arXiv subjects

Simone Scaringi

Publications and source records attributed to Simone Scaringi.

At least 55 records · Page 3Linked to original sources

arXiv2022

Hitting a New Low: The Unique 28 h Cessation of Accretion in the TESS Light Curve of YY Dra (DO Dra)

We present the Transiting Exoplanet Surveying Satellite (TESS) light curve of the intermediate polar YY Draconis (YY Dra, also known as DO Dra). The power spectrum indicates that while there is stream-fed accretion for most of the observational period, there is a day-long, flat-bottomed low state at the beginning of 2020 during which the only periodic signal is ellipsoidal variation and there is no appreciable flickering. We interpret this low state to be a complete cessation of accretion, a phenomenon that has been observed only once before in an intermediate polar. Simultaneous ground-based observations of this faint state establish that when accretion is negligible, YY Dra fades to $g=17.37\pm0.12$, which we infer to be the magnitude of the combined photospheric contributions of the white dwarf and its red dwarf companion. Using survey photometry, we identify additional low states in 2018-2019 during which YY Dra repeatedly fades to -- but never below -- this threshold. This implies relatively frequent cessations in accretion. Spectroscopic observations during future episodes of negligible accretion can be used to directly measure the field strength of the white dwarf by Zeeman splitting. Separately, we search newly available catalogs of variable stars in an attempt to resolve the long-standing dispute over the proper identifier of this system.

astro-ph.SR↗

arXiv2021

Rapid bursts of magnetically gated accretion in the intermediate polar V1025 Cen

Magnetically gated accretion has emerged as a proposed mechanism for producing extremely short, repetitive bursts of accretion onto magnetized white dwarfs in intermediate polars (IPs), but this phenomenon has not been detected previously in a confirmed IP. We report the 27-day TESS light curve of V1025 Cen, an IP that shows a remarkable series of twelve bursts of accretion, each lasting for less than six hours. The extreme brevity of the bursts and their short recurrence times (~1-3 days) are incompatible with the dwarf-nova instability, but they are natural consequences of the magnetic gating mechanism developed by Spruit & Taam to explain the Type II bursts of the accreting neutron star known as the Rapid Burster. In this model, the accretion flow piles up at the magnetospheric boundary and presses inward until it couples with the star's magnetic field, producing an abrupt burst of accretion. After each burst, the reservoir of matter at the edge of the magnetosphere is replenished, leading to cyclical bursts of accretion. A pair of recent studies applied this instability to the suspected IPs MV Lyr and TW Pic, but the magnetic nature of these two systems has not been independently confirmed. In contrast, previous studies have unambiguously established the white dwarf in V1025 Cen to be significantly magnetized. The detection of magnetically gated bursts in a confirmed IP therefore validates the extension of the Spruit & Taam instability to magnetized white dwarfs.

astro-ph.SR↗

arXiv2021

Searching for Diamagnetic Blob Accretion in the 74 day K2 Observation of V2400 Ophiuchi

Since its discovery in 1995, V2400 Ophiuchi (V2400 Oph) has stood apart from most known intermediate polar cataclysmic variables due to its proposed magnetic field strength (9-27 MG) and disk-less accretion. To date, the exact accretion mechanism of the system is still unknown, and standard accretion models fail to accurately predict the peculiar behavior of its lightcurve. We present the K2 Campaign~11 light curve of V2400 Oph recording 74.19 days of photometric data cadenced at 1 minute. The light curve is dominated by aperiodic flickering and quasi-periodic oscillations, which make the beat and spin signals inconspicuous on short timescales. Notably, a log-log full power spectrum shows a break frequency at $\sim10^2$ cycles~d$^{-1}$ similar to some disk-fed systems. Through power spectral analysis, the beat and spin periods are measured as $1003.4\pm0.2$ seconds and $ 927.7\pm 0.1$ seconds respectively. A power spectrum of the entire K2 observation demonstrates beat period dominance. However, time-resolved power spectra reveals a strong dependence between observation length and the dominant frequency of the light curve. For short observations (2-12 hrs) the beat, spin, or first beat harmonic can be observed as the dominant periodic signal. Such incoherence and variability indicate a dynamical accretion system more complex than current intermediate polar theories can explain. We propose that a diamagnetic blob accretion model may serve as a plausible explanation for the accretion mechanism.

