arXiv subjects

Pau Amaro-Seoane

Publications and source records attributed to Pau Amaro-Seoane.

At least 19 recordsLinked to original sources

arXiv2026

Electromagnetic alignment and jet precession around supermassive black holes: Quasi-periodic oscillations in tidal disruption events

We evaluate quasi-periodic oscillations and jet formation in tidal disruption events using the covariant formulation of electromagnetic angular-momentum transfer. General-relativistic frame-dragging tears apart misaligned transient accretion flows, forming an isolated inner mini-disk. The accumulation of magnetic flux on the event horizon powers a relativistic jet via the Blandford-Znajek mechanism. Because the magnetic field anchors to the precessing mini-disk, the jet axis rotates, generating geometric modulations in the observed X-ray and radio fluxes. To ensure physical consistency with the force-free magnetosphere required to launch a Blandford-Znajek jet, we model the electromagnetic back-reaction using a split-monopole magnetic field topology. By performing a small-spin expansion of the Noether current density over the event horizon, we derive a closed-form analytical reaction torque exerted by the electromagnetic field on the accretion plasma. We evaluate the resulting kinematics to show that the electromagnetic back-reaction induces a retrograde precession of the mini-disk, coupling with the prograde Lense-Thirring precession to dictate the global oscillation frequency. We formulate explicit predictions for observable transient signals and predict a monotonic attenuation of the peak-to-trough flux ratio as the mini-disk aligns, as well as a specific frequency drift signature characterized by an initial lengthening followed by an asymptotic shortening of the time interval between consecutive flares. We establish an analytical mechanism where magnetic flux depletion stalls alignment, predicting a constant residual modulation amplitude at late times. We formulate a methodology to extract the black hole spin and the magnetic flux density directly from the temporal derivatives of this predicted frequency drift, operating independently of spectral continuum fitting.

astro-ph.HE↗

arXiv2026

Gravitational Wave Astronomy With TianQin

The opening of the gravitational wave window has significantly enhanced our capacity to explore the universe's most extreme and dynamic sector. In the mHz frequency range, a diverse range of compact objects, from the most massive black holes at the farthest reaches of the Universe to the lightest white dwarfs in our cosmic backyard, generate a complex and dynamic symphony of gravitational wave signals. Once recorded by gravitational wave detectors, these unique fingerprints have the potential to decipher the birth and growth of cosmic structures over a wide range of scales, from stellar binaries and stellar clusters to galaxies and large-scale structures. The TianQin space-borne gravitational wave mission is scheduled for launch in the 2030s, with an operational lifespan of five years. It will facilitate pivotal insights into the history of our universe. This document presents a concise overview of the detectable sources of TianQin, outlining their characteristics, the challenges they present, and the expected impact of the TianQin observatory on our understanding of them.

astro-ph.GA↗

arXiv2025

The JWST Galactic Center Survey -- A White Paper

The inner hundred parsecs of the Milky Way hosts the nearest supermassive black hole, largest reservoir of dense gas, greatest stellar density, hundreds of massive main and post main sequence stars, and the highest volume density of supernovae in the Galaxy. As the nearest environment in which it is possible to simultaneously observe many of the extreme processes shaping the Universe, it is one of the most well-studied regions in astrophysics. Due to its proximity, we can study the center of our Galaxy on scales down to a few hundred AU, a hundred times better than in similar Local Group galaxies and thousands of times better than in the nearest active galaxies. The Galactic Center (GC) is therefore of outstanding astrophysical interest. However, in spite of intense observational work over the past decades, there are still fundamental things unknown about the GC. JWST has the unique capability to provide us with the necessary, game-changing data. In this White Paper, we advocate for a JWST NIRCam survey that aims at solving central questions, that we have identified as a community: i) the 3D structure and kinematics of gas and stars; ii) ancient star formation and its relation with the overall history of the Milky Way, as well as recent star formation and its implications for the overall energetics of our galaxy's nucleus; and iii) the (non-)universality of star formation and the stellar initial mass function. We advocate for a large-area, multi-epoch, multi-wavelength NIRCam survey of the inner 100\,pc of the Galaxy in the form of a Treasury GO JWST Large Program that is open to the community. We describe how this survey will derive the physical and kinematic properties of ~10,000,000 stars, how this will solve the key unknowns and provide a valuable resource for the community with long-lasting legacy value.

