Optical Low States Research Articles

Optical identification and follow-up observations of SRGA J213151.5+491400

Context. The paper is comprised of optical identification and multiwavelength studies of a new X-ray source discovered by the Spectrum Roentgen-Gamma (SRG) observatory during the ART-XC survey and its follow-up optical and X-ray observations. Aims. We aim to identify SRGA J213151.5+491400 in the optical wavelengths. We determine spectra and light curves in the optical high and low states to find periodicities in the light curves and resolve emission lines in the system using optical ground-based data. We intend to study the spectral and temporal X-ray characteristics of the new source using the SRG surveys in the high and low states and NICER data in the low state. Methods. We present optical data from telescopes in Türkiye (RTT-150 and T100 at the TÜBİTAK National Observatory) and in Russia (6-m and 1-m at SAO RAS), together with the X-ray data obtained with ART-XC and eROSITA telescopes aboard SRG and the NICER observatory. Using the optical data, we performed astrometry, photometry, spectroscopy, and power spectral analysis of the optical time series. We present optical Doppler tomography along with X-ray data analysis producing light curves and spectra. Results. We detected SRGA J213151.5+491400 in a high state in 2020 (17.9 mag) that decreased by about 3 mag into a low state (21 mag) in 2021. We find only one significant period using optical photometric time series analysis, which reveals the white dwarf spin (orbital) period to be 0.059710(1) days (85.982 min). The long slit spectroscopy in the high state yields a power-law continuum increasing towards the blue with a prominent He II line along with the Balmer line emissions with no cyclotron humps, which is consistent with a magnetic cataclysmic variable (MCV) nature. Doppler Tomography confirms the polar nature revealing ballistic stream accretion along with magnetic stream during the high state. These characteristics show that the new source is a polar-type MCV source. ART-XC detections yield an X-ray flux of (4.0−7.0) × 10−12 erg s−1 cm−2 in the high state. eROSITA detects a dominating hot plasma component (kTmax > 21 keV in the high state) declining to (4.0−6.0) × 10−13 erg s−1 cm−2 in 2021 (low state). The NICER data obtained in the low state reveal a two-pole accretor showing a soft X-ray component at (6−7)σ significance with a blackbody temperature of 15−18 eV. A soft X-ray component has never been detected for a polar in the low state before.

Optical spectroscopy of blazars for the Cherenkov Telescope Array – III

Context. Blazars, which include BL Lacs and flat-spectrum radio quasars, represent the brightest persistent extragalactic sources in the high-energy (HE; 10 MeV–100 GeV) and very-high-energy (VHE; E > 100 GeV) γ-ray sky. Due to their almost featureless optical/UV spectra, it is challenging to measure the redshifts of BL Lacs. As a result, about 50% of γ-ray BL Lacs lack a firm measurement of this property, which is fundamental for population studies, indirect estimates of the extragalactic background light, and fundamental physics probes (e.g., searches for Lorentz-invariance violation or axion-like particles). Aims. This paper is the third in a series of papers aimed at determining the redshift of a sample of blazars selected as prime targets for future observations with the next generation, ground-based VHE γ-ray astronomy observatory, Cherenkov Telescope Array Observatory (CTAO). The accurate determination of the redshift of these objects is an important aid in source selection and planning of future CTAO observations. Methods. Promising targets were selected following a sample selection obtained with Monte Carlo simulations of CTAO observations. The selected targets were expected to be detectable with CTAO in observations of 30 h or less. We performed deep spectroscopic observations of 41 of these blazars using the Keck II, Lick, SALT, GTC, and ESO/VLT telescopes. We carefully searched for spectral lines in the spectra and whenever features of the host galaxy were detected, we attempted to model the properties of the host galaxy. The magnitudes of the targets at the time of the observations were also compared to their long-term light curves. Results. Spectra from 24 objects display spectral features or a high signal-to-noise ratio (S/N). From these, 12 spectroscopic redshifts were determined, ranging from 0.2223 to 0.7018. Furthermore, 1 tentative redshift (0.6622) and 2 redshift lower limits at z > 0.6185 and z > 0.6347 were obtained. The other 9 BL Lacs showed featureless spectra, despite the high S/N (≥100) observations. Our comparisons with long-term optical light curves tentatively suggest that redshift measurements are more straightforward during an optical low state of the active galactic nucleus. Overall, we have determined 37 redshifts and 6 spectroscopic lower limits as part of our programme thus far.

