We report a possible detection of an ∼4.6-h quasi-periodic oscillation (QPO) in the 0.3-10 keV emission of the high-energy peaked blazar PKS 2155−304 from a 64 ks observation by the XMM-Newton EPIC/pn detector. We identify a total modulation of ∼5% in the light curve and confirm that nominal period by periodogram, structure function, and wavelet analyses. The limited light curve duration allows the capture of only 3.8 cycles of this oscillation and thus precludes a very strong claim for this QPO, despite a nominally high ( 3σ) statistical significance. We briefly discuss models capable of producing an X-ray QPO of such a period in a blazar.

We selected a sample of 24 XMM-Newton light curves (LCs) of four high energy peaked blazars, PKS 0548−322, ON 231, 1ES 1426+428 and PKS 2155−304. These data comprise continuous light curves of 7.67h to 18.97h in length. We searched for possible quasi-periodic oscillations (QPO) and intra-day variability (IDV) timescales in the LCs of these blazars. We found a likely QPO in one LC of PKS 2155−304 which was reported elsewhere (Lachowicz et al. 2009). In the remaining 23 LCs we found hints of possible weak QPOs in one LC of each of ON 231 and PKS 2155−304, but neither is statistically significant. We found IDV timescales that ranged from 15.7 ks to 46.8 ks in 8 LCs. In 13 LCs any variability timescales were longer than the length of the data. Assuming the possible weak QPO periods in the blazars PKS 2155−304 and ON 231 are real and are associated with the innermost portions of their accretion disk, we can estimate that their central black hole masses exceed 1.2 × 10 7 M ⊙ . Emission models for radio-loud active galactic nuclei (AGN) that could explain our results are briefly discussed.

We present a comprehensive analysis of long-term periodic variability of Cyg X-1 using the method of multiharmonic analysis of variance (mhAoV) applied to available monitoring data since 1969, in X-rays from Vela 5B, Ariel 5, Ginga, CGRO and RXTE satellites and in radio from the Ryle and Green Bank telescopes. We confirm a number of previously obtained results, and, for the first time, find an orbital modulation at 15 GHz in the soft state and show the detailed non-sinusoidal shape of that modulation in the hard state of both the 15-GHz emission and the X-rays from the RXTE/ASM. We find the CGRO/BATSE data are consistent with the presence of a weak orbital modulation, in agreement with its theoretical modelling as due to Compton scattering in the companion wind. We then confirm the presence of a ~150-d superorbital period in all of the data since ~1976, finding it in particular for the first time in the Ariel 5 data. Those data sets, covering >65 superorbital cycles, show a remarkable constancy of both the period and the phase. On the other hand, we confirm the presence of a ~290-d periodicity in the 1969-1979 Vela 5B data, indicating a switch from that period to its first harmonic at some time <~1980. We find the superorbital modulation is compatible with accretion disc precession. Finally, we find a significant modulation in the RXTE/ASM data at a period of 5.82-d, which corresponds to the beat between the orbital and superorbital modulations provided the latter is prograde.Comment: MNRAS, in pres

Aims. In the past four decades, it has been observed that solar flares display quasi-periodic pulsations (QPPs) from the lowest, i.e. radio, to the highest, i.e. gamma-ray, frequencies in the electromagnetic spectrum. It remains unclear which mechanism creates these QPPs. In this paper, we analyze four bright solar flares that display compelling signatures of quasi-periodic behavior and were observed with the Gamma-Ray Burst Monitor (GBM) onboard the Fermi satellite. Because GBM covers over three decades in energy (8 keV to 40 MeV), it is regarded as a key instrument in our attempt to understand the physical processes that drive solar flares. Methods. We tested for periodicity in the time series of the solar flares observed by GBM by applying a classical periodogram analysis. However, in contrast to previous authors, we did not detrend the raw light curve before creating the power spectral density (PSD) spectrum. To assess the significance of the frequencies, we used a method that is commonly applied to X-ray binaries and Seyfert galaxies. This technique takes into account the underlying continuum of the PSD, which for all of these sources has a P( f ) ∼ f −α dependence and is typically labeled red-noise. Results. We checked the reliability of this technique by applying it to observations of a solar flare that had been observed by the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI ). These data contain, besides any potential periodicity from the Sun, a 4 s rotational period caused by the rotation of the spacecraft about its axis. We were unable to identify any intrinsic solar quasi-periodic pulsation but we did manage to reproduce the instrumental periodicity. Moreover, with the method adopted here, we do not detect significant QPPs in the four bright solar flares observed by GBM. We stress that for this kind of analyses it is of utmost importance to account appropriately for the red-noise component in the PSD of these astrophysical sources.