astro-ph.GA↗

arXiv2024

Neutrino follow-up with the Zwicky Transient Facility: Results from the first 24 campaigns

The Zwicky Transient Facility (ZTF) performs a systematic neutrino follow-up program, searching for optical counterparts to high-energy neutrinos with dedicated Target-of-Opportunity (ToO) observations. Since first light in March 2018, ZTF has taken prompt observations for 24 high-quality neutrino alerts from the IceCube Neutrino Observatory, with a median latency of 12.2 hours from initial neutrino detection. From two of these campaigns, we have already reported tidal disruption event (TDE) AT 2019dsg and likely TDE AT 2019fdr as probable counterparts, suggesting that TDEs contribute >7.8% of the astrophysical neutrino flux. We here present the full results of our program through to December 2021. No additional candidate neutrino sources were identified by our program, allowing us to place the first constraints on the underlying optical luminosity function of astrophysical neutrino sources. Transients with optical absolutes magnitudes brighter that $-21$ can contribute no more than 87% of the total, while transients brighter than $-22$ can contribute no more than 58% of the total, neglecting the effect of extinction and assuming they follow the star formation rate. These are the first observational constraints on the neutrino emission of bright populations such as superluminous supernovae. None of the neutrinos were coincident with bright optical AGN flares comparable to that observed for TXS 0506+056/IC170922A, with such optical blazar flares producing no more than 26% of the total neutrino flux. We highlight the outlook for electromagnetic neutrino follow-up programs, including the expected potential for the Rubin Observatory.

astro-ph.HE↗

arXiv2024

LISA Definition Study Report

The Laser Interferometer Space Antenna (LISA) is the first scientific endeavour to detect and study gravitational waves from space. LISA will survey the sky for Gravitational Waves in the 0.1 mHz to 1 Hz frequency band which will enable the study of a vast number of objects ranging from Galactic binaries and stellar mass black holes in the Milky Way, to distant massive black-hole mergers and the expansion of the Universe. This definition study report, or Red Book, presents a summary of the very large body of work that has been undertaken on the LISA mission over the LISA definition phase.

astro-ph.CO↗

arXiv2021

Revised event rates for extreme and extremely large mass-ratio inspirals

One of the main targets of the Laser Interferometer Space Antenna (LISA) is the detection of extreme mass-ratio inspirals (EMRIs) and extremely large mass-ratio inspirals (X-MRIs). Their orbits are expected to be highly eccentric and relativistic when entering the LISA band. Under these circumstances, the inspiral time-scale given by Peters' formula loses precision and the shift of the last-stable orbit (LSO) caused by the massive black hole spin could influence the event rates estimate. We re-derive EMRIs and X-MRIs event rates by implementing two different versions of a Kerr loss-cone angle that includes the shift in the LSO, and a corrected version of Peters' time-scale that accounts for eccentricity evolution, 1.5 post-Newtonian hereditary fluxes, and spin-orbit coupling. The main findings of our study are summarized as follows: (1) implementing a Kerr loss-cone changes the event rates by a factor ranging between 0.9 and 1.1; (2) the high-eccentricity limit of Peters' formula offers a reliable inspiral time-scale for EMRIs and X-MRIs, resulting in an event rate estimate that deviates by a factor of about 0.9 to 3 when compared to event rates computed with the corrected version of Peters' time-scale and the usual loss-cone definition. (3) Event rates estimates for systems with a wide range of eccentricities should be revised. Peters' formula overestimates the inspiral rates of highly eccentric systems by a factor of about 8 to 30 compared to the corrected values. Besides, for e$_0 \lesssim$0.8, implementing the corrected version of Peters' formula would be necessary to obtain accurate estimates.