Long-term optical variability of the Be/X-ray binary GRO J2058+42

Context. GRO J2058+42 is a transient hard X-ray pulsar that occasionally goes into outburst. The optical counterpart is a poorly studied OB-type companion. Aims. We investigate the long-term optical variability of the Be/X-ray binary GRO J2058+42 and the possible connection with periods of enhanced X-ray activity. Methods. We performed an optical spectroscopic and photometric analysis on data collected during about 18 yr. We also present the first optical polarimetric observations of this source. Results. The long-term optical light curves in the BVRI bands and the evolution of the Hα equivalent width display a sinusoidal pattern with maxima and minima that repeat every ∼9.5 yr. The amplitude of this variability increases as the wavelength increases from 0.3 mag in the B band to 0.7 in the I band. The Hα equivalent width varied from about −0.3 to −15 Å. We found a significant decrease in the polarization degree during the low optical state. The intrinsic polarization degree changed by ∼1% from maximum to minimum. The optical maxima occur near periods of enhanced X-ray activity and are followed by a drop in the optical emission. Unlike many other Be/X-ray binaries, GRO J2058+42 does not display V/R variability. Conclusions. The long-term optical variability agrees with the standard model of a Be/X-ray binary, where the circumstellar disk of the Be star grows and dissipates on timescales of 9−10 yr. We find that the dissipation of the disk started after a major X-ray outburst. However, the stability of the Hα line shape as a double-peak profile and the lack of asymmetries suggest the absence of a warped disk and argue against the presence of a highly distorted disk during major X-ray outbursts.

A robust model for the origin of optical quasi-periodic variability in supersoft X-ray sources

Supersoft X-ray sources (SSSs) are known as possible progenitors of Type Ia supernovae. The quasi-periodic variability has been detected in the optical light curves of SSSs. However, the exact origin of such quasi-periodic observable features remains a mystery. In this paper, we aim to reproduce the observed optical quasi-periodic variability of SSSs by proposing a white dwarf (WD) accretion model with a periodic mass transfer caused by the irradiation of supersoft X-ray onto the companion star. Methods. Assuming that a periodic mass transfer from the companion star to the WD can be caused while the supersoft X-ray irradiates the companion star, we used MESA to simulate the WD accretion process and the subsequent WD evolution by adopting a periodic jagged accretion rate. Comparing our results to the optical light curves of a well-observed SSS RX J0513.9-6951, we find that our models can reproduce the quasi-periodic transition between the optical high and low states of RX J0513.9-6951 because the periodic accretion rate can lead to the WD photosphere expands and contracts periodically in our models. In addition, we find that the transitional periods of the SSSs in our models strongly depend on the mass of the accreting WDs. The more massive the WD mass is, the shorter the transitional period. Based on our results, we suggest that the periodic mass transfer caused by the irradiation of supersoft X-ray onto the companion star may be the origin of the observed optical quasi-periodic variability in SSSs. In addition, our results indicate that the observed optical transition period of a SSS may be useful for the rough estimate of the mass of an accreting WD.

Phase evolution of all-inorganic perovskite nanowires during its growth from quantum dots

