arXiv subjects

N. Castro Segura

Publications and source records attributed to N. Castro Segura.

At least 37 records · Page 2Linked to original sources

arXiv2024

Discovery of Persistent Quasi-Periodic Oscillations in Accreting White Dwarfs: A New Link to X-ray Binaries

Almost all accreting black hole and neutron star X-ray binary systems (XRBs) exhibit prominent brightness variations on a few characteristic time-scales and their harmonics. These quasi-periodic oscillations (QPOs) are thought to be associated with the precession of a warped accretion disc, but the physical mechanism that generates the precessing warp remains uncertain. Relativistic frame dragging (Lense-Thirring precession) is one promising candidate, but a misaligned magnetic field is an alternative, especially for neutron star XRBs. Here, we report the discovery of 5 accreting white dwarf systems (AWDs) that display strong optical QPOs with characteristic frequencies and harmonic structures that suggest they are the counterpart of the QPOs seen in XRBs. Since AWDs are firmly in the classical (non-relativistic) regime, Lense-Thirring precession cannot account for these QPOs. By contrast, a weak magnetic field associated with the white dwarf can drive disc warping and precession in these systems, similar to what has been proposed for neutron star XRBs. Our observations confirm that magnetically driven warping is a viable mechanism for generating QPOs in disc-accreting astrophysical systems, certainly in AWDs and possibly also in (neutron star) XRBs. Additionally, they establish a new way to estimate magnetic field strengths, even in relatively weak-field systems where other methods are not available.

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arXiv2024

Rapid Mid-Infrared Spectral-Timing with JWST. I. The prototypical black hole X-ray Binary GRS 1915+105 during a MIR-bright and X-ray-obscured state

We present mid-infrared (MIR) spectral-timing measurements of the prototypical Galactic microquasar GRS 1915+105. The source was observed with the Mid-Infrared Instrument (MIRI) onboard JWST in June 2023 at a MIR luminosity L(MIR)~10^{36} erg/s exceeding past IR levels by about a factor of 10. By contrast, the X-ray flux is much fainter than the historical average, in the source's now-persistent 'obscured' state. The MIRI low-resolution spectrum shows a plethora of emission lines, the strongest of which are consistent with recombination in the hydrogen Pfund (Pf) series and higher. Low amplitude (~1%) but highly significant peak-to-peak photometric variability is found on timescales of ~1,000 s. The brightest Pf(6-5) emission line lags the continuum. Though difficult to constrain accurately, this lag is commensurate with light-travel timescales across the outer accretion disc or with expected recombination timescales inferred from emission line diagnostics. Using the emission line as a bolometric indicator suggests a moderate (~5-30% Eddington) intrinsic accretion rate. Multiwavelength monitoring shows that JWST caught the source close in-time to unprecedentedly bright MIR and radio long-term flaring. Assuming a thermal bremsstrahlung origin for the MIRI continuum suggests an unsustainably high mass-loss rate during this time unless the wind remains bound, though other possible origins cannot be ruled out. PAH features previously detected with Spitzer are now less clear in the MIRI data, arguing for possible destruction of dust in the interim. These results provide a preview of new parameter space for exploring MIR spectral-timing in XRBs and other variable cosmic sources on rapid timescales.

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arXiv2024

Evolution of Spin in the Intermediate Polar CC Sculptoris

We report on spin variations in the intermediate polar and cataclysmic variable CC Scl, as seen by the Transiting Exoplanet Survey Satellite (TESS). By studying both the spin period and its harmonic, we find that the spin has varied since it was first observed in 2011. We find the latest spin value for the source to be 389.473(6)s, equivalent to 0.00450779(7) days, 0.02s shorter than the first value measured. A linear fit to these and intermediate data give a rate of change of spin ~-4.26(2.66)e10^-11 and a characteristic timescale tau~2.90e10^5 years, in line with other known intermediate polars with varying spin. The spin profile of this source also matches theoretical spin profiles of high-inclination intermediate polars, and furthermore, appears to have changed in shape over a period of three years. Such `spin-up' in an intermediate polar is considered to be from mass accretion onto the white dwarf (the primary), and we note the presence of dwarf nova eruptions in this source as being a possible catalyst of the variations.