astro-ph.SR↗

arXiv2021

ASAS J071404+7004.3 -- a close, bright nova-like cataclysmic variable with gusty winds

Despite being bright ($V=12$) and nearby ($d=212$ pc) ASAS J071404+7004.3 has only recently been identified as a nova-like cataclysmic variable. We present time-resolved optical spectroscopy obtained at the Isaac Newton Telescope together with $\textit{Swift}$ X-ray and ultraviolet observations. We combined these with $\textit{TESS}$ photometry and find a period of 3.28h and a mass transfer rate of $4-9 \times 10^{-9} M_{sun}/yr$. Historical photometry shows at least one low state establishing the system as a VY Scl star. Our high-cadence spectroscopy also revealed rapidly changing winds emanating from the accretion disc. We have modelled these using the Monte Carlo PYTHON code and shown that all the emission lines could emanate from the wind - which would explain the lack of double-peaked lines in such systems. In passing,we discuss the effect of variability on the position of cataclysmic variables in the $\textit{Gaia}$ Hertzsprung-Russell diagram.

astro-ph.SR↗

arXiv2021

SU Lyn -- a transient symbiotic star

SU Lyn is a binary system composed of a white dwarf and a red giant star. Although it is known to be bright and variable at X-ray wavelengths, the optical counterpart of the source appeared as a single red giant without prominent emission lines. Because of the lack of optical features typical for interacting systems, the system was classified as a hidden symbiotic star. We present the results of optical monitoring of the system. While SU Lyn did not show substantial photometric variability, the spectroscopic observations revealed a complex behavior. The system showed strong emission line variability, including P Cygni profiles, changing line emission environments, and variable reddening. Both X-ray and optical observations indicate that the components of SU Lyn were interacting only for a short time during the last twelve years of monitoring. For the first time we showed that SU Lyn resembled a classical symbiotic star when it was X-ray bright, and remained hidden afterwards. We also discuss the current evolutionary status of the red giant, as well as possible future evolution of the system. We suggest that SU Lyn could be a progenitor of a classical, persistent symbiotic system.

astro-ph.SR↗

arXiv2021

TACOS: TESS AM~CVn Outbursts Survey

Using \emph{TESS} we are doing a systematic study of outbursting AM~CVn systems to place some limits on the current outbursts models. We present the \emph{TESS} light curve (LC) for 9 AM~CVns showing both superoutbursts (SOs) and normal outbursts (NOs). The continuous coverage of the outbursts with \emph{TESS} allows us to place stringent limits on the duration and structures of the SOs and the NOs. We present evidence that in at least some of the systems enhanced mass transfer (EMT) has to be taken into account to explain the observed LC of the SOs and rebrighthening phase after the SOs. For others, the colour evolution from simultaneous observations in $g$ and $r$ with ZTF differs from previously reported colour evolution of longer period AM~CVns where EMT is responsible for the SO. We also find that due to the lack of sufficiently high cadence coverage the duration of many systems might have been overestimated in previous ground-based surveys. We report the SO duration for 6 AM~CVns. We also found that precursors are a common feature of SOs in AM~CVns and are seen in the LC of 5 of the 6 reported SOs. Finally, the 10-minute and 2-minute cadence LCs from \emph{TESS} also allowed us to find two new candidate orbital periods of AM~CVns, both of which are in reasonably good agreement with the predictions for their periods based on their past outburst histories.

astro-ph.HE↗

arXiv2021

A characteristic optical variability timescale in astrophysical accretion disks

Accretion disks around supermassive black holes in active galactic nuclei produce continuum radiation at ultraviolet and optical wavelengths. Physical processes in the accretion flow lead to stochastic variability of this emission on a wide range of timescales. We measure the optical continuum variability observed in 67 active galactic nuclei and the characteristic timescale at which the variability power spectrum flattens. We find a correlation between this timescale and the black hole mass, extending over the entire mass range of supermassive black holes. This timescale is consistent with the expected thermal timescale at the ultraviolet-emitting radius in standard accretion disk theory. Accreting white dwarfs lie close to this correlation, suggesting a common process for all accretion disks.

astro-ph.GA↗

arXiv2021

Quasi-periodic oscillations in the TESS light curve of TX Col, a diskless intermediate polar on the precipice of forming an accretion disk