All-inorganic lead-halide perovskites have emerged as an exciting material owing to their excellent optoelectronic properties and high stability over hybrid organometallic perovskites. Nanowires of these materials, in particular, have shown great promise for optoelectronic applications due to their high optical absorption coefficient and low defect state density. However, the synthesis of the most promising alpha-Cesium lead iodide (α-CsPbI3) nanowires is challenging as it is metastable and spontaneously converts to a non-perovskite δ-phase. The hot-injection method is one of the most facile, well-controlled, and commonly used approaches for synthesizing CsPbX3 nanostructures. But the exact mechanism of growing these nanowires in this technique is not clear. Here, we show that the hot-injection method produces photoactive phases of quantum dots (QDs) and nanowires of CsPbBr3, and QDs of CsPbI3, but CsPbI3 nanowires are grown in their non-perovskite δ-phase. Monitoring the nanowire growth during the hot-injection technique and through detailed characterization, we establish that CsPbI3 nanowires are formed in the non-perovskite phase from the beginning rather than transforming after its growth from perovskite to a non-perovskite phase. We have discussed a possible mechanism of how non-perovskite nanowires of CsPbI3 grow at the expense of photoactive perovskite QDs. Our findings will help to synthesize nanostructures of all-inorganic perovskites with desired phases, which is essential for successful technological applications.

A novel design of all optical half-subtractor using a square lattice photonic crystals

In this research, an all-optical half-subtractor is designed and simulated using two-dimensional photonic crystals. First, a photonic crystal structure is created using Si rods in the air context to obtain the optical half-subtractor. Afterward, using point and line defects, two waveguides are created for the input and two waveguides are created for the outputs. A high logical value and a low logical value are defined based on the optical power in each port. The FDTD method is used in the simulation of light propagation in the structure. The simulation results show that the designed half-subtractor has high optical power values for logic “1” and low values for logic “0”. The small size of the designed structure is among the advantages of this structure. Moreover, given that this half-subtractor is devoid of ring resonators, it can be used in high-speed integrated optical circuits. Another advantage of the proposed half-subtractor is that the optical powers in the outputs are similar in the high and low optical states.

MAGIC detection of short-term variability of the high-peaked BL Lac object 1ES 0806+524

The high-frequency-peaked BL Lac (HBL) 1ES 0806+524 (z = 0.138) was discovered in VHE $\gamma$ rays in 2008. Until now, the broad-band spectrum of 1ES 0806+524 has been only poorly characterized, in particular at high energies. We analysed multiwavelength observations from $\gamma$ rays to radio performed from 2011 January to March, which were triggered by the high activity detected at optical frequencies. These observations constitute the most precise determination of the broad-band emission of 1ES 0806+524 to date. The stereoscopic MAGIC observations yielded a $\gamma$-ray signal above 250 GeV of $(3.7 \pm 0.7)$ per cent of the Crab Nebula flux with a statistical significance of 9.9 $\sigma$. The multiwavelength observations showed significant variability in essentially all energy bands, including a VHE $\gamma$-ray flare that lasted less than one night, which provided unprecedented evidence for short-term variability in 1ES 0806+524. The spectrum of this flare is well described by a power law with a photon index of $2.97 \pm 0.29$ between $\sim$150 GeV and 1 TeV and an integral flux of $(9.3 \pm 1.9)$ per cent of the Crab Nebula flux above 250 GeV. The spectrum during the non-flaring VHE activity is compatible with the only available VHE observation performed in 2008 with VERITAS when the source was in a low optical state. The broad-band spectral energy distribution can be described with a one-zone Synchrotron Self Compton model with parameters typical for HBLs, indicating that 1ES 0806+524 is not substantially different from the HBLs previously detected.

The quiescent state of the accreting X-ray pulsar SAX J2103.5+4545

We present an X-ray timing and spectral analysis of the Be/X-ray binary SAX J2103.5+4545 at a time when the Be star's circumstellar disk had disappeared and thus the main reservoir of material available for accretion had extinguished. In this very low optical state, pulsed X-ray emission was detected at a level of L_X~10^{33} erg/s. This is the lowest luminosity at which pulsations have ever been detected in an accreting pulsar. The derived spin period is 351.13 s, consistent with previous observations. The source continues its overall long-term spin-up, which reduced the spin period by 7.5 s since its discovery in 1997. The X-ray emission is consistent with a purely thermal spectrum, represented by a blackbody with kT=1 keV. We discuss possible scenarios to explain the observed quiescent luminosity and conclude that the most likely mechanism is direct emission resulting from the cooling of the polar caps, heated either during the most recent outburst or via intermittent accretion in quiescence.