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arXiv2023

Shedding far-ultraviolet light on the donor star and evolutionary state of the neutron-star LMXB Swift J1858.6-0814

The evolution of accreting X-ray binary systems is closely coupled to the properties of their donor stars. As a result, we can constrain the evolutionary track a system is by establishing the nature of its donor. Here, we present far-UV spectroscopy of the transient neutron-star low-mass X-ray binary Swift J1858 in three different accretion states (low-hard, high-hard and soft). All of these spectra exhibit anomalous N\,{\sc v}, C\,{\sc iv}, Si\,{\sc iv} and He\,{\sc ii} lines, suggesting that its donor star has undergone CNO processing. We also determine the donor's effective temperature, $T_{d} \simeq 5700$~K, and radius, $R_d \simeq 1.7~R_{\odot}$, based on photometric observations obtained during quiescence. Lastly, we leverage the transient nature of the system to set an upper limit of $\dot{M}_{\rm acc} \lesssim 10^{-8.5}~M_{\odot}~yr^{-1}$ on the present-day mass-transfer rate. Combining all these with the orbital period of the system, $P_{\rm orb} = 21.3$~hrs, we search for viable evolution paths. The initial donor masses in the allowed solutions span the range $1~M_{\odot} \lesssim M_{d,i} \lesssim 3.5~M_{\odot}$. All but the lowest masses in this range are consistent with the strong CNO-processing signature in the UV line ratios. The present-day donor mass in the permitted tracks are $0.5~M_{\odot}\lesssim M_{d,obs} \lesssim 1.3~M_{\odot}$, higher than suggested by recent eclipse modelling. Since $P_{\rm orb}$ is close to the so-called bifurcation period, both converging and diverging binary tracks are permitted. If Swift J1858 is on a converging track, it will end its life as an ultra-compact system with a sub-stellar donor star.

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arXiv2023

Variability and evolution of the optical polarization of a sample of gamma-ray blazars

We present a polarization variability analysis of a sample of 26 $γ$-ray blazars monitored by the Steward Observatory between 2008 and 2018 in the optical band. We investigate the properties and long-term variability of their optical polarization, searching for differences between blazar types. We observe that BL Lac objects are typically less polarized and less variable than flat spectrum radio quasars (FSRQs). Moreover, BL Lacs display a distribution of their polarization angle typically oriented in a preferential direction, contrary to the rather random distribution of FSRQs. For the latter blazar type, as well as those sources showing a bright stellar emission, we take into account the depolarizing effect introduced by the broad line region and the host galaxy on the measured polarization degree. In this sample we also observe that BL Lacs present an uncorrelated evolution of the flux and the polarization. Contrary, FSRQs show a correlation before the depolarization correction, that is lost however after considering this effect. In addition, we study the behaviour of the polarization angle, searching for angle rotations in its long-term evolution. We derive that the FSRQs studied here show rotations more frequently than BL Lac objects by a factor $\sim$1.5. During these periods we also observe a systematic decrease of the polarization fraction, as well as a marginal flux increase, not significant however to connect rotations with optical flares. We interpret these results within the extended shock-in-jet scenario, able to explain the overall features observed here for the polarization of the blazar sample.

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arXiv2023

A transient ultraviolet outflow in the short-period X-ray binary UW CrB

Accreting low mass X-ray binaries (LMXBs) are capable of launching powerful outflows such as accretion disc winds. In disc winds, vast amounts of material can be carried away, potentially greatly impacting the binary and its environment. Previous studies have uncovered signatures of disc winds in the X-ray, optical, near-infrared, and recently even the UV band, predominantly in LMXBs with large discs ($P_{orb}{\geq}20$ hrs). Here, we present the discovery of transient UV outflow features in UW CrB, a high-inclination ($i{\geq}77$°) neutron star LMXB with an orbital period of only $P_{orb}{\approx}111$ min. We present P-Cygni profiles detected for Si iv 1400Å and tentatively for N v 1240Å in one 15 min exposure, which is the only exposure covering orbital phase $ϕ{\approx}0.7{-}0.8$, with a velocity of ${\approx}1500$ km/s. We show that due to the presence of black body emission from the neutron star surface and/or boundary layer, a thermal disc wind can be driven despite the short $P_{orb}$, but explore alternative scenarios as well. The discovery that thermal disc winds may occur in NS-LMXBs with $P_{orb}$ as small as ${\approx}111$ min, and can potentially be transient on time scales as short as ${\approx}15$ min, warrants further observational and theoretical work.

