arXiv subjects

Aaron Tohuvavohu

Publications and source records attributed to Aaron Tohuvavohu.

35 records · Page 2Linked to original sources

arXiv2022

A very luminous jet from the disruption of a star by a massive black hole

Tidal disruption events (TDEs) are bursts of electromagnetic energy released when supermassive black holes (SMBHs) at the centers of galaxies violently disrupt a star that passes too close. TDEs provide a new window to study accretion onto SMBHs; in some rare cases, this accretion leads to launching of a relativistic jet, but the necessary conditions are not fully understood. The best studied jetted TDE to date is Swift J1644+57, which was discovered in gamma-rays, but was too obscured by dust to be seen at optical wavelengths. Here we report the optical discovery of AT2022cmc, a rapidly fading source at cosmological distance (redshift z=1.19325) whose unique lightcurve transitioned into a luminous plateau within days. Observations of a bright counterpart at other wavelengths, including X-rays, sub-millimeter, and radio, supports the interpretation of AT2022cmc as a jetted TDE containing a synchrotron "afterglow", likely launched by a SMBH with spin $a \gtrsim 0.3$. Using 4 years of Zwicky Transient Facility (ZTF) survey data, we calculate a rate of $0.02 ^{+ 0.04 }_{- 0.01 }$ Gpc$^{-3}$ yr$^{-1}$ for on-axis jetted TDEs based on the luminous, fast-fading red component, thus providing a measurement complementary to the rates derived from X-ray and radio observations. Correcting for the beaming angle effects, this rate confirms that about 1% of TDEs have relativistic jets. Optical surveys can use AT2022cmc as a prototype to unveil a population of jetted TDEs.

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arXiv2022

Harvesting BAT-GUANO with NITRATES (Non-Imaging Transient Reconstruction And TEmporal Search): Detecting and localizing the faintest GRBs with a likelihood framework

The detection of the gravitational wave counterpart GRB 170817A, underluminous compared to the cosmological GRB population by a factor of 10,000, motivates significant effort in detecting and localizing a dim, nearby, and slightly off-axis population of short GRBs. Swift/BAT is the most sensitive GRB detector in operation, and the only one that regularly localizes GRBs to arcminute precision, critical to rapid followup studies. However, the utility of BAT in targeted sub-threshold searches had been historically curtailed by the unavailability of the necessary raw data for analysis. The new availability of time-tagged event (TTE) data from the GUANO system (arXiv:2005.01751), motivates renewed focus on developing sensitive targeted search analysis techniques to maximally exploit these data. While computationally cheap, we show that the typical coded-mask deconvolution imaging is limited in its sensitivity due to several factors. We formalize a maximum likelihood framework for the analysis of BAT data wherein signals are forward modelled through the full instrument response, and -- coupled with the development of new response models -- demonstrate its superior sensitivity to typical imaging via archival comparisons, injection campaigns, and, after implementing as a targeted search, a large number of low-latency GRB discoveries and confirmed arcminute localizations to date. We also demonstrate independent localization of some out-of-FOV GRBs for the first time. NITRATES's increased sensitivity boosts the discovery rate of GRB 170817A-like events in BAT by a factor of at least $3-4$x, along with enabling joint analyses and searches with other GRB, GW, neutrino, and FRB instruments. We provide public access to the response functions and search pipeline code.

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arXiv2022

The First Short GRB Millimeter Afterglow: The Wide-Angled Jet of the Extremely Energetic SGRB 211106A

We present the discovery of the first millimeter afterglow of a short-duration $γ$-ray burst (SGRB) and the first confirmed afterglow of an SGRB localized by the GUANO system on Swift. Our Atacama Large Millimeter/Sub-millimeter Array (ALMA) detection of SGRB 211106A establishes an origin in a faint host galaxy detected in Hubble Space Telescope (HST) imaging at $0.7\lesssim z\lesssim1.4$. From the lack of a detectable optical afterglow, coupled with the bright millimeter counterpart, we infer a high extinction, $A_{\rm V}\gtrsim2.6$ mag along the line of sight, making this the one of the most highly dust-extincted SGRBs known to date. The millimeter-band light curve captures the passage of the synchrotron peak from the afterglow forward shock and reveals a jet break at $t_{\rm jet}=29.2^{+4.5}_{-4.0}$~days. For a presumed redshift of $z=1$, we infer an opening angle, $θ_{\rm jet}=(15.5\pm1.4)$~degrees, and beaming-corrected kinetic energy of $\log(E_{\rm K}/{\rm erg})=51.8\pm0.3$, making this one of the widest and most energetic SGRB jets known to date. Combining all published millimeter-band upper limits in conjunction with the energetics for a large sample of SGRBs, we find that energetic outflows in high density environments are more likely to have detectable millimeter counterparts. Concerted afterglow searches with ALMA should yield detection fractions of 24-40% on timescales of $\gtrsim2$~days at rates $\approx0.8$-1.6 per year, outpacing the historical discovery rate of SGRB centimeter-band afterglows.

