}$, duration of the super-Eddington phase $t_\mathrm{Edd}$, time $t_{>0.5\dot{M}_\mathrm{peak}}$ during which $\dot{M}>0.5\dot{M}_\mathrm{peak}$, early rise-time $τ_\mathrm{rise}$ and late-time slope $n_\infty$. We recover the trends of $\dot{M}_\mathrm{peak}$, $t_\mathrm{peak}$, $τ_\mathrm{rise}$ and $n_\infty$ with $β$, stellar mass and age, which were obtained in previous studies. We find that $t_\mathrm{Edd}$, at a fixed $β$, scales primarily with the stellar mass, while $t_{>0.5\dot{M}_\mathrm{peak}}$ scales with the compactness of stars. The effect of SMBH's rotation depends on the orientation of its rotational axis relative to the direction of the stellar motion on the initial orbit. Encounters on prograde orbits result in narrower $\dot{M}$ curves with higher $\dot{M}_\mathrm{peak}$, while the opposite occurs for retrograde orbits. We find that disruptions, at the same pericenter distance, are stronger in a relativistic tidal field than in a Newtonian. Therefore, relativistic $\dot{M}$ curves have higher $\dot{M}_\mathrm{peak}$, and shorter $t_\mathrm{peak}$ and $t_\mathrm{Edd}$.

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arXiv2022

Rubin Observatory LSST Transients and Variable Stars Roadmap

The Vera C. Rubin Legacy Survey of Space and Time holds the potential to revolutionize time domain astrophysics, reaching completely unexplored areas of the Universe and mapping variability time scales from minutes to a decade. To prepare to maximize the potential of the Rubin LSST data for the exploration of the transient and variable Universe, one of the four pillars of Rubin LSST science, the Transient and Variable Stars Science Collaboration, one of the eight Rubin LSST Science Collaborations, has identified research areas of interest and requirements, and paths to enable them. While our roadmap is ever-evolving, this document represents a snapshot of our plans and preparatory work in the final years and months leading up to the survey's first light.

astro-ph.IM↗

arXiv2022

From Data to Software to Science with the Rubin Observatory LSST

The Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) dataset will dramatically alter our understanding of the Universe, from the origins of the Solar System to the nature of dark matter and dark energy. Much of this research will depend on the existence of robust, tested, and scalable algorithms, software, and services. Identifying and developing such tools ahead of time has the potential to significantly accelerate the delivery of early science from LSST. Developing these collaboratively, and making them broadly available, can enable more inclusive and equitable collaboration on LSST science. To facilitate such opportunities, a community workshop entitled "From Data to Software to Science with the Rubin Observatory LSST" was organized by the LSST Interdisciplinary Network for Collaboration and Computing (LINCC) and partners, and held at the Flatiron Institute in New York, March 28-30th 2022. The workshop included over 50 in-person attendees invited from over 300 applications. It identified seven key software areas of need: (i) scalable cross-matching and distributed joining of catalogs, (ii) robust photometric redshift determination, (iii) software for determination of selection functions, (iv) frameworks for scalable time-series analyses, (v) services for image access and reprocessing at scale, (vi) object image access (cutouts) and analysis at scale, and (vii) scalable job execution systems. This white paper summarizes the discussions of this workshop. It considers the motivating science use cases, identified cross-cutting algorithms, software, and services, their high-level technical specifications, and the principles of inclusive collaborations needed to develop them. We provide it as a useful roadmap of needs, as well as to spur action and collaboration between groups and individuals looking to develop reusable software for early LSST science.

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arXiv2021

Multi-Messenger Astrophysics with THESEUS in the 2030s

Multi-messenger astrophysics is becoming a major avenue to explore the Universe, with the potential to span a vast range of redshifts. The growing synergies between different probes is opening new frontiers, which promise profound insights into several aspects of fundamental physics and cosmology. In this context, THESEUS will play a central role during the 2030s in detecting and localizing the electromagnetic counterparts of gravitational wave and neutrino sources that the unprecedented sensitivity of next generation detectors will discover at much higher rates than the present. Here, we review the most important target signals from multi-messenger sources that THESEUS will be able to detect and characterize, discussing detection rate expectations and scientific impact.