X-ray and UV observations of V751 Cygni in an optical high state

Aims: The VY Scl system (anti-dwarf nova) V751 Cyg is examined following a claim of a super-soft spectrum in the optical low state. Methods: A serendipitous XMM-Newton X-ray observation and, 21 months later, Swift X-ray and UV observations, have provided the best such data on this source so far. These optical high-state datasets are used to study the flux and spectral variability of V751 Cyg. Results: Both the XMM-Newton and Swift data show evidence for modulation of the X-rays for the first time at the known 3.467 hr orbital period of V751 Cyg. In two Swift observations, taken ten days apart, the mean X-ray flux remained unchanged, while the UV source brightened by half a magnitude. The X-ray spectrum was not super-soft during the optical high state, but rather due to multi-temperature optically thin emission, with significant (10^{21-22} cm^-2) absorption, which was higher in the observation by Swift than that of XMM-Newton. The X-ray flux is harder at orbital minimum, suggesting that the modulation is related to absorption, perhaps linked to the azimuthally asymmetric wind absorption seen previously in H-alpha.

Optical and X-ray properties of CAL 83 – I. Quasi-periodic optical and supersoft variability

We have studied the long-term (~ years) temporal variability of the prototype supersoft X-ray source (SSS) CAL 83 in the LMC, using data from the MACHO and OGLE projects. The CAL 83 light curve exhibits dramatic brightness changes of ~1 mag on timescales of ~450 days, and spends typically ~200 days in the optical low state. Combined with archival XMM-Newton X-ray observations these represent the most extensive X-ray/optical study to date of this system, and reveal in much greater detail that the X-ray light curve is anti-correlated with the optical behaviour. This is remarkably similar to the behaviour of the "transient" SSS, RX J0513.9-6951, where the SSS outbursts recur on a timescale of ~168 days, and also anti-correlate with the optical flux. We performed simple blackbody fits to both high and low state X-ray spectra, and find that the blackbody temperature and luminosity decrease when the optical counterpart brightens. We interpret these long-term variations in terms of the limit-cycle model of Hachisu & Kato (2003a), which provides further support for these systems containing massive (~1.3 Msun) white dwarfs. In addition, we have refined their orbital periods in the MACHO and OGLE-III light curves to values of 1.047529(1) days and 0.762956(5) days for CAL 83 and RX J0513.9-6951, respectively.

The multifrequency campaign on 3C 279 in January 2006

We present the results of a multifrequency campaign from radio to hard X-ray energies on the blazar 3C 279 during an optical high-state in January 2006. We give the observational results (multifrequency light curves and spectra) and compile an SED. This complements an SED from an optical low-state in June 2003. Surprisingly the two SEDs differ only in their high-energy synchrotron emission (near-IR - UV), while the low-energy inverse-Compton emission (X- to Gamma-rays) remained unchanged. By interpreting with a steady-state leptonic emission model, the variability among the SED can be reproduced by a change solely of the low-energy cutoff of the relativistic electron distribution. In an internal shock model for blazar emission, such a change could e.g. achieved through a varying relative Lorentz factor of colliding shells producing internal shocks in the jet.

The host galaxy of 3C 279

We have obtained a deep i-band image of the blazar 3C 279 while the target was in a low optical state. Due to the faintness of the optical nucleus we have made the first detection of the host galaxy. The host galaxy has an apparent I-band magnitude of 18.4 +- 0.3 and an effective radius of (2.7 +- 1.1) arcsec. The luminosity of the host galaxy M_R = -23.8 is consistent with the luminosities of other radio-loud quasar host galaxies. Using the empirical correlation between bulge luminosity and central black hole mass M_bh we estimate log(M_bh/M_Sun) = 8.9 +- 0.5, broadly consistent with values obtained by photoionization methods.