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arXiv2022

A low-cost ultraviolet-to-infrared absolute quantum efficiency characterization system of detectors

We present a low-cost ultraviolet to infrared absolute quantum efficiency detector characterization system developed using commercial off-the-shelf components. The key components of the experiment include a light source,a regulated power supply, a monochromator, an integrating sphere, and a calibrated photodiode. We provide a step-by-step procedure to construct the photon and quantum efficiency transfer curves of imaging sensors. We present results for the GSENSE 2020 BSI CMOS sensor and the Sony IMX 455 BSI CMOS sensor. As a reference for similar characterizations, we provide a list of parts and associated costs along with images of our setup.

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arXiv2021

Multi-messenger emission from the parsec-scale jet of the flat-spectrum radio quasar PKS 1502+106 coincident with high-energy neutrino IceCube-190730A

On July 30th, 2019 IceCube detected a high-energy astrophysical muon neutrino candidate, IC-190730A, with a $67\%$ probability of astrophysical origin. The flat spectrum radio quasar (FSRQ) PKS 1502+106 is in the error circle of the neutrino. Motivated by this observation, we study PKS 1502+106 as a possible source of IC-190730A. PKS 1502+106 was in a quiet state in terms of UV/optical/X-ray/gamma-ray flux at the time of the neutrino alert, we therefore model the expected neutrino emission from the source during its average long-term state, and investigate whether the emission of IC-190730A as a result of the quiet long-term emission of PKS 1502+106 is plausible. We analyse UV/optical and X-ray data and collect additional observations from the literature to construct the multi-wavelength spectral energy distribution of PKS 1502+106. We perform leptohadronic modelling of the multi-wavelength emission of the source and determine the most plausible emission scenarios and the maximum expected accompanying neutrino flux. A model in which the multi-wavelength emission of PKS 1502+106 originates beyond the broad-line region and inside the dust torus is most consistent with the observations. In this scenario, PKS 1502+106 can have produced up to of order one muon neutrino with energy exceeding 100 TeV in the lifetime of IceCube. An appealing feature of this model is that the required proton luminosity is consistent with the average required proton luminosity if blazars power the observed ultra-high-energy-cosmic-ray flux and well below the source's Eddington luminosity. If such a model is ubiquitous among FSRQs, additional neutrinos can be expected from other bright sources with energy $\gtrsim 10$ PeV.

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arXiv2020

Swift X-ray Follow-Up Observations of Gravitational Wave and High-Energy Neutrino Coincident Signals

Electromagnetic observations of gravitational wave and high-energy neutrino events are crucial in understanding the physics of their astrophysical sources. X-ray counterparts are especially useful in studying the physics of the jet, the energy of the outflow, and the particle acceleration mechanisms in the system. We present the Neil Gehrels Swift Observatory prompt searches for X-ray counterparts to the joint gravitational wave and high-energy neutrino coincident events that happened during the third observing run of LIGO/Virgo. Swift observed the overlap between gravitational wave and neutrino error regions for three of the considerable (p-value < 1%) joint gravitational wave and high-energy neutrino coincident alerts, which were generated by the IceCube Neutrino Observatory in realtime after triggering by the LIGO/Virgo gravitational wave public alerts. The searches did not associate any X-ray counterparts to any of the joint gravitational wave and high-energy neutrino coincident events, however, the follow-up of these alerts significantly improved the tiling techniques covering regions between the gravitational wave sky maps and neutrino's error regions, making the realtime system ready for the future potential discoveries. We will discuss the details of each follow-up procedure, the results of each search, and the plans for future searches.

