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Register a new userA Quasar microlensing event towards J1249+3449?
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We show that the optical flare event discovered by Graham et al. (2020) towards the active galactic nucleus J1249+3449 is fully consistent with being a quasar microlensing event due to a $simeq 0.1 M_{odot}$ star, although other explanations, such as that, mainly supported by Graham et al. (2020), of being the electromagnetic counterpart associated to a binary black hole merger, cannot be completely excluded at present.
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We report the discovery of an extreme X-ray flux rise (by a factor of > 20) of the weak-line quasar SDSS J153913.47+395423.4 (hereafter SDSS J1539+3954) at z = 1.935. SDSS J1539+3954 is the most-luminous object among radio-quiet type 1 AGNs where such dramatic X-ray variability has been observed. Before the X-ray flux rise, SDSS J1539+3954 appeared X-ray weak compared with the expectation from its UV flux; after the rise, the ratio of its X-ray flux and UV flux is consistent with the majority of the AGN population. We also present a contemporaneous HET spectrum of SDSS J1539+3954, which demonstrates that its UV continuum level remains generally unchanged despite the dramatic increase in the X-ray flux, and its C iv emission line remains weak. The dramatic change only observed in the X-ray flux is consistent with a shielding model, where a thick inner accretion disk can block our line of sight to the central X-ray source. This thick inner accretion disk can also block the nuclear ionizing photons from reaching the high-ionization broad emission-line region, so that weak high-ionization emission lines are observed. Under this scenario, the extreme X-ray variability event may be caused by slight variations in the thickness of the disk. This event might also be explained by gravitational light-bending effects in a reflection model.
We present evidence for ultraviolet/optical microlensing in the gravitationally lensed quasar Q0957+561. We combine new measurements from our optical monitoring campaign at the United States Naval Observatory, Flagstaff (USNO) with measurements from the literature and find that the time-delay-corrected r-band flux ratio m_A - m_B has increased by ~0.1 magnitudes over a period of five years beginning in the fall of 2005. We apply our Monte Carlo microlensing analysis procedure to the composite light curves, obtaining a measurement of the optical accretion disk size, log {(r_s/cm)[cos(i)/0.5]^{1/2}} = 16.2^{+0.5}_{-0.6}, that is consistent with the quasar accretion disk size - black hole mass relation.
We used the Tycho-Gaia Astrometric Solution catalogue, part of the Gaia Data Release 1, to search for candidate astrometric microlensing events expected to occur within the remaining lifetime of the Gaia satellite. Our search yielded one promising candidate. We predict that the nearby DQ type white dwarf LAWD 37 (WD 1142-645) will lens a background star and will reach closest approach on November 11th 2019 ($pm$ 4 days) with impact parameter $380pm10$ mas. This will produce an apparent maximum deviation of the source position of $2.8pm0.1$ mas. In the most propitious circumstance, Gaia will be able to determine the mass of LAWD 37 to $sim3%$. This mass determination will provide an independent check on atmospheric models of white dwarfs with helium rich atmospheres, as well as tests of white dwarf mass radius relationships and evolutionary theory.
Microlensing by stars within distant galaxies acting as strong gravitational lenses of multiply-imaged quasars, provides a unique and direct measurement of the internal structure of the lensed quasar on nano-arcsecond scales. The measurement relies on the temporal variation of high-magnification caustic crossings which vary on timescales of days to years. Multiwavelength observations provide information from distinct emission regions in the quasar. Through monitoring of these strong gravitational lenses, a full tomographic view can emerge with Astronomical-Unit scale resolution. Work to date has demonstrated the potential of this technique in about a dozen systems. In the 2020s there will be orders of magnitude more systems to work with. Monitoring of lens systems for caustic-crossing events to enable triggering of multi-platform, multi-wavelength observations in the 2020s will fulfill the potential of quasar microlensing as a unique and comprehensive probe of active black hole structure and dynamics.
We present a new approach in the study of the Initial Mass function (IMF) in external galaxies based on quasar microlensing observations. We use measurements of quasar microlensing magnifications in 24 lensed quasars to estimate the average mass of the stellar population in the lens galaxies without any a priori assumption on the shape of the IMF. The estimated mean mass of the stars is $langle M rangle =0.16^{+0.05}_{-0.08} M_odot$ (at 68% confidence level). We use this average mass to put constraints into two important parameters characterizing the IMF of lens galaxies: the low-mass slope, $alpha_2$, and the low-mass cutoff, $M_{low}$. Combining these constraints with prior information based on lensing, stellar dynamics, and absorption spectral feature analysis, we calculate the posterior probability distribution for the parameters $M_{low}$ and $alpha_2$. We estimate values for the low-mass end slope of the IMF $langle alpha_2rangle=-2.6pm 0.9$ (heavier than that of the Milky Way) and for the low-mass cutoff $langle M_{low}rangle=0.13pm0.07$. These results are in good agreement with previous studies on these parameters and remain stable against the choice of different suitable priors.
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