GRB Afterglow Parameters in the Era of TeV Observations: The Case of GRB 190114C

The afterglow of GRB 190114C has been observed at 60–1200 s after the burst in the sub-TeV range by the MAGIC Cerenkov telescope. The simultaneous observations in the X-ray range, which is presumed to be of synchrotron origin, and in the sub-TeV range, where the emission is presumed to be inverse Compton, provide new stringent constraints on the conditions within the emitting regions and their evolution in time. While the additional data contain a lot of new information, it turns out that fitting both the X-ray and the TeV emission is much more complicated than what was originally anticipated. We find that optical flux measurements provide important complementary information that, in combination with TeV measurements, breaks degeneracy in the parameter space. We present here a numerical fit to the multiwavelength observed spectrum using a new code that calculates the single-zone synchrotron including self-Compton emission, taking into account the exact Klein–Nishina cross sections, as well as pair production via absorption of the high-energy photons inside the emitting zone and the emission from the resulting secondary pairs. We also present a revised set of single-zone parameters and a method for fitting the data to the observations. Our model for GRB 190114C that fits all the observations, from the optical data point to the sub-TeV range, suggests that it is in the fast-cooling regime. The inferred parameters for observations at two separate moments of time show significant deviations from some of the common expectations in afterglow modeling but are all consistent with the predictions of the pair-balance model.

Dipper-like variability of the Gaia alerted young star V555 Ori

ABSTRACT V555 Ori is a T Tauri star, whose 1.5 mag brightening was published as a Gaia science alert in 2017. We carried out optical and near-infrared (NIR) photometric, and optical spectroscopic observations to understand the light variations. The light curves show that V555 Ori was faint before 2017, entered a high state for about a year, and returned to the faint state by mid-2018. In addition to the long-term flux evolution, quasi-periodic brightness oscillations were also evident, with a period of about 5 d. At optical wavelengths both the long-term and short-term variations exhibited colourless changes, while in the NIR they were consistent with changing extinction. We explain the brightness variations as the consequence of changing extinction. The object has a low accretion rate whose variation in itself would not be enough to reproduce the optical flux changes. This behaviour makes V555 Ori similar to the pre-main sequence star AA Tau, where the light changes are interpreted as periodic eclipses of the star by a rotating inner disc warp. The brightness maximum of V555 Ori was a moderately obscured (AV = 2.3 mag) state, while the extinction in the low state was AV = 6.4 mag. We found that while the Gaia alert hinted at an accretion burst, V555 Ori is a standard dipper, similar to the prototype AA Tau. However, unlike in AA Tau, the periodic behaviour was also detectable in the faint phase, implying that the inner disc warp remained stable in both the high and low states of the system.

Intra-night optical variability of γ −ray detected narrow-line Seyfert1 galaxies

We report the first attempt to systematically characterise intra-night optical variability (INOV) of the rare and enigmatic subset of Narrow-Line Seyfert1 galaxies (NLSy1s), which is marked by detection in the γ-ray band and is therefore endowed with Doppler boosted relativistic jets, like blazars. However, the central engines in these two types of AGN are thought to operate in different regimes of accretion rate. Our INOV search in a fairly large and unbiased sample of 15 γ-ray NLSy1s was conducted in 36 monitoring sessions, each lasting ≥ 3 hrs. In our analysis, special care has been taken to address the possible effect on the differential light curves, of any variation in the seeing disc during the session, since that might lead to spurious claims of INOV from such AGN due to the possibility of a significant contribution from the host galaxy to the total optical emission. From our observations, a duty cycle (DC) of INOV detection in the γ-ray NLSy1s is estimated to be around 25% - 30%, which is comparable to that known for blazars. This estimate of DC will probably need an upward revision, once it becomes possible to correct for the dilution of the AGN’s nonthermal optical emission by the (much steadier) optical emission contributed not only by the host galaxy but also the nuclear accretion disc in these high Eddington rate accretors. Finally, we also draw attention to the possibility that sharp optical flux changes on sub-hour time scale are less rare for γ-ray NLSy1s, in comparison to blazars.

Modelling of 35-d superorbital cycle of B and V light curves of IMXB HZ Her/Her X-1

ABSTRACT The X-ray binary Her X-1 consists of an accreting neutron star and the optical component HZ Her. The 35-d X-ray superorbital variability of this system is known, since its discovery in 1972 by the Uhuru satellite and is believed to be caused by forced precession of a warped accretion disc tilted to the orbital plane. We argue that the observed features of the 35-d optical variability of HZ Her can be explained by free precession of the neutron star with a period close to that of the forced disc. The model parameters include (a) the X-ray luminosity of the neutron star; (b) the optical flux from the accretion disc; and (c) the tilt of the inner and outer edges of the accretion disc. A possible synchronization mechanism based on the coupling between the neutron star free precession and the dynamical action of non-stationary gas streams is discussed.

