Brightening Episodes Research Articles

A Pilot Search for Gravitational Self-Lensing Binaries with the Zwicky Transient Facility

Binary systems containing a compact object may exhibit periodic brightening episodes due to gravitational lensing as the compact object transits the companion star. Such “self-lensing” signatures have been detected before for white dwarf binaries. We attempt to use these signatures to identify detached stellar-mass neutron star and black hole binaries using data from the Zwicky Transient Facility (ZTF). We present a systematic search for self-lensing signals in Galactic binaries from a subset of high-cadence ZTF data taken in 2018. We identify 12 plausible candidates from the search, although because each candidate is observed to only brighten once, other origins such as stellar flares are more likely. We discuss prospects for more comprehensive future searches of the ZTF data.

Topology-induced quantum transition in multiparticle systems in vicinity of a black hole

The qualitative change of the type of particle trajectory homotopy in close vicinity of general-relativistic gravitational singularity affects quantum statistics in systems of identical indistinguishable particles at passing the photon sphere rim of a Schwarzschild black hole. This causes a local departure from the Pauli exclusion principle, which results in some high energy effects manifesting themselves when the matter is falling into a black hole. The release of the energy can take place at the rim of the photon sphere of a black hole due to the decay of Fermi spheres in highly compressed fermion systems entering this region. The effect is observable at activity of quasars or at collapses of neutron star mergers. The related supplementation to conventional models of the accretion disc luminosity by additional radiation from close vicinity of the event horizon of a massive black hole powering super-luminous quasar is proposed and compared with observations. The source of short-lasting gamma-ray bursts has been also identified at collapses of unstable neutron star mergers due to the decay of Fermi sphere of neutrons. The duration and spectrum of the related e-m radiation bursts estimated by the Fermi golden rule for the Fermi sphere decay agree with observations. The quantum statistics effect contributes also to the radiation of transients of micro-quasars and short lasting brightening episodes in closer active galactic nuclei. The proposed quantum effect changes the premises for the current discussion of the information paradox and hypothesis of the black hole firewall.

A long hard look on multiwavelength properties of blazar OJ 287

We examined the behaviour of blazar OJ 287 based on 12 year observation data in optic, UV, X-ray, and gamma ray. The optical, UV, and X-ray data of OJ 287 were obtained from Swift archive, which provide more or less simultaneous optical–X-ray observations, while gamma ray data were obtained from Fermi archive. The occurrence of outburst episode in 2007 can be confirmed in Swift/UVOT data. In addition, episodes of significant brightening (possibly the outburst(s) of this decade) can also be seen in 2015–2016 and 2016–2017. Long-term flux variability in gamma ray and X-ray data is not always concurrent with those of optical and UV. Interestingly, the high peaks appeared almost simultaneous in optic, UV and X-ray in the episode of brightening in 2016–2017. X-ray spectral fittings show that non-thermal process predominates most of the X-ray spectra. Simultaneous optical–X-ray spectral fittings show the spectral evolution and suggest the combination of thermal and non-thermal component before brightening episode in 2015–2016. We obtained gamma ray spectra with broader peak at rather high frequency compared to that expected from simple synchrotron self-Compton mechanism. During brightening 2016–2017, the optical–X-ray fluxes are higher compared to those other periods. The shape of the X-ray and gamma ray spectra also have inverted trend than those of other periods.

Full orbital solution for the binary system in the northern Galactic disc microlensing event Gaia16aye

Gaia16aye was a binary microlensing event discovered in the direction towards the northern Galactic disc and was one of the first microlensing events detected and alerted to by the Gaia space mission. Its light curve exhibited five distinct brightening episodes, reaching up to I = 12 mag, and it was covered in great detail with almost 25 000 data points gathered by a network of telescopes. We present the photometric and spectroscopic follow-up covering 500 days of the event evolution. We employed a full Keplerian binary orbit microlensing model combined with the motion of Earth and Gaia around the Sun to reproduce the complex light curve. The photometric data allowed us to solve the microlensing event entirely and to derive the complete and unique set of orbital parameters of the binary lensing system. We also report on the detection of the first-ever microlensing space-parallax between the Earth and Gaia located at L2. The properties of the binary system were derived from microlensing parameters, and we found that the system is composed of two main-sequence stars with masses 0.57 ± 0.05 M⊙ and 0.36 ± 0.03 M⊙ at 780 pc, with an orbital period of 2.88 years and an eccentricity of 0.30. We also predict the astrometric microlensing signal for this binary lens as it will be seen by Gaia as well as the radial velocity curve for the binary system. Events such as Gaia16aye indicate the potential for the microlensing method of probing the mass function of dark objects, including black holes, in directions other than that of the Galactic bulge. This case also emphasises the importance of long-term time-domain coordinated observations that can be made with a network of heterogeneous telescopes.

