Evidence For Quasi-periodic Oscillations Research Articles

Characterizing Long-term Optical Variability Properties of γ-Ray-bright Blazars

Abstract Optical observations of a sample of 12 γ-ray-bright blazars from four optical data archives—American Association of Variable Star Observers, Small and Moderate Aperture Research Telescope System, Catalina, and Steward Observatory—are compiled to create densely sampled light curves spanning more than a decade. As a part of the blazar multiwavelength studies, several methods of analysis, e.g., flux distribution and rms–flux relation, are performed on the observations with the aim to compare the results with the similar ones in the γ-ray band presented in Bhatta & Dhital. It is found that, similar to the γ-ray band, blazars display significant variability in the optical band that can be characterized with lognormal flux distribution and a power-law dependence of rms on flux. It could be an indication of a possible inherent linear rms–flux relation, yet the scatter in the data does not allow to rule out other possibilities. When comparing variability properties in the two bands, the blazars in the γ-rays are found to exhibit stronger variability with a steeper possible linear rms–flux relation and a flux distribution that is more skewed toward higher fluxes. The cross-correlation study shows that except for source 3C 273, the overall optical and the γ-ray emission in the sources are highly correlated, suggesting a cospatial existence of the particles responsible for both the optical and γ-ray emission. Moreover, sources S5 0716+714, Mrk 421, Mrk 501, PKS 1424-418, and PKS 2155-304 revealed possible evidence of quasiperiodic oscillations in the optical emission with the characteristic timescales, which are comparable to those in the γ-ray band detected in our previous work.

NuSTAR and Swift observations of the extragalactic black hole X-ray binaries

ABSTRACT We present the results obtained from detailed spectral and timing studies of extragalactic black hole X-ray binaries LMC X–1 and LMC X–3, using simultaneous observations with Nuclear Spectroscopic Telescope Array (NuSTAR) and Neil Gehrels Swift observatories. The combined spectra in the 0.5–30 keV energy range, obtained between 2014 and 2019, are investigated for both sources. We do not find any noticeable variability in 0.5–30 keV light curves, with 0.1–10 Hz fractional rms estimated to be <2 per cent. No evidence of quasi-periodic oscillations is found in the power density spectra. The sources are found to be in the high soft state during the observations with disc temperature Tin ∼ 1 keV, photon index, Γ > 2.5 and thermal emission fraction, fdisc > 80 per cent. An Fe Kα emission line is detected in the spectra of LMC X–1, though no such feature is observed in the spectra of LMC X–3. From the spectral modelling, the spins of the black holes in LMC X–1 and LMC X–3 are estimated to be in the range of 0.92–0.95 and 0.19−0.29, respectively. The accretion efficiency is found to be, η ∼ 0.13 and η ∼ 0.04 for LMC X–1 and LMC X–3, respectively.

NICER observations of the black hole candidate MAXI J0637–430 during the 2019–2020 outburst

ABSTRACT We present detailed timing and spectral studies of the black hole candidate MAXI J0637–430 during its 2019–2020 outburst using observations with the Neutron Star Interior Composition Explorer (NICER) and the Neil Gehrels Swift Observatory. We find that the source evolves through the soft-intermediate, high-soft, hard-intermediate, and low-hard states during the outburst. No evidence of quasi-periodic oscillations is found in the power-density spectra of the source. Weak variability with fractional rms amplitude ${\lt}5{{\ \rm per\ cent}}$ is found in the softer spectral states. In the hard-intermediate and hard states, high variability with the fractional rms amplitude of ${\gt}20{{\ \rm per\ cent}}$ is observed. The 0.7–10 keV spectra with NICER are studied with a combined disc-black-body and nthcomp model along with the interstellar absorption. The temperature of the disc is estimated to be 0.6 keV in the rising phase and decreased slowly to 0.1 keV in the declining phase. The disc component was not detectable or absent during the low-hard state. From the state-transition luminosity and the inner edge of the accretion flow, we estimate the mass of the black hole to be in the range of 5–12 M⊙, assuming the source distance of d < 10 kpc.