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arXiv2023

A shared accretion instability for black holes and neutron stars

Accretion disks around compact objects are expected to enter an unstable phase at high luminosity. One instability may occur when the radiation pressure generated by accretion modifies the disk viscosity, resulting in the cyclic depletion and refilling of the inner disk on short timescales. Such a scenario, however, has only been quantitatively verified for a single stellar-mass black hole. Although there are hints of these cycles in a few isolated cases, their apparent absence in the variable emission of most bright accreting neutron stars and black holes has been a lingering puzzle. Here we report the presence of the same multiwavelength instability around an accreting neutron star. Moreover, we show that the variability across the electromagnetic spectrum-from radio to X-ray-of both black holes and neutron stars at high accretion rates can be explained consistently if the accretion disks are unstable, producing relativistic ejections during transitions that deplete or refill the inner disk. Such new association allows us to identify the main physical components responsible for the fast multiwavelength variability of highly accreting compact objects.

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arXiv2022

The Birth of a Relativistic Jet Following the Disruption of a Star by a Cosmological Black Hole

A black hole can launch a powerful relativistic jet after it tidally disrupts a star. If this jet fortuitously aligns with our line of sight, the overall brightness is Doppler boosted by several orders of magnitude. Consequently, such on-axis relativistic tidal disruption events (TDEs) have the potential to unveil cosmological (redshift $z>$1) quiescent black holes and are ideal test beds to understand the radiative mechanisms operating in super-Eddington jets. Here, we present multi-wavelength (X-ray, UV, optical, and radio) observations of the optically discovered transient \target at $z=1.193$. Its unusual X-ray properties, including a peak observed luminosity of $\gtrsim$10$^{48}$ erg s$^{-1}$, systematic variability on timescales as short as 1000 seconds, and overall duration lasting more than 30 days in the rest-frame are traits associated with relativistic TDEs. The X-ray to radio spectral energy distributions spanning 5-50 days after discovery can be explained as synchrotron emission from a relativistic jet (radio), synchrotron self-Compton (X-rays), and thermal emission similar to that seen in low-redshift TDEs (UV/optical). Our modeling implies a beamed, highly relativistic jet akin to blazars but requires extreme matter-domination, i.e, high ratio of electron-to-magnetic field energy densities in the jet, and challenges our theoretical understanding of jets.

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arXiv2022

A persistent ultraviolet outflow from an accreting neutron star binary transient

All disc-accreting astrophysical objects produce powerful outflows. In binaries containing neutron stars (NS) or black holes, accretion often takes place during violent outbursts. The main disc wind signatures during these eruptions are blue-shifted X-ray absorption lines, which are preferentially seen in disc-dominated "soft states". By contrast,optical wind-formed lines have recently been detected in "hard states", when a hot corona dominates the luminosity. The relationship between these signatures is unknown, and no erupting system has revealed wind-formed lines between the X-ray and optical bands yet, despite the many strong resonance transitions in this ultraviolet (UV) region. Here, we show that the transient NS binary Swift J1858.6-0814 exhibits wind-formed, blue-shifted absorption associated with C IV, N V and He II in time-resolved UV spectroscopy during a luminous hard state. This represents the first evidence for a warm, moderately ionized outflow component in this state. Simultaneously observed optical lines also display transient blue-shifted absorption. Decomposing the UV data into constant and variable components, the blue-shifted absorption is associated with the former. This implies that the outflow is not connect to the luminous flares in the data. The joint presence of UV and optical wind features reveals a multi-phase and/or stratified outflow from the outer disc. This type of persistent mass loss across all accretion states has been predicted by radiation-hydrodynamic simulations and helps to explain the shorter-than-expected outbursts duration.