Magnetic Field Dynamics and Varying Plasma Emission in Large-scale Coronal Loops

Abstract In this study we report detailed observations of magnetic environment at four footpoints of two warm coronal loops observed on 2016 May 5 in NOAA AR 12542 (Loop I) and 2015 December 17 in NOAA AR 12470 (Loop II). These loops were connecting a plage region with sunspot periphery (Loop I) and a sunspot umbra (Loop II). We used Solar Dynamics Observatory (SDO) and Goode Solar Telescope (GST) data to describe the phenomenon and understand its causes. The study indicates loop brightening episodes were associated with magnetic flux emergence and cancellation processes observed in SDO’s Helioseismic and Magnetic Imager and GST’s Near InfraRed Imaging Spectrapolarimeter data. The observed activity was driven by magnetic reconnection between small-scale emerging dipoles and large-scale pre-existing fields, suggesting that the reconnection occurred in the lower chromosphere at the edge of an extended plage region, where the loops were rooted. We suggest that plasma, evaporated during these reconnection events, gradually filled the loops and as it cooled the visible density front propagated from one footpoint of the loop to another at a rate of 90–110 km s−1. This study also indicates that at least some of the bright loops seen in SDO Atmospheric Imaging Assembly images rooted in sunspot umbra may be heated due to magnetic activity taking place at the remote (nonsunspot) footpoint.

HIGHLY VARIABLE EXTINCTION AND ACCRETION IN THE JET-DRIVING CLASS I-TYPE YOUNG STAR PTF 10nvg (V2492 Cyg, IRAS 20496+4354)

We report extensive new photometry and spectroscopy of the highly variable young stellar object PTF 10nvg including optical and near-infrared time series data as well as mid-infrared and millimeter data. Following the previously reported 2010 rise, during 2011 and 2012 the source underwent additional episodes of brightening and dimming events including prolonged faint states. The observed high-amplitude variations are largely consistent with extinction changes having a 220 day quasi-periodic signal. Spectral evolution includes not only changes in the spectral slope but correlated variation in the prominence of TiO/VO/CO bands and atomic line emission, as well as anticorrelated variation in forbidden line emission which, along with H_2, dominates optical and infrared spectra at faint epochs. Neutral and singly-ionized atomic species are likely formed in an accretion flow and/or impact while the origin of zero-velocity atomic LiI 6707 in emission is unknown. Forbidden lines, including several rare species, exhibit blueshifted emission profiles and likely arise from an outflow/jet. Several of these lines are also seen spatially offset from the continuum source position, presumably in a shocked region of an extended jet. CARMA maps resolve on larger scales a spatially extended outflow in mm-wavelength CO. We attribute the observed photometric and spectroscopic behavior in terms of occultation of the central star as well as the bright inner disk and the accretion/outflow zones that renders shocked gas in the inner part of the jet amenable to observation at the faint epochs. We discuss PTF 10nvg as a source exhibiting both accretion-driven (perhaps analogous to V1647 Ori) and extinction-driven (perhaps analogous to UX Ori or GM Cep) high-amplitude variability phenomena.

The X-ray lightcurve of Sagittarius A* over the past 150 years inferred from Fe-Kαline reverberation in Galactic centre molecular clouds

We examine the temporal and spectral properties of nine Fe-Ka bright molecular clouds within about 30 pc of Sgr A*, in order to understand and constrain the primary energising source of the Fe fluorescence. Significant Fe-Ka variability was detected, with a spatial and temporal pattern consistent with that reported in previous studies. The main breakthrough that sets our paper apart from earlier contributions on this topic is the direct measurement of the column density and the Fe abundance of the MCs in our sample. We used the EW measurements to infer the average Fe abundance within the clouds to be 1.6$\pm$0.1 times solar. The cloud column densities derived from the spectral analysis were typically of the order of 10$^{23}$ cm$^{-2}$, which is significantly higher than previous estimates. This in turn has a significant impact on the inferred geometry and time delays within the cloud system. Past X-ray activity of Sgr A* is the most likely source of ionisation within the molecular clouds in the innermost 30 pc of the Galaxy. In this scenario, the X-ray luminosity required to excite these reflection nebulae is of the order of 10$^{37}-10^{38}$ erg s$^{-1}$, significantly lower than that estimated for the Sgr B2 molecular cloud. Moreover, the inferred Sgr A* lightcurve over the past 150 years shows a long-term downwards trend punctuated by occasional counter-trend brightening episodes of at least 5 years duration. Finally, we found that contributions to the Fe fluorescence by X-ray transient binaries and cosmic-ray bombardment are very likely, and suggest possible ways to study this latter phenomenon in the near future.