One of the fundamental properties of an intermediate polar is the dynamical nature of the accretion flow as it encounters the white dwarf's magnetosphere. Many works have presumed a dichotomy between disk-fed accretion, in which the WD accretes from a Keplerian disk, and stream-fed accretion, in which the matter stream from the donor star directly impacts the WD's magnetosphere without forming a disk. However, there is also a third, poorly understood regime in which the accretion flow consists of a torus of diamagnetic blobs that encircles the WD. This mode of accretion is expected to exist at mass-transfer rates below those observed during disk-fed accretion, but above those observed during pure stream-fed accretion. We invoke the diamagnetic-blob regime to explain the exceptional TESS light curve of the intermediate polar TX Col, which transitioned into and out of states of enhanced accretion during Cycles 1 and 3. Power-spectral analysis reveals that the accretion was principally stream-fed. However, when the mass-transfer rate spiked, large-amplitude quasi-periodic oscillations (QPOs) abruptly appeared and dominated the light curve for weeks. The QPOs have two striking properties: they appear in a stream-fed geometry at elevated accretion rates, and they occur preferentially within a well-defined range of frequencies (~10-25 cycles per day). We propose that during episodes of enhanced accretion, a torus of diamagnetic blobs forms near the binary's circularization radius and that the QPOs are beats between the white dwarf's spin frequency and unstable blob orbits within the WD's magnetosphere. We discuss how such a torus could be a critical step in producing an accretion disk in a formerly diskless system.

astro-ph.SR↗

arXiv2021

Exploring the tilted accretion disc of AQ Men with TESS

AQ Men is a nova-like variable which is presumed to have a tilted, precessing accretion disc. Grazing eclipses in this system have been speculated to be useful in exploring the geometry of its accretion disc. In this work we analysed TESS observations of AQ Men, which provide the best light curve of this object thus far. We show that the depths of the eclipses are changing with the orientation of the accretion disc, which means that they can serve as a direct test of the tilted accretion disc models. The precession period of the accretion disc is increasing during the TESS observations. However, it is still shorter than the period determined in the previous studies. The amplitude of the variability related to the precession of the accretion disc varies, and so does the shape of this variability. Moreover, we have detected a positive superhump that was previously unseen in AQ Men. Interestingly, the positive superhump has a strongly non-sinusoidal shape, which is not expected for a nova-like variable.

astro-ph.SR↗

arXiv2021

Discovery of UV millisecond pulsations and moding in the low mass X-ray binary state of transitional millisecond pulsar J1023+0038

PSR J1023+0038 is a rapidly-spinning neutron star with a low-mass-binary companion that switches between a radio pulsar and low-luminosity disk state. In 2013, it transitioned to its current disk state accompanied by brightening at all observed wavelengths. In this state, PSR J1023+0038 now shows optical and X-ray pulsations and abrupt X-ray luminosity switches between discrete 'low' and 'high' modes. Continuum radio emission, denoting an outflow, is also present and brightens during the X-ray low modes. Here, we present a simultaneous optical, ultraviolet (UV) and X-ray campaign comprising Kepler ($400-800$ nm), Hubble Space Telescope ($180-280$ nm), XMM-Newton ($0.3-10$ keV) and NuSTAR ($3 - 79$ keV). We demonstrate that low and high luminosity modes in the UV band are strictly simultaneous with the X-ray modes and change the UV brightness by a factor of $\sim25$\% on top of a much brighter persistent UV component. We find strong evidence for UV pulsations (pulse fraction of $0.82\pm0.19$\%) in the high-mode, with a similar waveform as the X-ray pulsations making it the first known UV millisecond pulsar. Lastly, we find that the optical mode changes occur synchronously with the UV/X-ray mode changes, but optical modes are inverted compared to the higher frequencies. There appear to be two broad-band emission components: one from radio to near-infrared/optical that is brighter when the second component from optical to hard X-rays is dimmer (and vice-versa). We suggest that these components trace switches between accretion into the neutron star magnetosphere (high-energy high-mode) versus ejection of material (low-energy high-mode). Lastly, we propose that optical/UV/X-ray pulsations can arise from a shocked accretion flow channeled by the neutron star's magnetic field.

