Context. The radio quasar 3C 454.3 underwent an exceptional optical outburst lasting more than 1 year and culminating in spring 2005. The maximum brightness detected was R = 12.0, which represents the most luminous quasar state thus far observed (M B ∼ −31.4). Aims. In order to follow the emission behaviour of the source in detail, a large multiwavelength campaign was organized by the Whole Earth Blazar Telescope (WEBT). Methods. Continuous optical, near-IR and radio monitoring was performed in several bands. ToO pointings by the Chandra and INTEGRAL satellites provided additional information at high energies in May 2005. Results. The historical radio and optical light curves show different behaviours. Until about 2001.0 only moderate variability was present in the optical regime, while prominent and long-lasting radio outbursts were visible at the various radio frequencies, with higher-frequency variations preceding the lower-frequency ones. After that date, the optical activity increased and the radio flux is less variable. This suggests that the optical and radio emissions come from two separate and misaligned jet regions, with the inner optical one acquiring a smaller viewing angle during the 2004−2005 outburst. Moreover, the colour-index behaviour (generally redder-when-brighter) during the outburst suggests the presence of a luminous accretion disc. A huge mm outburst followed the optical one, peaking in June−July 2005. The high-frequency (37−43 GHz) radio flux started to increase in early 2005 and reached a maximum at the end of our observing period (end of September 2005). VLBA observations at 43 GHz during the summer confirm the brightening of the radio core and show an increasing polarization. An exceptionally bright X-ray state was detected in May 2005, corresponding to the rising mm flux and suggesting an inverse-Compton nature of the hard X-ray spectrum. Conclusions. A further multifrequency monitoring effort is needed to follow the next phases of this unprecedented event.

We report on NICER observations of the magnetar SGR1935+2154, covering its 2020 burst storm and long-term persistent emission evolution up to ∼90 days postoutburst. During the first 1120s taken on April 28 00:40:58 UTC, we detect over 217 bursts, corresponding to a burst rate of >0.2 burstss −1. Three hours later, the rate was 0.008burstss −1 , remaining at a comparatively low level thereafter. The T 90 burst duration distribution peaks at 840 ms; the distribution of waiting times to the next burst is fit with a lognormal with an average of 2.1 s. The 1-10 keV burst spectra are well fit by a blackbody, with an average temperature and area of kT=1.7 keV and R 2 =53 km 2. The differential burst fluence distribution over ∼3 orders of magnitude is well modeled with a power-law form dN/dF∝F −1.5±0.1. The source persistent emission pulse profile is double-peaked hours after the burst storm. We find that the burst peak arrival times follow a uniform distribution in pulse phase, though the fast radio burst associated with the source aligns in phase with the brighter peak. We measure the source spin-down from heavy-cadence observations covering days 21-39 postoutburst, n =-´-3.72 3 10 12 ()  Hzs −1 , a factor of 2.7 larger than the value measured after the 2014 outburst. Finally, the persistent emission flux and blackbody temperature decrease rapidly in the early stages of the outburst, reaching quiescence 40 days later, while the size of the emitting area remains unchanged.

We report the analysis result of UV/X-ray emission from AR Scorpii, which is an intermediate polar (IP) composed of a magnetic white dwarf and a M-type star, with the XMM-Newton data. The X-ray/UV emission clearly shows a large variation over the orbit, and their intensity maximum (or minimum) is located at the superior conjunction (or inferior conjunction) of the M-type star orbit. The hardness ratio of the X-ray emission shows a small variation over the orbital phase, and shows no indication of the absorption by an accretion column. These properties are naturally explained by the emission from the M-type star surface rather than from the accretion column on the WD's star similar to the usual IPs. Beside, the observed X-ray emission also modulates with WD's spin with a pulse fraction of ∼ 14%. The peak position is aligned in the optical/UV/X-ray band. This supports the hypothesis that the electrons in AR Scorpii are accelerated to a relativistic speed, and emit nonthermal photons via the synchrotron radiation. In the X-ray bands, the evidence of the power-law spectrum is found in the pulsed component, although the observed emission is dominated by the optically thin thermal plasma emissions with several different temperatures. It is considered that the magnetic dissipation/reconnection process on the M-type star surface heats up the plasma to a temperature of several keV, and also accelerates the electrons to the relativistic speed. The relativistic electrons are trapped in the WD's closed magnetic field lines by the magnetic mirror effect. In this model, the observed pulsed component is explained by the emissions from the first magnetic mirror point.

The ultra-compact Low Mass X-ray Binary (LMXB) X1916-053, composed of a neutron star and a semi-degenerated white dwarf, exhibits periodic X-ray dips with variable width and depth. We have developed new methods to parameterize the dip to systematically study its variations. This helps to further understand binary and accretion disk behaviors. The RXTE 1998 observations clearly show a 4.87d periodic variation of the dip width. This is probably due to the nodal precession of the accretion disk, although there are no significant sidebands in the spectrum from the epoch folding search. From the negative superhump model (Larwood et. al. 1996), the mass ratio can be estimated as q = 0.045. Combined with more than 24 years of historical data, we found an orbital period derivative ofṖ orb /P orb = (1.62 ± 0.48) × 10 −7 yr −1 and established a quadratic ephemeris for the X-ray dips. The period derivative seems inconsistent with the prediction of the standard model of binary orbital evolution proposed by Rappaport et. al. (1987). On the other hand, the radiation-driven model (Tavani et. al. 1991) may properly interpret the period derivative even though the large mass outflow predicted by this model has never been observed in this system. With the best ephemeris, we obtained that the standard deviation of primary dips are smaller than that of secondary dips. This means that the primary dips are more stable than the secondary dips. Thus, we conclude that the primary dips of X1916-053 occur from the bulge at the rim instead of the ring of the disk proposed by Frank et. al. (1987).

Here we report the results of searching millisecond pulsar (MSP) candidates from the Fermi LAT second source catalog (2FGL). Seven unassociated γ−ray sources in this catalog are identified as promising MSP candidates based on their γ-ray properties. Through the X-ray analysis, we have detected possible X-ray counterparts, localized to an arcsecond accuracy. We have systematically estimated their X-ray fluxes and compared with the corresponding γ-ray fluxes. The X-ray to γ-ray flux ratios for 2FGL J1653.6-0159 and 2FGL J1946.4-5402 are comparable with the typical value for pulsars. For 2FGL J1625.2-0020, 2FGL J1653.6-0159 and 2FGL J1946.4-5402, their candidate X-ray counterparts are bright enough for performing a detailed spectral and temporal analysis to discriminate their thermal/non-thermal nature and search for the periodic signal. We have also searched for possible optical/IR counterparts at the X-ray positions. For the optical/IR source coincident with the brightest X-ray object that associated with 2FGL J1120.0-2204, its spectral energy distribution is comparable with a late-type star. Evidence for the variability has also been found by examining its optical light curve. All the aforementioned 2FGL sources resemble a pulsar in one or more aspects, which make them as the promising targets for follow-up investigations.