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Register a new userThe EUV variability of the luminous QSO HS 1700+6416
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We report on the first observations of variations in UV (intrinsic EUV 330 A) flux of the luminous QSO HS 1700+6416 (z=2.72) over a decade. The amplitude of variations increases from +/-0.1 mag in the optical (R) to up to a factor of 3 at 1250 A. This is apparently an extension of the increase in amplitude of variations towards shorter wavelengths observed with IUE in low z AGN (Paltani & Courvoisier, 1996) to the EUV. The time-scale for variations with the largest amplitudes is >= 1/2 yr to years. We briefly discuss the consequences of the observed variations on the ionizing metagalactic UV background.
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It is found that feii emission contributes significantly to the optical and ultraviolet spectra of most active galactic nuclei. The origin of the optical/UV feii emission is still a question open to debate. The variability of feii would give clues to this origin. Using 7.5 yr spectroscopic monitoring data of one Palomer-Green (PG) quasi-stellar object (QSO), PG 1700+518, with strong optical feii emission, we obtain the light curves of the continuum lv, feii, the broad component of hb, and the narrow component of hb by the spectral decomposition. Through the interpolation cross-correlation method, we calculate the time lags for light curves of feii, the total hb, the broad component of hb, and the narrow component of hb with respect to the continuum light curve. We find that the feii time lag in PG1700+518 is $209^{+100}_{-147}$ days, and the hb time lag cannot be determined. Assuming that feii and hb emission regions follow the virial relation between the time lag and the FWHM for the hb and feii emission lines, we can derive that the hb time lag is $148^{+72}_{-104}$ days. The hb time lag calculated from the empirical luminosity--size relation is 222 days, which is consistent with our measured feii time lag. Considering the optical feii contribution, PG 1700+518 shares the same characteristic on the spectral slope variability as other 15 PG QSOs in our previous work, i.e., harder spectrum during brighter phase.
We present the results of spectroscopic, narrow-band and X-ray observations of a z=2.30 protocluster in the field of the QSO HS 1700+643. Using a sample of BX/MD galaxies, which are selected to be at z~2.2-2.7 by their rest-frame ultraviolet colours, we find that there are 5 protocluster AGN which have been identified by characteristic emission-lines in their optical/near-IR spectra; this represents an enhancement over the field significant at ~98.5 per cent confidence. Using a ~200 ks Chandra/ACIS-I observation of this field we detect a total of 161 X-ray point sources to a Poissonian false-probability limit of 4x10^{-6} and identify 8 of these with BX/MD galaxies. Two of these are spectroscopically confirmed protocluster members and are also classified as emission-line AGN. When compared to a similarly selected field sample the analysis indicates this is also evidence for an enhancement of X-ray selected BX/MD AGN over the field, significant at ~99 per cent confidence. Deep Lya narrow-band imaging reveals that a total of 4/123 Lya emitters (LAEs) are found to be associated with X-ray sources, with two of these confirmed protocluster members and one highly likely member. We do not find a significant enhancement of AGN activity in this LAE sample over that of the field (result significant at only 87 per cent confidence). The X-ray emitting AGN fractions for the BX/MD and LAE samples are found to be 6.9_{-4.4}^{+9.2} and 2.9_{-1.6}^{+2.9} per cent, respectively, for protocluster AGN with L_{2-10 keV}>4.6x10^{43} erg s^{-1} at z=2.30. These findings are similar to results from the z=3.09 protocluster in the SSA 22 field found by Lehmer et al. (2009), in that both suggest AGN activity is favoured in dense environments at z>2.
The relationship between variability, luminosity and redshift in the South Galactic Pole QSO sample is examined in an effort to disentangle the effects of luminosity and redshift in the amplitude of the optical variations. The anticorrelation between variability and luminosity found by other authors is confirmed. Our analysis also supports claims that variability increases with redshift, most likely due to an anticorrelation between variability and wavelength. In particular, our parametric fits show that the QSO variability-wavelength relation is consistent with that observed in low-luminosity nearby active galactic nuclei. The results are used to constrain Poissonian-type models. We find that if QSO variability results from a random superposition of pulses, then the individual events must have B-band energies between $sim 10^{50}$ and a few times $10^{51}$ erg and time-scales of $sim 2$ yr. Generalized Poissonian models in which the pulse energy and lifetime scale with luminosity are also discussed.
One of the defining characteristics of a solar flare is the impulsive formation of very high temperature plasma. The properties of the thermal emission are not well understood, however, and the analysis of solar flare observations is often predicated on the assumption that the flare plasma is isothermal. The EUV Variability Experiment (EVE) on the Solar Dynamics Observatory (SDO) provides spectrally resolved observations of emission lines that span a wide range of temperatures (e.g., Fe XV-Fe XXIV) and allow for thermal flare plasma to be studied in detail. In this paper we describe a method for computing the differential emission measure distribution in a flare using EVE observations and apply it to several representative events. We find that in all phases of the flare the differential emission measure distribution is broad. Comparisons of EVE spectra with calculations based on parameters derived from the GOES soft X-ray fluxes indicate that the isothermal approximation is generally a poor representation of the thermal structure of a flare.
We present a combined analysis of the optical spectral variability for two samples of QSOs, 42 objects at $z<0.4$ monitored at the Wise Observatory (Giveon et al 1999), plus 59 objects up to $zsim 3$ in the field of the Magellanic Clouds, detected and/or monitored within the MACHO Project database (Geha et al 2003). Our analysis shows some increase of the observed spectral variability as a function of redshift, with a large scatter. These data are compared with a model based on the addition of flares of different temperatures to a stationary quasar SED, taking into account also the intrinsic scatter of the SEDs.
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