astro-ph.GA↗

arXiv2021

Improved gravitational radiation time-scales II: spin-orbit contributions and environmental perturbations

Peters' formula is an analytical estimate of the time-scale of gravitational wave (GW)-induced coalescence of binary systems. It is used in countless applications, where the convenience of a simple formula outweighs the need for precision. However, many promising sources of the Laser Interferometer Space Antenna (LISA), such as supermassive black hole binaries and extreme mass-ratio inspirals (EMRIs), are expected to enter the LISA band with highly eccentric ($e \gtrsim$ 0.9) and highly relativistic orbits. These are exactly the two limits in which Peters' estimate performs the worst. In this work, we expand upon previous results and give simple analytical fits to quantify how the inspiral time-scale is affected by the relative 1.5 post-Newtonian (PN) hereditary fluxes and spin-orbit couplings. We discuss several cases that demand a more accurate GW time-scale. We show how this can have a major influence on quantities that are relevant for LISA event-rate estimates, such as the EMRI critical semi-major axis. We further discuss two types of environmental perturbations that can play a role in the inspiral phase: the gravitational interaction with a third massive body and the energy loss due to dynamical friction and torques from a surrounding gas medium ubiquitous in galactic nuclei. With the aid of PN corrections to the time-scale in vacuum, we find simple analytical expressions for the regions of phase space in which environmental perturbations are of comparable strength to the effects of any particular PN order, being able to qualitatively reproduce the results of much more sophisticated analyses.

astro-ph.GA↗

arXiv2021

Merging stellar and intermediate-mass black holes in dense clusters: implications for LIGO, LISA and the next generation of gravitational wave detectors

We study the formation of intermediate-mass ratio inspirals (IMRIs) triggered by the interactions between two stellar black holes (BHs) and an intermediate-mass BH (IMBH) inhabiting the centre of a dense star cluster. We exploit $N$-body models varying the IMBH mass, the stellar BH mass spectrum, and the star cluster properties. These simulations are coupled with a semi-analytic procedure to characterise the evolution of the remnant IMBH. The IMRIs formation probability attains values $\sim 5-50\%$, with larger values corresponding to larger IMBH masses. IMRIs map out the stellar BH mass spectrum, thus they might be used to unravel BH populations in star clusters harboring an IMBH. After the IMRI phase, an IMBH initially nearly maximal(almost non-rotating) tends to decrease(increase) its spin. If IMBHs grow mostly via repeated IMRIs, we show that only IMBH seeds sufficiently massive ($M_{\rm seed} > 300$ M$_\odot$) can grow up to $M_{\rm imbh} >10^3$ M$_\odot$ in dense globular clusters. Assuming that these seeds form at a redshift $z\sim 2-6$, we find that around $1-5\%$ of them would reach masses $\sim 500-1500$ M$_\odot$ at redshift $z=0$ and would exhibit low-spins, $S_{\rm imbh} < 0.2$. Measuring the mass and spin of IMBHs involved in IMRIs could help unravelling their formation mechanisms. We show that LISA can detect IMBHs in Milky Way globular clusters with a signal-to-noise ratio SNR$=10-100$, or in the Large Magellanic Cloud with an SNR$=8-40$. We provide the IMRIs merger rate for LIGO ($Γ_{\rm LIG} = 0.003-1.6$ yr$^{-1}$), LISA ($Γ_{\rm LIS} = 0.02-60$ yr$^{-1}$), ET ($Γ_{\rm ET} = 1-600$ yr$^{-1}$), and DECIGO ($Γ_{\rm DEC} = 6-3000$ yr$^{-1}$). Our simulations show that IMRIs' mass and spin encode crucial insights on the mechanisms that regulate IMBH formation and that the synergy among different detectors would enable us to fully unveil them. (Abridged)

astro-ph.GA↗

arXiv2021

The missing link in gravitational-wave astronomy: A summary of discoveries waiting in the decihertz range