Detection of the host galaxy of S5 0716+714

We have acquired a deep i-band image of the BL Lacertae object S5 0716+714 while the target was in an low optical state. Due to the faintness of the nucleus, we were able to detect the underlying host galaxy. The host galaxy is measured to have an I-band magnitude of 17.5 +- 0.5 and an effective radius of (2.7 +- 0.8) arcsec. Using the host galaxy as a ``standard candle'', we derive z = 0.31 +- 0.08 (1 sigma error) for the host galaxy of S5 0716+714. This redshift is consistent with the redshift z = 0.26 determined by spectroscopy for 3 galaxies close to S5 0716+714. The effective radius at z = 0.31 would be 12 +- 4 kpc, which is consistent with values obtained for BL Lac host galaxies. An optical spectrum acquired during the same epoch shows no identifiable spectral lines.

XMM spectroscopy of the transient supersoft source RX J0513.9 — 6951: probing the dynamic white dwarf photosphere

The highly luminous (>1037 erg s?1) supersoft X-ray sources (SSSs) are believed to be Eddington-limited accreting white dwarfs undergoing surface hydrogen burning. The current paradigm for SSSs involves thermally unstable mass transfer from a 1–2M companion. However, this model has never been directly confirmed and yet is crucial for the evolution of cataclysmic variables (CVs) in general, and for the establishment of SSSs as progenitors of Type Ia supernovae in particular. The key SSS is RX J0513.9?6951 which has recurrent X-ray outbursts every 100–200 d (lasting for ?40 d) during which the optical brightness declines by 1 mag.We present the firstXMM observations of RX J0513.9?6951 through one of its optical low states. Our results show that as the optical low state progresses, the temperature and X-ray luminosity decrease, behaviour that is anti-correlated with the optical and ultraviolet (UV) emission. We find that as the optical (and UV) intensity recovers, the radius implied by the spectral fits increases. The high-resolution spectra show evidence of deep-absorption features which vary during the optical low state. Our results are consistent with the predictions of the white dwarf photospheric contraction model proposed by Southwell et al.

A Limit Cycle Model For Long‐Term Optical Variations of V Sagittae: The Second Example of Accretion Wind Evolution

V Sagittae shows quasi-periodic optical high (soft X-ray-off) and low (soft X-ray-on) states with the total period of ~300 days. A binary model is presented to explain orbital light curves for both the high and low states, as well as the transition mechanism between them. The binary model consists of a white dwarf (WD), a disk around the WD, and a lobe-filling main-sequence companion. In the optical low state, the mass-transfer rate to the WD is small and the size of the disk is as small as its Roche lobe size. In the optical high state, the mass-transfer rate to the WD exceeds the critical rate of ~1 × 10-6 M☉ yr-1 and the WD blows an optically thick, massive wind. Surface layers of the disk are blown in the wind, and the disk surface extends to the companion or over. As a result, optical luminosity of the disk increases by a magnitude because of its large irradiation effect. The massive wind completely obscures soft X-rays. This corresponds to the optical high/soft X-ray off state. The transition between optical high and low states is driven by an attenuation of the mass transfer from the secondary. During the optical high state, the wind from the WD hits the companion and strips off its surface layer. The mass transfer from the companion gradually reduces and stops. As the mass supply stops, the WD wind weakens and eventually stops. The disk shrinks to a Roche lobe size, and the optical magnitude drops. This phase corresponds to the optical low/soft X-ray on state. Then a rapid mass transfer resumes. The mass of the WD envelope increases and blows a wind again. This cycle is repeated like a limit cycle. During this intermittent wind phase, the WD can grow in mass at the critical rate and eventually reach the Chandrasekhar mass limit. This process is called wind evolution, which is a key evolutionary process in a recently developed evolutionary scenario of Type Ia supernovae. This evolutionary process was first confirmed in the LMC supersoft X-ray source RX J0513.9-6951, although it commonly occurs in the supersoft X-ray sources when the mass-transfer rate exceeds the critical rate. Thus, V Sge is the second example of accretion wind evolution.