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arXiv2020

The Prospects of Observing Tidal Disruption Events with the LSST

The upcoming Large Synoptic Survey Telescope (LSST) will observe 18 000 deg$^2$ of the Southern sky and is expected to discover thousands of transients every night due to its large coverage of the sky and its observing strategy. In this work we address the prospects of the LSST in discovering Tidal Disruption Events (TDEs) and in probing the supermassive black hole (SMBH) mass distribution in the Universe. We used the LSST simulation framework and defined TDE catalogs on 20 fields of 20.25 deg$^2$ size. TDE properties were defined by randomly chosen impact factors and SMBH masses drawn from six different mass distributions. Observations of TDEs in 10 years of LSST operations were simulated by querying the simulated observing strategy database minion_1016. Based on the results of our simulations we estimate that the LSST should discover between 35 000 and 80 000 TDEs in 10 years of operations, depending on the assumed SMBH mass distribution. We also find that probing the SMBH mass distribution with TDE observations will not be straightforward due to the fact that TDEs caused by low mass black holes ($10^ 5 M_\odot$) are expected to be less luminous and shorter than TDEs by heavier SMBHs ($> 10^6 M_\odot$), and therefore will mostly be missed by the irregular LSST cadence minion_1016.

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arXiv2019

A Reverse Shock in GRB 181201A

We present comprehensive multiwavelength radio to X-ray observations of GRB 181201A spanning from $\approx150$ s to $\approx163$ days after the burst, comprising the first joint ALMA-VLA-GMRT observations of a gamma-ray burst (GRB) afterglow. The radio and mm-band data reveal a distinct signature at $\approx3.9$ days, which we interpret as reverse shock (RS) emission. Our observations present the first time that a single radio-frequency spectral energy distribution can be decomposed directly into RS and forward shock (FS) components. We perform detailed modeling of the full multiwavelength data set, using Markov Chain Monte Carlo sampling to construct the joint posterior density function of the underlying physical parameters describing the RS and FS synchrotron emission. We uncover and account for all degeneracies in the model parameters. The joint RS-FS modeling reveals a weakly magnetized ($σ\approx3\times10^{-3}$), mildly relativistic RS, from which we derive an initial bulk Lorentz factor of $Γ_0\approx103$ for the GRB jet. Our results support the hypothesis that low-density environments are conducive to the observability of RS emission. We compare our observations to other events with strong RS detections, and find a likely observational bias selecting for longer lasting, non-relativistic reverse shocks. We present and begin to address new challenges in modeling posed by the present generation of comprehensive, multi-frequency data sets.

astro-ph.HE↗

arXiv2019

An Unexpectedly Small Emission Region Size Inferred from Strong High-Frequency Diffractive Scintillation in GRB 161219B

We present Karl G. Jansky Very Large Array radio observations of the long gamma-ray burst GRB 161219B ($z=0.147$) spanning $1-37$ GHz. The data exhibit unusual behavior, including sharp spectral peaks and minutes-timescale large-amplitude variability centered at $20$ GHz and spanning the full frequency range. We attribute this behavior to scattering of the radio emission by the turbulent ionized Galactic interstellar medium (ISM), including both diffractive and refractive scintillation. However, the scintillation is much stronger than predicted by a model of the Galactic electron density distribution (NE2001); from the measured variability timescale and decorrelation bandwidth we infer a scattering measure of $SM\approx {(8-70)\times 10^{-4}}$ kpc m$^{-20/3}$ (up to ${25}$ times larger than predicted in NE2001) and a scattering screen distance of $d_{\rm scr}\approx {0.2-3}$ kpc. We infer an emission region size of $θ_s \approx {0.9-4}$ $μ$as ($\approx {(1-4)}\times 10^{16}$ cm) at $\approx4$ days, and find that prior to 8 days the source size is an order of magnitude smaller than model predictions for a uniformly illuminated disk or limb-brightened ring, indicating a slightly off-axis viewing angle or significant substructure in the emission region. Simultaneous multi-hour broadband radio observations of future GRB afterglows will allow us to characterize the scintillation more completely, and hence to probe the observer viewing angle, the evolution of the jet Lorentz factor, the structure of the afterglow emission regions, and ISM turbulence at high Galactic latitudes.

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arXiv2018

TDEs with LSST

We investigate the prospects of observing Tidal Disruption Events (TDEs) with different LSST cadences proposed with the White Paper call. We study their detection rates, the quality of their light curves and discuss which cadences sample TDEs better before or during the peak of the light curve. We suggest some constraints on the observing cadences that we believe will increase the number of reliable classifications of TDEs, in particular a WFD survey with 2 visits in different filters every night or at least every second night, observing the extra-galactic sky.