Since 2015 the gravitational-wave observations of LIGO and Virgo have transformed our understanding of compact-object binaries. In the years to come, ground-based gravitational-wave observatories such as LIGO, Virgo, and their successors will increase in sensitivity, discovering thousands of stellar-mass binaries. In the 2030s, the space-based LISA will provide gravitational-wave observations of massive black holes binaries. Between the $\sim 10$-$10^3~\mathrm{Hz}$ band of ground-based observatories and the $\sim10^{-4}$-$10^{-1}~\mathrm{Hz}$ band of LISA lies the uncharted decihertz gravitational-wave band. We propose a Decihertz Observatory to study this frequency range, and to complement observations made by other detectors. Decihertz observatories are well suited to observation of intermediate-mass ($\sim10^2$-$10^4 M_\odot$) black holes; they will be able to detect stellar-mass binaries days to years before they merge, providing early warning of nearby binary neutron star mergers and measurements of the eccentricity of binary black holes, and they will enable new tests of general relativity and the Standard Model of particle physics. Here we summarise how a Decihertz Observatory could provide unique insights into how black holes form and evolve across cosmic time, improve prospects for both multimessenger astronomy and multiband gravitational-wave astronomy, and enable new probes of gravity, particle physics and cosmology.

gr-qc↗

arXiv2020

LION :Laser Interferometer On the mooN

Gravitational wave astronomy has now left its infancy and has become an important tool for probing the most violent phenomena in our universe. The LIGO/Virgo-KAGRA collaboration operates ground based detectors which cover the frequency band from 10 Hz to the kHz regime, meanwhile the pulsar timing array and the soon to launch LISA mission will cover frequencies below 0.1 Hz, leaving a gap in detectable gravitational wave frequencies. Here we show how a Laser Interferometer On the mooN (LION) gravitational wave detector would be sensitive to frequencies from sub Hz to kHz. We find that the sensitivity curve is such that LION can measure compact binaries with masses between 10 and 100M at cosmological distances, with redshifts as high as z= 100 and beyond, depending on the spin and the mass ratio of the binaries. LION can detect binaries of compact objects with higher-masses, with very large signal-to-noise ratios, help us tounderstand how supermassive black holes got their colossal masses on the cosmological landscape, and it can observe in detail intermediate-mass ratio inspirals at distances as large as at least 100 Gpc. Compact binaries that never reach the LIGO/Virgo sensitivity band can spend significantamounts of time in the LION band, while sources present in the LISA band can be picked up by the detector and observed until their final merger. Since LION covers the deci-Hertz regime with such large signal-to-noise ratios, it truly achieves the dream of multi messenger astronomy

astro-ph.IM↗

arXiv2020

The gravitational capture of compact objects by massive black holes

The gravitational capture of a stellar-mass compact object (CO) by a supermassive black hole is a unique probe of gravity in the strong field regime. Because of the large mass ratio, we call these sources extreme-mass ratio inspirals (EMRIs). In a similar manner, COs can be captured by intermediate-mass black holes in globular clusters or dwarf galaxies. The mass ratio in this case is lower, and hence we refer to the system as an intermediate-mass ratio inspiral (IMRI). Also, sub-stellar objects such as a brown dwarf, with masses much lighter than our Sun, can inspiral into supermassive black holes such as Sgr A* at our Galactic centre. In this case, the mass ratio is extremely large and, hence, we call this system ab extremely-large mass ratio inspirals (XMRIs). All of these sources of gravitational waves will provide us with a collection of snapshots of spacetime around a supermassive black hole that will allow us to do a direct mapping of warped spacetime around the supermassive black hole, a live cartography of gravity in this extreme gravity regime. E/I/XMRIs will be detected by the future space-borne observatories like LISA. There has not been any other probe conceived, planned or even thought of ever that can do the science that we can do with these inspirals. We will discuss them from a viewpoint of relativistic astrophysics.

gr-qc↗

arXiv2020

The Missing Link in Gravitational-Wave Astronomy: Discoveries waiting in the decihertz range