astro-ph.HE↗

arXiv2020

Periodicities in the K2 lightcurve of HP Librae

We analyse Kepler/K2 lightcurve data of the AM CVn system HP Librae. We detect with confidence four photometric periodicities in the system: the orbital frequency, both positive and negative superhumps, and the positive apsidal precession frequency of the accretion disc. This is only the second time that the apsidal precession frequency has ever been directly detected in the photometry of a helium accreting system, after SDSS J135154.46-064309.0. We present phase-folded lightcurves and sliding power spectra of each of the four periodicities. We measure rates of change of the positive superhump period of $10^{-7}$ days/day. We also redetect a QPO at 300 cyc/day, a feature that has been stable over decades, and show that it is harmonically related to two other QPOs, the lowest of which is centered on the superhump/orbital frequency. The continuum power spectrum is consistent with a single power law with no evidence of any breaks within our observed frequency range.

astro-ph.HE↗

arXiv2020

Astrometric excess noise in Gaia DR2 and the search for X-ray emitting binaries

Astrometric noise in excess of parallax and proper motion could be a signature of orbital wobble of individual components in binary star systems. The combination of X-ray selection with astrometric noise can then be a powerful tool for identifying accreting binaries. Here, we mine the Gaia DR2 catalogue for Galactic sources with significant values of astrometric noise over the parameter space expected for known and candidate X-ray binaries (XRBs). Cross-matching our sample with the Chandra Source Catalogue CSC2.0 returns a primary sample of ~1,500 X-ray sources with significant excess noise, constituting ~0.04% of the initial Gaia sample. By contrast, the fraction of matched X-ray sources in a control sample with smaller excess noise is a factor of about 7 lower. The primary sample branches off the main sequence much more than control objects in colour-mag space, shows a distinct spatial distribution in terms of Galactic latitudes, comprises more objects with an Halpha excess and larger X-ray-to-optical flux ratios, and includes a higher fraction of known binaries, variables and young stellar object class types. However, for individual XRBs with known system parameters, excess noise can exceed expectations, especially for small wobble. It is likely that other factors (possibly attitude and modelling uncertainties, as well as source variability) currently dominate the observed noise in such systems. Confirmation must therefore await future Gaia releases. The full Chandra/Gaia matched catalogue is released here to enable legacy follow-up.

astro-ph.HE↗

arXiv2020

Optical Variability of the Dwarf AGN NGC 4395 from the Transiting Exoplanet Survey Satellite

We present optical light curves from the Transiting Exoplanet Survey Satellite (TESS) for the archetypical dwarf active galactic nucleus (AGN) in the nearby galaxy NGC 4395 hosting a $\sim 10^5\,M_\odot$ supermassive black hole (SMBH). Significant variability is detected on timescales from weeks to hours before reaching the background noise level. The $\sim$month-long, 30 minute-cadence, high-precision TESS light curve can be well fit by a simple damped random walk (DRW) model, with the damping timescale $τ_{\rm DRW}$ constrained to be $2.3_{-0.7}^{+1.8}$~days ($1σ$). NGC 4395 lies almost exactly on the extrapolation of the $τ_{\rm DRW}-M_{\rm BH}$ relation measured for AGNs with BH masses that are more than three orders of magnitude larger. The optical variability periodogram can be well fit by a broken power law with the high-frequency slope ($-1.88\pm0.15$) and the characteristic timescale ($τ_{\rm br}\equiv 1/(2πf_{\rm br})=1.4_{-0.5}^{+1.9}\,$days) consistent with the DRW model within 1$σ$. This work demonstrates the power of TESS light curves in identifying low-mass accreting SMBHs with optical variability, and a potential global $τ_{\rm DRW}-M_{\rm BH}$ relation that can be used to estimate SMBH masses with optical variability measurements.

astro-ph.GA↗

arXiv2020

EX Draconis: Using Eclipses to Separate Outside-In and Inside-Out Outbursts

We present a study of the eclipses in the accreting white dwarf EX Dra during TESS Cycles 14 and 15. During both of the two outbursts present in this dataset, the eclipses undergo a hysteretic loop in eclipse-depth/out-of-eclipse-flux space. In each case, the direction in which the loops are executed strongly suggests an outburst which is triggered near the inner edge of the accretion disk and propagates outwards. This in turn suggests that the outbursts in EX Dra are 'Inside Out' outbursts; events predicted by previous hydrodynamic studies of dwarf nova accretion disks and confirmed spectroscopically in a number of other accreting white dwarf systems. We therefore propose that the direction of the loop executed in eclipse-depth/out-of-eclipse flux space be used as a test to phenomenologically distinguish between 'inside out' and 'outside in' outbursts in other eclipsing dwarf novae; a reliable and purely photometric test to differentiate between these phenomena.