X-ray spectral and timing evolution of MAXI J1727–203 with NICER

ABSTRACT We present a detailed X-ray spectral and variability study of the full 2018 outburst of MAXI J1727–203 using NICER observations. The outburst lasted approximately four months. Spectral modelling in the 0.3–10 keV band shows the presence of both a soft thermal and a hard Comptonised component. The analysis of these components shows that MAXI J1727–203 evolved through the soft, intermediate, and hard spectral states during the outburst. We find that the soft (disc) component was detected throughout almost the entire outburst, with temperatures ranging from ∼0.4 keV, at the moment of maximum luminosity, to ∼0.1 keV near the end of the outburst. The power spectrum in the hard and intermediate states shows broad-band noise up to 20 Hz, with no evidence of quasi-periodic oscillations. We also study the rms spectra of the broad-band noise at 0.3−10 keV of this source. We find that the fractional rms increases with energy in most of the outburst except during the hard state, where the fractional rms remains approximately constant with energy. We also find that, below 3 keV, the fractional rms follows the same trend generally observed at energies >3 keV, a behaviour known from previous studies of black holes and neutron stars. The spectral and timing evolution of MAXI J1727–203, as parametrised by the hardness–intensity, hardness–rms, and rms–intensity diagrams, suggest that the system hosts a black hole, although we could not rule out a neutron star.

Evidence for quasi-periodic oscillations in the optical polarization of the blazar PKS 2155–304

Abstract Evidence for the presence of quasi-periodic oscillations (QPOs) in the optical polarization of the blazar PKS 2155–304, during a period of enhanced gamma-ray brightness, is presented. The periodogram of the polarized flux revealed the existence of a prominent peak at T ∼ 13 min, detected at >99.7 per cent significance, and T ∼ 30 min, which was nominally significant at >99 per cent. This is the first evidence of QPOs in the polarization of an active galactic nucleus, potentially opening up a new avenue of studying this phenomenon.

Detection of superhumps in the Z Camelopardalis-type dwarf nova AT Cnc at standstill

We present results of our photometry of the Z Camelopardalis-type dwarf nova AT Cnc. The observational data were obtained during 13 nights in February and March 2003 when AT Cnc was in its long standstill. Two sets of our data reveal brightness variations with quasi-periods of (4.65 ± 0.02) and of (4.74 ± 0.02) h. The semiamplitude of these variations was observed in the range 5-9 mmag, showing changes from night to night. This signal varies in period and in phase on a time-scale of weeks, as is typical of superhumps. A comparison with the orbital period obtained by Nogami et al. (1999) from the radial velocity measurements of AT Cnc, which equals (4.826 ± 0.014) h, shows that this signal can be a negative superhump since the average period of brightness variations is approximately 3% shorter than the orbital period. These results make AT Cnc a permanent superhump system with a rather large orbital period and a large mass ratio. This is also the first detection of superhumps in a Z Camelopardalis-type system. In addition, we have found signs of an unstable signal with a period of about 2.3 h, which may represent the second harmonic of the 4.7-h signal or the second orbital sideband of the wobble frequency of the tilted precessing accretion disc. The average power spectrum of AT Cnc reveals a broad hump at frequencies in the range 0.4-0.7 mHz, which is evidence of quasiperiodic oscillations. The hump crest corresponds to a period of 1700-1800 s, which is remarkably close to 10% of the orbital period. This may indicate that this system contains a magnetic white dwarf creating the tilted accretion disc.

ITAL]ROSSI X-RAY TIMING EXPLORER[/ITAL] Observations of LMC X-1

The luminous X-ray binary and black hole candidate LMC X-1 has been observed with the Rossi X-Ray Timing Explorer to search for quasi-periodic oscillations (QPOs) previously reported in its high state. The source was observed monthly in a series of nine observations. Analysis of the temporal variations shows no evidence for QPOs or other periodic changes, but correlations between the high-energy light curve and hardness of the spectrum are described. Spectral fits with two-component models demonstrate that the hardness variations come from changes in the intensity of the high-energy power-law tail.