The gravitational-wave astronomical revolution began in 2015 with LIGO's observation of the coalescence of two stellar-mass black holes. Over the coming decades, ground-based detectors like LIGO will extend their reach, discovering thousands of stellar-mass binaries. In the 2030s, the space-based LISA will enable gravitational-wave observations of the massive black holes in galactic centres. Between LISA and ground-based observatories lies the unexplored decihertz gravitational-wave frequency band. Here, we propose a Decihertz Observatory to cover this band, and complement observations made by other gravitational-wave observatories. The decihertz band is uniquely suited to observation of intermediate-mass ($\sim 10^2$-$10^4 M_\odot$) black holes, which may form the missing link between stellar-mass and massive black holes, offering a unique opportunity to measure their properties. Decihertz observations will be able to detect stellar-mass binaries days to years before they merge and are observed by ground-based detectors, providing early warning of nearby binary neutron star mergers, and enabling measurements of the eccentricity of binary black holes, providing revealing insights into their formation. Observing decihertz gravitational-waves also opens the possibility of testing fundamental physics in a new laboratory, permitting unique tests of general relativity and the Standard Model of particle physics. Overall, a Decihertz Observatory will answer key questions about how black holes form and evolve across cosmic time, open new avenues for multimessenger astronomy, and advance our understanding of gravitation, particle physics and cosmology.

gr-qc↗

arXiv2020

Improved gravitational radiation time-scales: significance for LISA and LIGO-Virgo sources

We present a revised version of Peters' (1964) time-scale for the gravitational-wave (GW) induced decay of two point masses. The new formula includes the effects of the first-order post-Newtonian perturbation and additionally provides a simple fit to account for the Newtonian self-consistent evolution of the eccentricity. The revised time-scale is found by multiplying Peters' estimate by two factors, $R(e_0)= 8^{1-\sqrt{1-e_0}}$ and $Q_{\rm f}(p_0) = \exp \left(2.5 (r_{\rm S}/p_0) \right)$, where $e_0$ and $p_0$ are the initial eccentricity and periapsis, respectively, and $r_{\rm S}$ the Schwarzschild radius of the system. Their use can correct errors of a factor of 1-10 that arise from using the original Peters' formula. We apply the revised time-scales to a set of typical sources for existing ground-based laser interferometers and for the future Laser Interferometer Space Antenna (LISA), at the onset of their GW driven decay. We argue that our more accurate model for the orbital evolution will affect current event- and detection-rate estimates for mergers of compact object binaries, with stronger deviations for eccentric LISA sources, such as extreme and intermediate mass-ratio inspirals. We propose the correction factors $R$ and $Q_{\rm f}$ as a simple prescription to quantify decay time-scales more accurately in future population synthesis models. We also suggest that the corrected time-scale may be used as a computationally efficient alternative to numerical integration in other applications that include the modelling of radiation reaction for eccentric sources.

astro-ph.GA↗

arXiv2020

Phase shift of gravitational waves induced by aberration

The velocity of a gravitational wave (GW) source provides crucial information about its formation and evolution processes. Previous studies considered the Doppler effect on the phase of GWs as a potential signature of a time-dependent velocity of the source. However, the Doppler shift only accounts for the time component of the wave vector, and in principle motion also affects the spatial components. The latter effect, known as ``aberration'' for light, is analyzed in this paper for GWs and applied to the waveform modeling of an accelerating source. We show that the additional aberrational phase shift could be detectable in two astrophysical scenarios, namely, a recoiling binary black hole (BBH) due to GW radiation and a BBH in a triple system. Our results suggest that adding the aberrational phase shift in the waveform templates could significantly enhance the detectability of moving sources.

astro-ph.HE↗

arXiv2020

Rates of Stellar Tidal Disruption

Tidal disruption events occur rarely in any individual galaxy. Over the last decade, however, time-domain surveys have begun to accumulate statistical samples of these flares. What dynamical processes are responsible for feeding stars to supermassive black holes? At what rate are stars tidally disrupted in realistic galactic nuclei? What may we learn about supermassive black holes and broader astrophysical questions by estimating tidal disruption event rates from observational samples of flares? These are the questions we aim to address in this Chapter, which summarizes current theoretical knowledge about rates of stellar tidal disruption, and compares theoretical predictions to the current state of observations.