Multi-wavelength photometry during the 2018 superoutburst of the WZ Sge-type dwarf nova EG Cancri

We report on the multi-wavelength photometry of the 2018 superoutburst in EG Cnc. We have detected stage A superhumps and long-lasting late-stage superhumps via the optical photometry and have constrained the binary mass ratio and its possible range. The median value of the mass ratio is 0.048 and the upper limit is 0.057, which still implies that EG Cnc is one of the possible candidates for period bouncers. This object also showed multiple rebrightenings in this superoutburst which are the same as those in its previous superoutburst in 1996–1997, despite the difference in the main superoutburst. This would represent that the rebrightening type is inherent to each object and is independent of the initial disk mass at the beginning of superoutbursts. We also found that B − I and J − Ks colors were unusually red just before the rebrightening phase and became bluer during the quiescence between rebrightenings, which would mean that the low-temperature mass reservoir at the outermost disk accreted with time after the main superoutburst. Also, the ultraviolet flux was sensitive to rebrightenings as well as the optical flux, and the U − B color became redder during the rebrightening phase, which would indicate that the inner disk became cooler when this object repeated rebrightenings. Our results thus basically support the idea that the cool mass reservoir in the outermost disk is responsible for rebrightenings.

XMM–Newton study of X-ray sources in the field of Willman 1 dwarf spheroidal galaxy★

ABSTRACT We present the results of the analysis of three XMM–Newton observations of the Willman 1 dwarf spheroidal galaxy (Wil 1). X-ray sources are classified on the basis of spectral analysis, hardness ratios, X-ray-to-optical flux ratio, X-ray variability, and cross-correlation with available catalogues in optical and infrared wavelengths. We catalogued 97 sources in the field of Wil 1. Our classification shows the presence of a β-type symbiotic star in Wil 1. We classified one M dwarf foreground star in the field of Wil 1. Moreover, 54 sources are classified as background active galactic nuclei and galaxies. Our study shows that the luminosity of the X-ray sources of Wil 1 does not exceed ∼1034 erg s−1 in the energy range of 0.2–12.0 keV, which is similar to observed luminosities of sources in nearby dwarf spheroidal galaxies.

Is HR 6819 a triple system containing a black hole?

Context. HR 6819 was recently proposed to be a triple system consisting of an inner B-type giant plus black hole (BH) binary with an orbital period of 40 d and an outer Be tertiary. This interpretation is mainly based on two inferences: that the emission attributed to the outer Be star is stationary and that the inner star, which is used as mass calibrator for the BH, is a B-type giant. Aims. We re-investigate the properties of HR 6819 to search for a possibly simpler alternative explanation for HR 6819, which does not invoke the presence of a triple system with a BH in the inner binary. Methods. Based on an orbital analysis, the disentangling of the spectra of the two visible components and the atmosphere analysis of the disentangled spectra, we investigate the configuration of the system and the nature of its components. Results. Disentangling implies that the Be component is not a static tertiary, but rather a component of the binary in the 40 d orbit. The inferred radial velocity amplitudes of K1 = 60.4 ± 1.0 km s−1 for the B-type primary and K2 = 4.0 ± 0.8 km s−1 for the Be-type secondary imply an extreme mass ratio of M2/M1 = 15 ± 3. We find that the B-type primary, which we estimate to contribute about 45% to the optical flux, has an effective temperature of Teff = 16 ± 1 kK and a surface gravity of log g = 2.8 ± 0.2 [cgs], while the Be secondary, which contributes about 55% to the optical flux, has Teff = 20 ± 2 kK and log g = 4.0 ± 0.3 [cgs]. We infer spectroscopic masses of 0.4−0.1+0.3and 6−3+5 for the primary and secondary which agree well with the dynamical masses for an inclination of i = 32°. This indicates that the primary might be a stripped star rather than a B-type giant. Evolutionary modelling suggests that a possible progenitor system would be a tight (Pi ≈ 2 d) B+B binary system that experienced conservative mass transfer. While the observed nitrogen enrichment of the primary conforms with the predictions of the evolutionary models, we find no indications for the predicted He enrichment. Conclusions. We suggest that HR 6819 is a binary system consisting of a stripped B-type primary and a rapidly-rotating Be star that formed from a previous mass-transfer event. In the framework of this interpretation, HR 6819 does not contain a BH. Interferometry can distinguish between these two scenarios by providing an independent measurement of the separation between the visible components.