astro-ph.IM↗

arXiv2016

A Reverse Shock in GRB 160509A

We present the second multi-frequency radio detection of a reverse shock in a $γ$-ray burst. By combining our extensive radio observations of the Fermi-LAT GRB 160509A at $z = 1.17$ up to $20$ days after the burst with Swift X-ray observations and ground-based optical and near-infrared data, we show that the afterglow emission comprises distinct reverse shock and forward shock contributions: the reverse shock emission dominates in the radio band at $\lesssim10~$days, while the forward shock emission dominates in the X-ray, optical, and near-infrared bands. Through multi-wavelength modeling, we determine a circumburst density of $n_0\approx10^{-3}~$cm$^{-3}$, supporting our previous suggestion that a low-density circumburst environment is conducive to the production of long-lasting reverse shock radiation in the radio band. We infer the presence of a large excess X-ray absorption column, $N_{\rm H} \approx 1.5\times10^{22}~$cm$^{-2}$, and a high rest-frame optical extinction, $A_{\rm V}\approx3.4~$mag. We identify a jet break in the X-ray light curve at $t_{\rm jet}\approx6~$d, and thus derive a jet opening angle of $θ_{\rm jet}\approx4~$deg, yielding a beaming-corrected kinetic energy and radiated $γ$-ray energy of $E_{\rm K}\approx4\times10^{50}~$erg and $E_γ\approx1.3\times10^{51}~$erg ($1$-$10^4~$keV, rest frame), respectively. Consistency arguments connecting the forward and reverse shocks suggest a deceleration time of $t_{\rm dec} \approx 460~$s$~\approx T_{90}$, a Lorentz factor of $Γ(t_{\rm dec})\approx330$, and a reverse shock to forward shock fractional magnetic energy density ratio of $R_{\rm B}\equivε_{\rm B,RS}/ε_{\rm B,FS}\approx8$.

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arXiv2015

Relativistic Positioning System in Perturbed Space-time

We present a variant of a Global Navigation Satellite System called a Relativistic Positioning System (RPS), which is based on emission coordinates. We modelled the RPS dynamics in a space-time around Earth, described by a perturbed Schwarzschild metric, where we included the perturbations due to Earth multipoles (up to the 6th), the Moon, the Sun, Venus, Jupiter, solid tide, ocean tide, and Kerr rotation effect. The exchange of signals between the satellites and a user was calculated using a ray-tracing method in the Schwarzschild space-time. We find that positioning in a perturbed space-time is feasible and is highly accurate already with standard numerical procedures: the positioning algorithms used to transform between the emission and the Schwarzschild coordinates of the user are very accurate and time efficient -- on a laptop it takes 0.04 s to determine the user's spatial and time coordinates with a relative accuracy of $10^{-28}-10^{-26}$ and $10^{-32}-10^{-30}$, respectively.

gr-qc↗

arXiv2014

An Autonomous Reference Frame for Relativistic GNSS

Current GNSS systems rely on global reference frames which are fixed to the Earth (via the ground stations) so their precision and stability in time are limited by our knowledge of the Earth dynamics. These drawbacks could be avoided by giving to the constellation of satellites the possibility of constituting by itself a primary and autonomous positioning system, without any a priori realization of a terrestrial reference frame. Our work shows that it is possible to construct such a system, an Autonomous Basis of Coordinates, via emission coordinates. Here we present the idea of the Autonomous Basis of Coordinates and its implementation in the perturbed space-time of Earth, where the motion of satellites, light propagation, and gravitational perturbations are treated in the formalism of general relativity.

gr-qc↗

arXiv2013

GRB 081007 and GRB 090424: the surrounding medium, outflows and supernovae

We discuss the results of the analysis of multi-wavelength data for the afterglows of GRB 081007 and GRB 090424, two bursts detected by Swift. One of them, GRB 081007, also shows a spectroscopically confirmed supernova, SN 2008hw, which resembles SN 1998bw in its absorption features, while the maximum luminosity is only about half as large as that of SN 1998bw. Bright optical flashes have been detected in both events, which allows us to derive solid constraints on the circumburst-matter density profile. This is particularly interesting in the case of GRB 081007, whose afterglow is found to be propagating into a constant-density medium, yielding yet another example of a GRB clearly associated with a massive star progenitor which did not sculpt the surroundings with its stellar wind. There is no supernova component detected in the afterglow of GRB 090424, likely due to the brightness of the host galaxy, comparable to the Milky Way. We show that the afterglow data are consistent with the presence of both forward- and reverse-shock emission powered by relativistic outflows expanding into the interstellar medium. The absence of optical peaks due to the forward shock strongly suggests that the reverse shock regions should be mildly magnetized. The initial Lorentz factor of outflow of GRB 081007 is estimated to be Γ~ 200, while for GRB 090424 a lower limit of Γ> 170 is derived. We also discuss the prompt emission of GRB 081007, which consists of just a single pulse. We argue that neither the external forward-shock model nor the shock-breakout model can account for the prompt emission data and suggest that the single-pulse-like prompt emission may be due to magnetic energy dissipation of a Poynting-flux dominated outflow or to a dissipative photosphere.

astro-ph.HE↗