Time delays for eleven gravitationally lensed quasars revisited

Abstract

Aims. We test the robustness of published time delays for 11 lensed quasars by using two techniques to measure time shifts in their light curves. Methods. We chose to use two fundamentally different techniques to determine time delays in gravitationally lensed quasars: a method based on fitting a numerical model and another one derived from the minimum dispersion method introduced by Pelt and collaborators. To analyse our sample in a homogeneous way and avoid bias caused by the choice of the method used, we apply both methods to 11 different lensed systems for which delays have been published: JVAS B0218+357, SBS 0909+523, RX J0911+0551, FBQS J0951+2635, HE 1104-1805, PG 1115+080, JVAS B1422+231, SBS 1520+530, CLASS B1600+434, CLASS B1608+656, and HE 2149-2745 Results. Time delays for three double lenses, JVAS B0218+357, HE 1104-1805, and CLASS B1600+434, as well as the quadruply lensed quasar CLASS B1608+656 are confirmed within the error bars. We correct the delay for SBS 1520+530. For PG 1115+080 and RX J0911+0551, the existence of a second solution on top of the published delay is revealed. The time delays in four systems, SBS 0909+523, FBQS J0951+2635, JVAS B1422+231, and HE 2149-2745 prove to be less reliable than previously claimed. Conclusions. If we wish to derive an estimate of H 0 based on time delays in gravitationally lensed quasars, we need to obtain more robust light curves for most of these systems in order to achieve a higher accuracy and robustness on the time delays.

FERMI LARGE AREA TELESCOPE DETECTION OF GRAVITATIONAL LENS DELAYED γ-RAY FLARES FROM BLAZAR B0218+357

“…The lags obtained from Gaussian fits to the peaks were Δt γ = 11.52 ± 0.31 and Δt γ = 11.38 ± 0.28 days, respectively, confirming the delay value and small uncertainty for the full interval, thus indicating that we obtained a robust measurement with the LAT. The small uncertainty in Δt γ is comparable to the best determined radio measurements for B0218+357 although the former is marginally larger by Δt γ − Δt r = 1.0 ± 0.3 and 1.4 ± 0.8 days (1σ ) than the Biggs et al (1999) and Cohen et al (2000) values, respectively, but consistent with the Eulaers & Magain (2011) values. If the radio/γ -ray delays are intrinsically different due to an offset between the respective emitting regions, the implied offset in a singular isothermal sphere lens model is ∼70 pc (projected) for a ∼10% difference in the time delay.…”

“…There are ∼20 gravitational lenses from these surveys out of >10 4 radio sources studied with 30 mJy at 8 GHz and, so far, the two radio brightest are detected γ -ray sources PKS1830-211 (below) and B0218+357 (out of ∼10 3 known γ -ray blazars; Nolan et al 2012). The other fainter lensed systems are typically less variable at radio frequencies, making delay measurements difficult (e.g., Jackson 2007;Eulaers & Magain 2011) and while they are not yet reported γ -ray sources, the all-sky monitoring of Fermi-LAT will allow the detection of short-timescale flaring γ -ray activity in which to attempt delay measurements. Importantly, γ -ray measurements constrain lens parameters free of propagation effects like scintillation (Heeschen 1984;Lovell et al 2008) that can hamper radio delay attempts (Winn et al 2004), although microlensing may be an important limiting factor because γ -ray emitting regions are expected to be more compact than in the radio.…”

“…Later independent (but contemporaneous) dual-frequency VLA observations further refined the delay, Δt r = 10.5 ± 0.2 (Biggs et al 1999) and 10.1 ± 0.8 days (Cohen et al 2000). Interestingly, Eulaers & Magain (2011) analyzed the latter's measurements and found LAT light curves over a wide dynamic range: one week bins over the first five years of the Fermi mission (top), 1 day bins for a 265 day flaring interval (middle), and 1.6 hr orbit bins during the 7 day Fermi ToO (bottom). The pre, post, and three active episodes outlined in the middle panel are further detailed in Figure 2.…”

A 5.5-year robotic optical monitoring of Q0957+561: substructure in a non-local cD galaxy

“…2. Although slow microlensing was detected in light curves of several lensed quasars (e.g., Gaynullina et al 2005;Fohlmeister et al 2007;Shalyapin et al 2009;Eulaers & Magain 2011), typical gradients are too small to play a role in brightness records over relatively short time segments. For example, Hainline et al (2012) used LQLM I data and more recent measurements from the United States Naval Observatory (USNO) to study the r-band flux ratio of Q0957+561.…”

Microlensingconstraintson the size of the gamma-ray emission region in blazar B0218+357

“…The system includes the lensing galaxy at the redshift z ≈ 0.68 (Browne et al 1993) and a more distant blazar at z ≈ 0.94 (Cohen et al 2003). The gravitational time delay between the two images of the source is determined in the radio, τ rad = 10.5 ± 0.2 d (Biggs et al 1999), τ rad = 10.1 ± 0.8 d (Cohen et al 2000; see also Eulaers & Magain 2011) and in the γ-ray band -τ γ ≈ 11.46 ± 0.16 days (all uncertainties correspond to 68% confidence interval). The flux ratio between the two images changes from 2 at 1.65 GHz frequency up to 4 at 15 GHz, presumably because of the freefree absorption in a giant molecular cloud (GMC), which is located in the lensing galaxy in front of the image A (Mittal et al 2007).…”