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arXiv2022

A statistical study of the optical spectral variability in gamma-ray blazars

Blazars optical emission is generally dominated by relativistic jets, although the host galaxy, accretion disk and broad line region (BLR) may also contribute significantly. Disentangling their contributions has been challenging for years due to the dominance of the jet. To quantify the contributions to the spectral variability, we use the statistical technique for dimensionality reduction Non-Negative Matrix Factorization on a spectroscopic data set of 26 $γ$-ray blazars. This technique allows to model large numbers of spectra in terms of a reduced number of components.We use a priori knowledge to obtain components associated to meaningful physical processes. The sources are classified according to their optical spectrum as host-galaxy dominated BL Lac objects (BL Lacs), BL Lacs, or Flat Spectrum Radio Quasars (FSRQs). Host-galaxy sources show less variability, as expected, and bluer-when-brighter trends, as the other BL Lacs. For FSRQs, more complicated colour-flux behaviours are observed: redder-when-brighter for low states saturating above a certain level and, in some cases, turning to bluer-when-brighter. We are able to reproduce the variability observed during 10 years using only 2 to 4 components, depending on the type. The simplest scenario corresponds to host-galaxy blazars, whose spectra are reconstructed using the stellar population and a power law for the jet. BL Lac spectra are reproduced using from 2 to 4 power laws. Different components can be associated to acceleration/cooling processes taking place in the jet. The reconstruction of FSRQs also incorporates a QSO-like component to account for the BLR, plus a very steep power law, associated to the accretion disk.

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arXiv2021

Dips and eclipses in the X-ray binary Swift J1858.6-0814 observed with NICER

We present the discovery of eclipses in the X-ray light curves of the X-ray binary Swift J1858.6-0814. From these, we find an orbital period of $P=76841.3_{-1.4}^{+1.3}$ s ($\approx21.3$ hours) and an eclipse duration of $t_{\rm ec}=4098_{-18}^{+17}$ s ($\approx1.14$ hours). We also find several absorption dips during the pre-eclipse phase. From the eclipse duration to orbital period ratio, the inclination of the binary orbit is constrained to $i>70^\circ$. The most likely range for the companion mass suggests that the inclination is likely to be closer to this value than $90^\circ$. The eclipses are also consistent with earlier data, in which strong variability ('flares') and the long orbital period prevent clear detection of the period or eclipses. We also find that the bright flares occurred preferentially in the post-eclipse phase of the orbit, likely due to increased thickness at the disc-accretion stream interface preventing flares being visible during the pre-eclipse phase. This supports the notion that variable obscuration is responsible for the unusually strong variability in Swift J1858.6-0814.

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arXiv2020

The Tidal Disruption Event AT 2018hyz I: Double-peaked emission lines and a flat Balmer decrement

We present results from spectroscopic observations of AT 2018hyz, a transient discovered by the ASAS-SN survey at an absolute magnitude of $M_V\sim -20.2$ mag, in the nucleus of a quiescent galaxy with strong Balmer absorption lines. AT 2018hyz shows a blue spectral continuum and broad emission lines, consistent with previous TDE candidates. High cadence follow-up spectra show broad Balmer lines and He I in early spectra, with He II making an appearance after $\sim70-100$ days. The Balmer lines evolve from a smooth broad profile, through a boxy, asymmetric double-peaked phase consistent with accretion disc emission, and back to smooth at late times. The Balmer lines are unlike typical AGN in that they show a flat Balmer decrement (H$α$/H$β\sim1.5$), suggesting the lines are collisionally excited rather than being produced via photo-ionisation. The flat Balmer decrement together with the complex profiles suggest that the emission lines originate in a disc chromosphere, analogous to those seen in cataclysmic variables. The low optical depth of material due to a possible partial disruption may be what allows us to observe these double-peaked, collisionally excited lines. The late appearance of He II may be due to an expanding photosphere or outflow, or late-time shocks in debris collisions.

