No abstract

No abstract

We present results on a systematic study of flux variability on hourly time-scales in a large sample of active galactic nuclei (AGN) in the 3−79 keV band using data from Nuclear Spectroscopic Telescope Array. Our sample consists of 4 BL Lac objects (BL Lacs), 3 flat spectrum radio quasars (FSRQs) 24 Seyfert 1, 42 Seyfert 2 and 8 narrow line Seyfert 1 (NLSy1) galaxies. We find that in the 3−79 keV band, about 65% of the sources in our sample show significant variations on hourly time scales. Using Mann-Whitney U-test and Kolmogorov-Smirnov test, we find no difference in the variability behaviour between Seyfert 1 and 2 galaxies. The blazar sources (FSRQs and BL Lacs) in our sample, are more variable than Seyfert galaxies that include Seyfert 1 and Seyfert 2 in the soft (3−10 keV), hard (10−79 keV) and total (3−79 keV) bands. NLSy1 galaxies show the highest duty cycle of variability (87%), followed by BL Lacs (82%), Seyfert galaxies (56%) and FSRQs (23%). We obtained flux doubling/halving time in the hard X-ray band less than 10 min in 11 sources. The flux variations between the hard and soft bands in all the sources in our sample are consistent with zero lag.

We present measurement of the cut-off energy, a proxy for the temperature of the corona in the nuclear continuum of the Seyfert 1 galaxy 3C 120 using ∼120 ks of observation from NuSTAR. The quality broad band spectrum from 3−79 keV has enabled us to measure the Compton reflection component (R) and to constrain the temperature of the coronal plasma. Fitting one of the advanced Comptonization models, compPS to the observed broad band spectrum we derived the kinetic temperature of the electrons in the corona to be kT e = 25±2 keV with Compton y parameter of y = 2.2±0.1 for a slab geometry and kT e = 26 +2 −0 keV with a y of 2.99 +2.99 −0.18 assuming a spherical geometry. We noticed excess emission from ∼10−35 keV arising due to Compton reflection and a broad Fe Kα line at 6.43 keV with an equivalent width of 60 ± 5 eV. The variations in count rates in the soft (3−10 keV) band is found to be more compared to the hard (10−79 keV) band with mean fractional variability amplitudes of 0.065±0.002 and 0.052±0.003 for the soft and hard bands respectively. 3C 120 is known to have a strong jet, however, our results indicate that it is either dormant or its contribution if any to the X-ray emission is negligible during the epoch of NuSTAR observation.

Context. The observed nuclear X-ray emission in the radio-quiet category of active galactic nuclei (AGN) is believed to be from a compact region, the corona, which is situated in the vicinity of central supermassive black holes. The shape of the X-ray continuum depends on, among other factors, the temperature of the corona (kTe). The launch of the Nuclear Spectroscopic Telescope Array (NuSTAR) has led to the determination of the high energy cutoff (Ecut), and thereby kTe, in many AGN. In a handful of sources, multiple observations with NuSTAR have also revealed changes in Ecut. Aims. In this work we aimed to investigate the variation in kTe in three AGN, namely NGC 3227, NGC 5548, and MR 2251−178, using more than one epoch of data on a source from NuSTAR. Methods. We carried out a spectral analysis of multiple epochs of data acquired using NuSTAR on the three sources, including a few new observations that had not yet been published. By fitting a Comptonization model to the data, we determined the temperature of the corona and investigated changes in kTe, if there were any, in these sources. Results. In NGC 3227, we found evidence for variation in kTe. We found no correlation of kTe, photon index (Γ), reflection fraction (R), or optical depth (τ) with flux, while τ is found to anti-correlate with kTe. This could be due to more than one physical process at work in the source causing the change in kTe. Conclusive evidence for the variation in kTe is not found in MR 2251−178 or NGC 5548.