We report the detection of a type C quasi-periodic oscillation (QPO) along with an upper harmonic in the commensurate ratio of 1:2 in two observations of the low-mass black hole transient H 1743–322 jointly observed by XMM-Newton and NuSTAR during the 2016 outburst. We find that the QPO and the upper harmonic exhibit shifts in their centroid frequencies in the second observation with respect to the first one. The hardness intensity diagram implies that in contrast to the 2008 and 2014 failed outbursts, the 2016 outburst was a successful one. We also detect the presence of a broad iron Kα line at ∼6.5 keV and a reflection hump in the energy range 15–30 keV in both of the observations. Along with the shape of the power density spectra, the nature of the characteristic frequencies and the fractional rms amplitude of the timing features imply that the source stayed in the low/hard state during these observations. Moreover, the photon index and other spectral parameters also indicate the low/hard state behavior of the source. Unlike the soft lag detected in this source during the 2008 and 2014 failed outbursts, we observe hard time lags of 0.40 ± 0.15 s and 0.32 ± 0.07 s in the 0.07–0.4 Hz frequency range in the two observations during the 2016 outburst. The correlation between the photon index and the centroid frequency of the QPO is consistent with the previous results. Furthermore, the high value of the Comptonized fraction and the weak thermal component indicate that the QPO is being modulated by the Comptonization process.

We perform comprehensive temporal and spectral analysis of the newly discovered X-ray transient MAXI J1803–298 using an AstroSat target of opportunity observation on 2021 May 11 during its outburst. The source was found to be in the hard-intermediate state. We detect type C quasi-periodic oscillations (QPOs) at the frequencies of ∼5.4 and ∼6.3 Hz along with a subharmonic at ∼2.8 Hz in the 3–15 keV band. The frequency and fractional rms amplitude of the QPO in the 15–30 keV band are found to be higher than those in the 3–15 keV band. We find soft lags of ∼3.8 and ∼6.8 ms for the respective QPOs at ∼5.4 and ∼6.3 Hz, whereas a soft lag of ∼4.7 ms is found at the subharmonic frequency. The increase in the soft lags at the QPO frequencies with energy is also observed in other black hole transients and attributed to the inclination dependence of the lags. The rms energy spectra indicate the power-law component to be more variable than the disk and reflection components. We find a broad iron line with an equivalent width of ∼0.17–0.19 keV and a reflection hump above ∼12 keV in the energy spectrum. Based on the X-ray spectroscopy and considering the distance to the source as 8 kpc, the estimated mass (∼8.5–16 M ⊙) and spin (a ≳ 0.7) of the black hole suggest that the source is likely to be a stellar mass Kerr black hole X-ray binary.

Motivated by the unsettled conclusion on whether there are any transit timing variations (TTVs) for the exoplanet Qatar-1b, 10 new transit light curves are presented and a TTV analysis with a baseline of 1400 epochs is performed. Because the linear model provides a good fit with a reduced chi-square of and the false-alarm probabilities of the possible TTV frequencies are as large as 35%, our results are consistent with a null-TTV model. Nevertheless, a new ephemeris with a reference time of T 0 = 2455647.63360 ± 0.00008 (BJD) and a period of P = 1.4200236 ± 0.0000001 (day) is obtained. In addition, the updated orbital semimajor axis and planetary radius in units of stellar radius are provided, and the lower limit of the modified stellar tidal quality factor is also determined.

Considering the importance of investigating the transit timing variations (TTVs) of transiting exoplanets, we present a follow-up study of HAT-P-12b. We include six new light curves observed between 2011 and 2015 from three different observatories, in association with 25 light curves taken from the published literature. The sample of the data used thus covers a time span of ∼ 10.2 years with a large coverage of epochs (1160) for the transiting events of the exoplanet HAT-P-12b. The light curves are utilized to determine the orbital parameters and conduct an investigation of possible TTVs. The new linear ephemeris shows a large value of reduced χ 2, i.e. χ red 2 ( 23 ) = 7.93, and the sinusoidal fitting using the prominent frequency coming from a periodogram shows a reduced χ 2 around 4. Based on these values and the corresponding O – C diagrams, we suspect the presence of a possible non-sinusoidal TTV in this planetary system. Finally, we find that a scenario with an additional non-transiting exoplanet could explain this TTV with an even smaller reduced χ 2 value of around 2.

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