astro-ph.HE↗

arXiv2019

2020 Vision: Towards a Sustainable OIR System

Open-access telescopes of all apertures are needed to operate a competitive and efficient national science program. While larger facilities contribute light-gathering power and angular resolution, smaller ones dominate for field of view, time-resolution, and especially, total available observing time, thereby enabling our entire, diversely-expert community. Smaller aperture telescopes therefore play a critical and indispensable role in advancing science. Thus, the divestment of NSF support for modest-aperture (1 - 4 m) public telescopes poses a serious threat to U.S. scientific leadership, which is compounded by the unknown consequences of the shift from observations driven by individual investigators to survey-driven science. Given the much higher cost efficiency and dramatic science returns for investments in modest aperture telescopes, it is hard to justify funding only the most expensive facilities. We therefore urge the Astro2020 panel to explicitly make the case for modest aperture facilities, and to recommend enhancing this funding stream to support and grow this critical component of the OIR System. Further study is urgently needed to prioritize the numerous exciting potential capabilities of smaller facilities,and to establish sustainable, long-term planning for the System.

astro-ph.IM↗

arXiv2019

Astro2020 Science White Paper: Understanding the evolution of close white dwarf binaries

Interacting binaries containing white dwarfs can lead to a variety of outcomes that range from powerful thermonuclear explosions, which are important in the chemical evolution of galaxies and as cosmological distance estimators, to strong sources of low frequency gravitational wave radiation, which makes them ideal calibrators for the gravitational low-frequency wave detector LISA mission. However, current theoretical evolution models still fail to explain the observed properties of the known populations of white dwarfs in both interacting and detached binaries. Major limitations are that the existing population models have generally been developed to explain the properties of sub-samples of these systems, occupying small volumes of the vast parameter space, and that the observed samples are severely biased. The overarching goal for the next decade is to assemble a large and homogeneous sample of white dwarf binaries that spans the entire range of evolutionary states, to obtain precise measurements of their physical properties, and to further develop the theory to satisfactorily reproduce the properties of the entire population. While ongoing and future all-sky high- and low-resolution optical spectroscopic surveys allow us to enlarge the sample of these systems, high-resolution ultraviolet spectroscopy is absolutely essential for the characterization of the white dwarfs in these binaries. The Hubble Space Telescope is currently the only facility that provides ultraviolet spectroscopy, and with its foreseeable demise, planning the next ultraviolet mission is of utmost urgency.

astro-ph.SR↗

arXiv2018

The Plane's The Thing: The Case for Wide-Fast-Deep Coverage of the Galactic Plane and Bulge

We argue that the exclusion of the Galactic Plane and Bulge from the uniform wide-fast-deep (WFD) LSST survey cadence is fundamentally inconsistent with two of the main science drivers of LSST: Mapping the Milky Way and Exploring the Transient Optical Sky. We outline the philosophical basis for this claim and then describe a number of important science goals that can only be addressed by WFD-like coverage of the Plane and Bulge.

astro-ph.GA↗

arXiv2014

On the Raman O VI and related lines in classical novae

We critically examine the recent claimed detection of Raman scattered O VI at around 6830Å in the iron curtain stage spectra of the classical CO nova V339 Del. The observed line variations are compatible in profile and timing of emission line strength with an excited state transition of neutral carbon. Line formation in classical nova ejecta is physically very different from that in symbiotic binaries, in which the O VI emission line is formed within the wind of the companion red giant at low differential velocity. The ejecta velocity and density structure prevent the scattering from producing analogous features. There might , however, be a broadband spectropolarimetric signature of the Raman process and also Rayleigh scattering at some stage in the expansion. We show that the neutral carbon spectrum, hitherto under-exploited for novae, is especially useful as a probe of the structure of the ejecta during the early, optically thick stages of the expansion

astro-ph.SR↗