Anomalous Fluctuations in Observations of Q0957+561 A,B: Smoking Gun of a Cosmic String?

TL;DR

The paper tackles anomalous, synchronous brightness fluctuations observed in the Q0957+561 A,B gravitational lens system. It develops a theoretical framework for gravitational lensing by oscillating cosmic string loops, deriving explicit lens equations and a magnification formula, and compares this with a binary-mass lens alternative. The authors show that for a Grand Unified Theory (GUT) string with tension $\mu \sim 10^{22}\ \mathrm{g\,cm^{-1}}$, the observed $\sim 4\%$ amplitude with a $\sim 100$-day period can be reproduced, whereas a binary-lens explanation would require unrealistically massive, nearby objects. The results suggest a potential observational signature of cosmic strings and motivate searches for similar loop-induced microlensing effects in other lens systems, with implications for string cosmology and the distribution of loops in galactic halos.

Abstract

We report the detection of anomalous brightness fluctuations in the multiple image Q0957+561 A,B gravitational lens system, and consider whether such anomalies have a plausible interpretation within the framework of cosmic string theory. We study a simple model of gravitational lensing by an asymmetrical rotating string. An explicit form of the lens equation is obtained and approximate relations for magnification are derived. We show that such a model with typical parameters of the GUT string can quantitatively reproduce the observed pattern of brightness fluctuations. On the other hand, explanation involving a binary star system as an alternative cause requires an unacceptably large massive object at a small distance. We also discuss possible observational manifestations of cosmic strings within our lens model.

Figures (6)

• Figure 1: Brightness of the two quasar images displayed with no correction for gravitational lens time delay. The brightness of quasar image A (upper record with square data markers), has been fitted with a sine curve having 0.04 magnitude amplitude. The lower record, with triangular markers, appears to have the same sinusoidal brightness curve with zero lag, even though at most epochs data for the gravitationally lensed images show a lag of 417 days.
• Figure 2: Quasar brightness displayed for measured time delay. The upper record shows the same data and fit for image A as displayed in Fig. \ref{['fig1']}. The lower data markers (triangles) are the brightness measurements for image B measured 417 days (the gravitational lens time delay) later, but with 417 subtracted from the Julian dates for plotting. If the image A brightness fluctuations are intrinsic to the quasar, they should be seen also in image B 417 days later, but the two are seen not to match as well as the 0 lag comparison in Fig. \ref{['fig1']}.
• Figure 3: Quasar brightness with 417-day lag for the opposite image pair. The lower record is the Fig. \ref{['fig1']} data for image B, with the data for image A measured 417 days previously. The agreement is seen to be poor, especially near the end of the observational period, even though the theory of gravitational lenses shows that the time delay must produce agreement for 417 days. If the measured sinusoidal oscillation seen in both images A and B (Fig. \ref{['fig1']}) is a chance coincidence of two quasar oscillations separated by 417 days, there must be agreement for both A data with corrected B data (Fig. \ref{['fig2']}) and B data with corrected A data (this figure). Comparing Fig. \ref{['fig1']} with Figs. \ref{['fig2']} and \ref{['fig3']} shows best agreement for 0 lag, contrary to gravitational lens time delay theory.
• Figure 4: Oscillations of quasar image brightness caused by the loop depending: a) on mass per unit length $\mu$ in g/cm, b) on transverse velocity of loop $v_{\perp}$ in units of the speed of light, c) on minimal distance $\rho_0$ between image and center of the loop in units of $R$, d) on angle $\varphi_0$ between loop direction and direction from the loop center to the image at $t=0$ in degrees.
• Figure 5: Oscillations of quasar image magnification predicted by the cosmic string model. Upper and lower curves are shifted up and down by $0.05$ and fitted to image A and B brightness records, respectively.