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arXiv2020

Discovery of thermonuclear (Type I) X-ray bursts in the X-ray binary Swift J1858.6-0814 observed with NICER and NuSTAR

Swift J1858.6-0814 is a recently discovered X-ray binary notable for extremely strong variability (by factors $>100$ in soft X-rays) in its discovery state. We present the detection of five thermonuclear (Type I) X-ray bursts from Swift J1858.6-0814, implying that the compact object in the system is a neutron star. Some of the bursts show photospheric radius expansion, so their peak flux can be used to estimate the distance to the system. The peak luminosity, and hence distance, can depend on several system parameters; for the most likely values, a high inclination and a helium atmosphere, $D=12.8_{-0.6}^{+0.8}$ kpc, although systematic effects allow a conservative range of $9-18$ kpc. Before one burst, we detect a QPO at $9.6\pm0.5$ mHz with a fractional rms amplitude of $2.2\pm0.2$% ($0.5-10$ keV), likely due to marginally stable burning of helium; similar oscillations may be present before the other bursts but the light curves are not long enough to allow their detection. We also search for burst oscillations but do not detect any, with an upper limit in the best case of 15% fractional amplitude (over $1-8$ keV). Finally, we discuss the implications of the neutron star accretor and this distance on other inferences which have been made about the system. In particular, we find that Swift J1858.6-0814 was observed at super-Eddington luminosities at least during bright flares during the variable stage of its outburst.

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arXiv2020

Bow-shocks, nova shells, disc winds and tilted discs: The Nova-Like V341 Ara Has It All

V341 Ara was recently recognised as one of the closest (d ~ 150 pc) and brightest (V~ 10) nova-like cataclysmic variables. This unique system is surrounded by a bright emission nebula, likely to be the remnant of a recent nova eruption. Embedded within this nebula is a prominent bow-shock, where the system's accretion disc wind runs into its own nova shell. In order to establish its fundamental properties, we present the first comprehensive multi-wavelength study of the system. Long-term photometry reveals quasi-periodic, super-orbital variations with a characteristic time-scale of 10-16 days and typical amplitude of ~1 mag. High-cadence photometry from TESS reveals for the first time both the orbital period and a "negative superhump" period. The latter is usually interpreted as the signature of a tilted accretion disc. We propose a recently developed disc instability model as a plausible explanation for the photometric behaviour. In our spectroscopic data, we clearly detect anti-phased absorption and emission line components. Their radial velocities suggest a high mass ratio, which in turn implies an unusually low white dwarf mass. We also constrain the wind mass-loss rate of the system from the spatially resolved [O iii] emission produced in the bow-shock; this can be used to test and calibrate accretion disc wind models. We suggest a possible association between V341 Ara and a "guest star" mentioned in Chinese historical records in AD1240. If this marks the date of the system's nova eruption, V341 Ara would be the oldest recovered nova of its class and an excellent laboratory for testing nova theory.

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arXiv2020

Soft X-ray emission lines in the X-ray binary Swift J1858.6-0814 observed with XMM-Newton-RGS: disc atmosphere or wind?

We find soft X-ray emission lines from the X-ray binary Swift J1858.6-0814 in data from XMM-Newton-RGS: N VII, O VII and O VIII, as well as notable residuals short of a detection at Ne IX and other higher ionisation transitions. These could be associated with the disc atmosphere, as in accretion disc corona sources, or with a wind, as has been detected in Swift J1858.6-0814 in emission lines at optical wavelengths. Indeed, the N VII line is redshifted, consistent with being the emitting component of a P-Cygni profile. We find that the emitting plasma has an ionisation parameter $\log(ξ)=1.35\pm0.2$ and a density $n>1.5\times10^{11}$ cm$^{-3}$. From this, we infer that the emitting plasma must be within $10^{13}$ cm of the ionising source, $\sim5\times10^{7}r_{\rm g}$ for a $1.4M_{\odot}$ neutron star, and from the line width that it is at least $10^4r_{\rm g}$ away ($2\times10^{9}(M/1.4M_{\odot})$ cm). We compare this with known classes of emission line regions in other X-ray binaries and active galactic nuclei.