astro-ph.HE↗

arXiv2019

Our Supermassive Black Hole Rivaled the Sun in the Ancient X-ray Sky

Sagittarius A* (SgrA*) lying in the Galactic Centre $8$ kpc from Earth, hosts the closest supermassive black hole known to us. It is now inactive, but there is evidence indicating that about six million years ago it underwent a powerful outburst where the luminosity could have approached the Eddington limit. Motivated by the fact that in extragalaxies the supermassive black holes with similar masses and near-Eddington luminosities are usually strong X-ray emitters, we calculate here the X-ray luminosity of SgrA*. For that, we assume that the outburst was due to accretion of gas or the tidal disruption of a star. We show that these cases could precipitate on Earth a hard X-ray (i.e. $hν>2~{\rm keV}$) flux comparable to that from the current quiescent sun. The flux in harder energy band $20~{\rm keV}<hν<100~{\rm keV}$, however, surpasses that from an X-class solar flare, and the irradiation timescale is also much longer, ranging from weeks to $10^5$ years depending on the outburst scenario. In the solar system gas giants will suffer the biggest impact in their atmospheres. Lower-mass planets such as Earth receive a level of radiation that might have played a role in the evolution of their primitive atmospheres, so that a detailed study of the consequences deserves further investigation. Planetary systems closer to SgrA* receive higher irradiance levels, making them more likely uninhabitable.

astro-ph.HE↗

arXiv2019

Motion deviation of test body induced by spin and cosmological constant in extreme mass ratio inspiral binary system

The future space-borne detectors will provide the possibility to detect gravitational waves emitted from extreme mass ratio inspirals of stellar-mass compact objects into supermassive black holes. It is natural to expect that the spin of the compact object and cosmological constant will affect the orbit of the inspiral process and hence lead to the considerable phase shift of the corresponding gravitational waves. In this paper, we investigate the motion of a spinning test particle in the spinning black hole background with a cosmological constant and give the order of motion deviation induced by the particle's spin and the cosmological constant by considering the corresponding innermost stable circular orbit. By taking the neutron star or kerr black hole as the small body, the deviations of the innermost stable circular orbit parameters induced by the particle's spin and cosmological constant are given. Our results show that the deviation induced by particle's spin is much larger than that induced by cosmological constant when the test particle locates not very far away from the black hole, the accumulation of phase shift during the inspiral from the cosmological constant can be ignored when compared to the one induced by the particle's spin. However when the test particle locates very far away from the black hole, the impact from the cosmological constant will increase dramatically. Therefore the accumulation of phase shift for the whole process of inspiral induced by the cosmological constant and the particle's spin should be handled with caution.

gr-qc↗

arXiv2019

Making Bright Giants Invisible At The Galactic Centre

Current observations of the Galactic Center (GC) seem to display a core-like distribution of bright stars from $\sim 5"$ inwards. On the other hand, we observe young, massive stars at the GC, with roughly 20-50\% of them in a disc, mostly in the region where the bright giants appear to be lacking. In a previous publication we put the idea forward that the missing stars are deeply connected to the presence of this disc. The progenitor of the stellar disc is very likely to have been a gaseous disc that at some point fragmented and triggered star formation. This caused the appearance of overdensity regions in the disc that had high enough densities to ensure stripping large giants of their atmospheres and thus rendering them very faint. In this paper we use a stellar evolution code to derive the properties that a red giant would display in a colour-magnitude diagram, as well as a non-linearity factor required for a correct estimate of the mass loss. We find that in a very short timescale, the red giants (RGs) leave their standard evolutionary track. The non-linearity factor has values that not only depend on the properties of the clumps, but also on the physical conditions the giant stars, as we predicted analytically. According to our results, envelope stripping works, moving stars on a short timescale from the giant branch to the white dwarf stage, thus rendering them invisible to observations.

astro-ph.SR↗