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arXiv2020

Rotation Optimized Filter for Longevity (ROFL): Increasing the lifetime of Swift/UVOT simply

I demonstrate that a very simple and safe change to the planning software filter assignment algorithm for the Ultraviolet/Optical Telescope (UVOT) onboard the Neil Gehrels Swift Observatory can reduce the number of filter wheel rotations by $>10\%$, and its adoption is thus likely to significantly extend the usable lifetime of the UVOT instrument. I recommend that such a scheme be implemented.

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arXiv2020

Multimessenger observations of counterparts to IceCube-190331A

High-energy neutrinos are a promising tool for identifying astrophysical sources of high and ultra-high energy cosmic rays (UHECR). Prospects of detecting neutrinos at high energies ($\gtrsim$TeV) from blazars have been boosted after the recent association of IceCube-170922A and TXS 0506+056. We investigate the high-energy neutrino, IceCube-190331A, a high-energy starting event (HESE) with a high likelihood of being astrophysical in origin. We initiated a Swift/XRT and UVOT tiling mosaic of the neutrino localisation, and followed up with ATCA radio observations, compiling a multiwavelength SED for the most likely source of origin. NuSTAR observations of the neutrino location and a nearby X-ray source were also performed. We find two promising counterpart in the 90% confidence localisation region and identify the brightest as the most likely counterpart. However, no Fermi/LAT $γ$-ray source and no prompt Swift/BAT source is consistent with the neutrino event. At this point it is unclear whether any of the counterparts produced IceCube-190331A. We note that the Helix Nebula is also consistent with the position of the neutrino event, and we calculate that associated particle acceleration processes cannot produce the required energies to generate a high-energy HESE neutrino.

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arXiv2020

A Demonstration of Extremely Low Latency $γ$-ray, X-Ray & UV Follow-Up of a Millisecond Radio Transient

We report results of a novel high-energy follow-up observation of a potential Fast Radio Burst. The radio burst was detected by VLA/realfast and followed-up by the Neil Gehrels Swift Observatory in very low latency utilizing new operational capabilities of Swift (arXiv:2005.01751), with pointed soft X-ray and UV observations beginning at T0+32 minutes, and hard X-ray/gamma-ray event data saved around T0. These observations are $>10$x faster than previous X-ray/UV follow-up of any radio transient to date. No emission is seen coincident with the FRB candidate at T0, with a 0.2s fluence $5σ$ upper limit of $1.35\times10^{-8}$ erg cm$^{-2}$ (14-195 keV) for a SGR 1935+2154-like flare, nor at T0+32 minutes down to $3σ$ upper limits of 22.18 AB mag in UVOT u band, and $3.33\times10^{-13}$ erg cm$^{-2}$ s$^{-1}$ from 0.3-10 keV for the 2 ks observation. The candidate FRB alone is not significant enough to be considered astrophysical, so this note serves as a technical demonstration. These new Swift operational capabilities will allow future FRB detections to be followed up with Swift at even lower latencies than demonstrated here: 15-20 minutes should be regularly achievable, and 5-10 minutes occasionally achievable. We encourage FRB detecting facilities to release alerts in low latency to enable this science.

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arXiv2020

Gamma-ray Urgent Archiver for Novel Opportunities (GUANO): Swift/BAT event data dumps on demand to enable sensitive sub-threshold GRB searches

We introduce a new capability of the Neil Gehrels Swift Observatory, to provide event data from the Burst Alert Telescope (BAT) on demand in response to transients detected by other instruments. These event data are not continuously available due to the large telemetry load, but are critical to recovering weak or sub-threshold GRBs that are not triggered onboard, such as the likely counterparts to GW-detected off-axis binary neutron star mergers. We show that the availability of event data can effectively increase the rate of detections, and arcminute localizations, of GRB 170817-like bursts by >400%. We describe a spacecraft commanding pipeline purpose built to enable this science; to our knowledge the first fully autonomous extremely-low-latency commanding of a space telescope for scientific purposes. This pipeline has been successfully run in its complete form since early 2020, and has resulted in the recovery of BAT event data for >700 externally triggered events to date (GWs, neutrinos, GRBs triggered by other facilities, FRBs), now running with a success rate of ~90%. We exemplify the utility of this new capability by using the resultant data to (1) set the most sensitive (8 sigma) upper limits of $8.1\times10^{-8}$ erg cm$^{-2}$ s$^{-1}$ (14-195 keV) on prompt 1s duration short GRB-like emission within $\pm$ 15s of the unmodelled GW burst candidate S200114f, and (2) provide arcminute localizations for short GRB 200325A and other bursts. We show that using data from GUANO to localize GRBs discovered elsewhere, we can increase the net rate of arcminute localized GRBs by 10-20% per year. Along with the scientific yield of more sensitive searches for sub-threshold GRBs, the new capabilities designed for this project will enable further rapid response Target of Opportunity capabilities for Swift, and have implications for the design of future rapid-response space telescopes.