Are the outflows in FU Orionis systems driven by the stellar magnetic field?

FU Orionis (FUOR) outbursts are major optical brightening episodes in low-mass protostars that correspond to rapid mass-accretion events in the innermost region of a protostellar disc. The outbursts are accompanied by strong outflows, with the inferred mass outflow rates reaching ~10% of the mass inflow rates. Shu et al. proposed that the outflows represent accreted disc material that is driven centrifugally from the spun-up surface layers of the protostar by the stellar magnetic field. This model was critiqued by Calvet et al., who argued that it cannot reproduce the photospheric absorption-line shifts observed in the prototype object FU Ori. Calvet et al. proposed that the wind is launched, instead, from the surface of the disc on scales of a few stellar radii by a non-stellar magnetic field. In this paper we present results from numerical simulations of disc accretion on to a slowly rotating star with an aligned magnetic dipole moment that gives rise to a kilogauss-strength surface field. We demonstrate that, for parameters appropriate to FU Ori, such a system can develop a strong, collimated disc outflow of the type previously identified by Romanova et al. in simulations of protostars with low and moderate accretion rates. At the high accretion rate that characterizes FUOR outbursts, the radius at which the disc is truncated by the stellar magnetic field moves much closer to the stellar surface, but the basic properties of the outflow, which is launched from the vicinity of the truncation radius along opened-up stellar magnetic field lines, remain the same, and are distinct from those of the mechanism proposed by Shu et al. We show that the simulated outflow can in principle account for the main observed characteristics of FUOR winds, including the photospheric line shifts measured in FU Ori.

A POSSIBLE PERIOD FOR THE K -BAND BRIGHTENING EPISODES OF GX 17+2

The low mass X-ray binary and Z source GX 17+2 undergoes infrared K-band brightening episodes of at least 3.5 magnitudes. The source of these episodes is not known. Prior published K-band magnitudes and new K-band measurements acquired between 2006 and 2008 suggest that the episodes last at least 4 hours and have a period of 3.01254 $\pm$ 0.00002 days. Future bright episodes can be predicted using the ephemeris JD_{max} (n) = 2454550.79829 + (3.01254 $\pm$ 0.00002)(n) days. A growing body of evidence suggests that the GX 17+2 could have a synchrotron jet, which could cause this activity.

Two‐Component Jet Models of Gamma‐Ray Burst Sources

Recent observational and theoretical studies have raised the possibility that the collimated outflows in gamma-ray burst (GRB) sources have two distinct components: a narrow (opening half-angle {theta}{sub n}), highly relativistic (initial Lorentz factor {eta}{sub n} & 10{sup 2}) outflow, from which the {gamma}-ray emission originates, and a wider ({theta}{sub w} {approx} E{sub n}, as expected for the decoupled neutron and proton components, respectively, in an initially neutron-rich, hydromagnetically accelerated jet model, then the narrow component only dominates the early afterglow and the wide component takes over after its nominalmore » deceleration time t{sub dec,w} (typically {approx} 0.1-1 days). For characteristic parameter values t{sub dec,w} is comparable to the jet break time t{sub jet,n} of the narrow component. In this case the emergence of the wide component at t{sub dec,w} may mask the jet break in the narrow component at t{sub jet,n} if E{sub w} & E{sub n}, which in turn may lead to an overestimate of the {gamma}-ray energy emitted by the source and hence of the required {gamma}-ray emission efficiency. We apply this scheme also to X-ray flash sources, which we interpret as GRB jets viewed at an angle {theta}{sub obs} > {theta}{sub n}. Finally, we argue that a neutron-rich hydromagnetic outflow may naturally give rise to repeated brightening episodes in the afterglow lightcurve as observed in GRB 021004 and GRB 030329.« less