A New Clue to the Transition Mechanism between Optical High and Low States of the Supersoft X‐Ray Source RX J0513.9−6951, Implied By the Recurrent Nova CI Aquilae 2000 Outburst Model

We have found a new clue to the transition mechanism between optical high/X-ray off and optical low/X-ray on states of the LMC supersoft X-ray source RX J0513.9-6951. A sharp ~1 mag drop is common to the CI Aql 2000 outburst. These drops are naturally attributed to the cessation of optically thick winds on white dwarfs. A detailed light-curve analysis of CI Aql indicates that the size of a disk drastically shrinks when the wind stops. This causes a ~1-2 mag drop in the optical light curve. In RX J0513.9-6951, the same mechanism reproduces sharp ~1 mag drop from optical high to low states. We predict that this mechanism also works on the transition from low to high states. Interaction between the wind and the companion star attenuates the mass transfer and drives full cycles of low and high states.

CH Cygni. I. Observational Evidence for a Disk‐Jet Connection

We investigate the role of accretion in the production of jets in the symbiotic star CH Cygni. Assuming that the rapid stochastic optical variations in CH Cygni come from the accretion disk, as in cataclysmic variables, we use changes in this flickering to diagnose the state of the disk in 1997. At that time, CH Cyg dropped to a very low optical state, and Karovska et al. report that a radio jet was produced. For approximately one year after the jet production, the amplitude of the fastest (time scales of minutes) variations was significantly reduced, although smooth, hour-time-scale variations were still present. This light curve evolution indicates that the inner disk may have been disrupted, or emission from this region suppressed, in association with the mass-ejection event. We describe optical spectra which support this interpretation of the flickering changes. The simultaneous state change, jet ejection, and disk disruption suggests a comparison between CH Cygni and some black-hole-candidate X-ray binaries that show changes in the inner disk radius in conjunction with discrete ejection events on a wide range of time scales (e.g., the microquasar GRS 1915+105 and XTE J1550-564).

A New Orbital Ephemeris and Reinterpretation of Spectroscopic Data for the Supersoft X-Ray Binary RX J0513.9−6951

We have analyzed nearly eight years of MACHO optical photometry of the supersoft X-ray binary RX J0513.9-6951 and derived a revised orbital period and ephemeris. Previously published velocities are reinterpreted using the new ephemeris. We show that the spectroscopic characteristics of the system depend strongly on whether the system is in a high or low optical state. We also discuss the properties of the source's high and low optical states and its long-term light curve. Evidence for an 83.3 day periodicity in the photometry is presented.

X-ray off states and optical variability in CAL 83

CAL 83 was one of the rst supersoft X-ray binaries (SSBs) to be discovered and is considered to be the prototype of its class. In 15 X-ray observations between 1983{1997 it was observed to have nearly constant X-ray luminosity and temperature, with the exception of one o-state in 1996 (Kahabka et al. 1996). We report on a second X-ray o-state, discovered with a Chandra ACIS-S observation in November 1999. We consider the long-term X-ray and MACHO optical light curves. We nd that, during more than 7 years of monitoring by the MACHO team, CAL 83 has exhibited distinct and well-dened low, intermediate, and high optical states. Transitions between states are not accompanied by color variations. We also nd that both X-ray o states were observed during optical high states and were followed by optical low states within50 days. We discuss possible explanations for the observed optical and X-ray variations. While photospheric adjustments might account for the variations in soft X-ray flux, optical variations can be explained only by invoking changes in the accretion disk, which is the primary source of optical radiation.

BZ Camelopardalis during its 1999/2000 optical low state

We report optical observations of the VY Scl star BZ Cam during its previous optical low state in 1999/2000. We nd drastic variations in the line proles. Narrow-band imaging observations show that its nebula extends farther than previously known and seems to be composed of two components. We determine the (O III) line intensity of BZ Cam's nebula to 4:8 10 13 erg/cm 2 /s. We discover a proper motion of BZ Cam of 25 2 mas/yr which together with the systemic radial velocity yields a space velocity of 125 km s 1 . We re-interpret the nebula as being photo-ionized by hypothesized transient, luminous, supersoft X-ray emission during optical low states, and shaped by the transverse motion of BZ Cam.