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arXiv2020

The Gravitational Wave Treasure Map: A Tool to Coordinate, Visualize, and Assess the Electromagnetic Follow-Up of Gravitational Wave Events

We present the Gravitational Wave Treasure Map, a tool to coordinate, visualize, and assess the electromagnetic follow-up of gravitational wave (GW) events. With typical GW localization regions of hundreds to thousands of square degrees and dozens of active follow-up groups, the pursuit of electromagnetic (EM) counterparts is a challenging endeavor, but the scientific payoff for early discovery of any counterpart is clear. With this tool, we provide a website and API interface that allows users to easily see where other groups have searched and better inform their own follow-up search efforts. A strong community of Treasure Map users will increase the overall efficiency of EM counterpart searches and will play a fundamental role in the future of multi-messenger astronomy.

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arXiv2020

Multi-Epoch Modeling of TXS 0506+056 and Implications for Long-Term High-Energy Neutrino Emission

The IceCube report of a $\sim 3.5σ$ excess of $13\pm5$ neutrino events in the direction of the blazar TXS 05056+056 in 2014-2015 and the 2017 detection of a high-energy neutrino, IceCube-170922A, during a gamma-ray flare from the same blazar, have revived the interest in scenarios for neutrino production in blazars. We perform comprehensive analyses on the long-term electromagnetic emission of TXS 05056+056 using optical, X-ray, and gamma-ray data from the All-Sky Automated Survey for Supernovae (ASAS-SN), the Neil Gehrels Swift Observatory (Swift), the Monitor of All-sky X-ray Image (MAXI), and the Fermi Large Area Telescope (Fermi-LAT). We also perform numerical modeling of the spectral energy distributions (SEDs) in four epochs prior to 2017 with contemporaneous gamma-ray and lower energy (optical and/or X-ray) data. We find that the multi-epoch SEDs are consistent with a hybrid leptonic scenario, where the gamma-rays are produced in the blazar zone via external inverse Compton scattering of accelerated electrons, and high-energy neutrinos are produced via the photomeson production process of co-accelerated protons. The multi-epoch SEDs can be satisfactorily explained with the same jet parameters and variable external photon density and electron luminosity. Using the maximal neutrino flux derived for each epoch, we put an upper limit of $\sim0.4-2$ on the muon neutrino number in ten years of IceCube observations. Our results are consistent with the IceCube-170922A detection, which can be explained as an upper fluctuation from the average neutrino rate expected from the source, but in strong tension with the 2014-2015 neutrino flare.

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arXiv2019

Scheduling Discovery in the 2020s

The 2020s will be the most data-rich decade of astronomy in history. As the scale and complexity of our surveys increase, the problem of scheduling becomes more critical. We must develop high-quality scheduling approaches, implement them as open-source software, and begin linking the typically separate stages of observation and data analysis.

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arXiv2019

A Summary of Multimessenger Science with Neutron Star Mergers

Neutron star mergers, referring to both binary neutron star and neutron star black hole mergers, are the canonical multimessenger events. They have been detected across the electromagnetic spectrum, have recently been detected in gravitational waves, and are likely to produce neutrinos over several decades in energy. The non-thermal prompt and afterglow emission of short gamma-ray bursts and the quasi-thermal emission from the radioactively powered kilonovae provide distinct insights into the physics of neutron star mergers. When combined with direct information on coalescence from gravitational waves and neutrinos these sources may become the best understood astrophysical transients. Multimessenger observations of these cataclysmic events will determine sources of gravitational waves and astrophysical neutrinos, enable precision cosmology, and unique tests of fundamental physics, the origin of heavy elements, the behavior of relativistic jets, and the equation of state of supranuclear matter. In this white paper we present a summary of the science discoveries possible with multimessenger observations of neutron star mergers and provide recommendations to enable them in the new era of time-domain, multimessenger astronomy.

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arXiv2019

Swift spectra of AT2018cow: A White Dwarf Tidal Disruption Event?