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arXiv2020

A MeerKAT Survey of Nearby Novalike Cataclysmic Variables

We present the results of MeerKAT radio observations of eleven nearby novalike cataclysmic variables. We have detected radio emission from IM Eri, RW Sex, V3885 Sgr and V603 Aql. While RW Sex, V3885 Sgr and V603 Aql had been previously detected, this is the first reported radio detection of IM Eri. Our observations have doubled the sample of non-magnetic CVs with sensitive radio data. We observe that at our radio detection limits, a specific optical luminosity $\gtrsim 2.2\times 10^{18}$ erg/s/Hz (corresponding to $M_V \lesssim 6.0$) is required to produce a radio detection. We also observe that the X-ray and radio luminosities of our detected novalikes are on an extension of the $L_{X} \propto L_{R}^{\sim 0.7}$ power law originally proposed for non-pulsating neutron star low-mass X-ray binaries. We find no other correlations between the radio emission and emission in other wavebands or any other system parameters for the existing sample of radio-detected non-magnetic CVs. We measure in-band (0.9-1.7 GHz) radio spectral indices that are consistent with reports from earlier work. Finally, we constructed broad spectral energy distributions for our sample from published multi-wavelength data, and use them to place constraints on the mass transfer rates of these systems.

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arXiv2020

The variable radio counterpart of Swift J1858.6-0814

Swift J1858.6-0814 is a transient neutron star X-ray binary discovered in October 2018. Multi-wavelength follow-up observations across the electromagnetic spectrum revealed many interesting properties, such as erratic flaring on minute timescales and evidence for wind outflows at both X-ray and optical wavelengths, strong and variable local absorption, and an anomalously hard X-ray spectrum. Here, we report on a detailed radio observing campaign consisting of one observation at 5.5/9 GHz with the Australia Telescope Compact Array, and nine observations at 4.5/7.5 GHz with the Karl G. Jansky Very Large Array. A radio counterpart with a flat to inverted radio spectrum is detected in all observations, consistent with a compact jet being launched from the system. Swift J1858.6-0814 is highly variable at radio wavelengths in most observations, showing significant variability when imaged on 3-to-5-minute timescales and changing up to factors of 8 within 20 minutes. The periods of brightest radio emission are not associated with steep radio spectra, implying they do not originate from the launching of discrete ejecta. We find that the radio variability is similarly unlikely to have a geometric origin, be due to scintillation, or be causally related to the observed X-ray flaring. Instead, we find that it is consistent with being driven by variations in the accretion flow propagating down the compact jet. We compare the radio properties of Swift J1858.6-0814 with those of Eddington-limited X-ray binaries with similar X-ray and optical characteristics, but fail to find a match in radio variability, spectrum, and luminosity.

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arXiv2020

The Mystery of Photometric Twins DES17X1boj and DES16E2bjy

We present an analysis of DES17X1boj and DES16E2bjy, two peculiar transients discovered by the Dark Energy Survey (DES). They exhibit nearly identical double-peaked light curves which reach very different maximum luminosities (M$_\mathrm{r}$ = -15.4 and M$_\mathrm{r}$ = -17.9, respectively). The light curve evolution of these events is highly atypical and has not been reported before. The transients are found in different host environments: DES17X1boj was found near the nucleus of a spiral galaxy, while DES16E2bjy is located in the outskirts of a passive red galaxy. Early photometric data is well fitted with a blackbody and the resulting moderate photospheric expansion velocities (1800 km/s for DES17X1boj and 4800 km/s for DES16E2bjy) suggest an explosive or eruptive origin. Additionally, a feature identified as high-velocity CaII absorption (v $\approx$ 9400km/s) in the near-peak spectrum of DES17X1boj may imply that it is a supernova. While similar light curve evolution suggests a similar physical origin for these two transients, we are not able to identify or characterise the progenitors.

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