The bright transient AT2018cow has been unlike any other known type of transient. Its high brightness, rapid rise and decay and initially nearly featureless spectrum are unprecedented and difficult to explain using models for similar burst sources. We present evidence for faint gamma-ray emission continuing for at least 8 days, and featureless spectra in the ultraviolet bands -- both unusual for eruptive sources. The X-ray variability of the source has a burst-like character. The UV-optical spectrum does not show any CNO line but is well described by a blackbody. We demonstrate that a model invoking the tidal disruption of a 0.1 - 0.4 Msun Helium White Dwarf (WD) by a 100,000 to one million solar mass Black Hole (BH) located in the outskirts of galaxy Z~137-068 could provide an explanation for most of the characteristics shown in the multi-wavelength observations. A blackbody-like emission is emitted from an opaque photosphere, formed by the debris of the WD disruption. Broad features showing up in the optical/infrared spectra in the early stage are probably velocity broadened lines produced in a transient high-velocity outward moving cocoon. The asymmetric optical/infrared lines that appeared at a later stage are emission from an atmospheric layer when it detached from thermal equilibrium with the photosphere, which undergoes more rapid cooling. The photosphere shrinks when its temperature drops, and the subsequent infall of the atmosphere produced asymmetric line profiles. Additionally, a non-thermal jet might be present, emitting X-rays in the 10-150 keV band.

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arXiv2017

Improving science yield for NASA Swift with automated planning technologies

The Swift Gamma-Ray Burst Explorer is a uniquely capable mission, with three on-board instruments and rapid slewing capabilities. It serves as a fast-response satellite observatory for everything from gravitational-wave counterpart searches to cometary science. Swift averages 125 different observations per day, and is consistently over-subscribed, responding to about one-hundred Target of Oportunity (ToO) requests per month from the general astrophysics community, as well as co-pointing and follow-up agreements with many other observatories. Since launch in 2004, the demands put on the spacecraft have grown consistently in terms of number and type of targets as well as schedule complexity. To facilitate this growth, various scheduling tools and helper technologies have been built by the Swift team to continue improving the scientific yield of the Swift mission. However, these tools have been used only to assist humans in exploring the local pareto surface and for fixing constraint violations. Because of the computational complexity of the scheduling task, no automation tool has been able to produce a plan of equal or higher quality than that produced by a well-trained human, given the necessary time constraints. In this proceeding we formalize the Swift Scheduling Problem as a dynamic fuzzy Constraint Satisfaction Problem (DF-CSP) and explore the global solution space. We detail here several approaches towards achieving the goal of surpassing human quality schedules using classical optimization and algorithmic techniques, as well as machine learning and recurrent neural network (RNN) methods. We then briefly discuss the increased scientific yield and benefit to the wider astrophysics community that would result from the further development and adoption of these technologies.

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arXiv2017

GW170817: $\textit{Swift}$ UV detection of a blue kilonova, and improving the search in O3

$\textit{Swift}$'s rapid slewing, flexible planning, and multi-wavelength instruments make it the most capable space-based follow-up engine for finding poorly localized sources. During O1 and O2 $\textit{Swift}$ successfully tiled hundreds of square-degrees of sky in the LVC localization regions, searching for, and identifying, possible X-ray and UV/O transients in the field. $\textit{Swift}$ made important contributions to the discovery and characterization of the kilonova AT 2017gfo, discovering the UV emission and providing the deepest X-ray upper limits in the first 24 hours after the trigger, strongly constraining the dynamics and geometry of the counterpart. $\textit{Swift}$ tiled 92% of the galaxy convolved error region down to average X-ray flux sensitivities of $10^{-12}$ erg $\mbox{cm}^{-2}$ $\mbox{s}^{-1}$, significantly increasing our confidence that AT 2017gfo is indeed the counterpart to GW 170817 and sGRB 170817. However, there remains significant room for improvement of $\textit{Swift}$'s follow-up in preparation for O3. This will take the form of both revised observation strategy based on detailed analysis of the results from O2, and significant changes to $\textit{Swift}$'s operational capabilities. These improvements are necessary both for maximizing the likelihood that $\textit{Swift}$ finds a counterpart, and minimizing the impact that follow-up activities have on other $\textit{Swift}$ science priorities. We outline areas of improvement to the observing strategy itself for optimal tiling of the LVC localization regions. We also discuss ongoing work on operational upgrades that will decrease latency in our response time, and minimize impact on pre-planned observations, while maintaining spacecraft health and safety.

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