X-ray flux ratio changes in the two gravitationally lensed images of the blazar PKS 1830-211

ATel #12737; J. Wilms (Remeis-Observatory and ECAP, D), S. Buson (Wuerzburg, D, UMBC, USA), A. Gokus (Wuerzburg and Remeis-Observatory & ECAP, D), T. C. Cheung (NRL, USA), M. A. Nowak (WUSTL, USA), S. Ciprini (INFN Roma Tor Vergata and ASI, I)
on 8 May 2019; 22:56 UT
Credential Certification: Joern Wilms (j.wilms@sternwarte.uni-erlangen.de)

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PKS 1830-211 is a blazar at z~2.5 (Lidman et al., 1999, ApJ 514, L57) which is gravitationally lensed by a foreground spiral galaxy at z~0.886 (Wiklind and Combes, 1996, Nature 379, 139, Winn et al., 2005, ApJ 575, 103). The source is currently undergoing a record-setting gamma-ray outburst (ATEL #12541, #12601, #12685, #12705 and therein). In order to facilitate broad-band monitoring of the source, we initiated Swift monitoring of the source, which also revealed potential flux variations in the X-rays (ATEL #12696). Enhanced radio flaring (ATEL #12667) and hard X-ray activity (ATEL #12659) are also seen.

Motivated by the large source brightness, we triggered Chandra DDT observations, which were performed 2018-04-19, 06:55-11:26 UT (12.8ks exposure) and 2018-05-07, 12:25-18:41 UT (18.2ks exposure). To avoid pile up, ACIS-S was operated with the HETGS in the optical path.

Both Chandra images clearly show the two gravitationally lensed images as well as diffuse emission which appears to coincide with the Einstein ring that is seen in the radio (Jauncey et al., 1991, Nature 351, 132).

Between the two observations, the northern image decreased in flux and the southern image increased in flux. The count rate ratio (north/south) changed from 1.39±0.09 to 1.03±0.05. These changes are likely due to source intrinsic variability combined with the 26d light travel time delay and gravitational lensing effects (Lovell et al, 1998 ApJ 508, 51).

A preliminary spectral analysis shows slight indications for changes in the absorbing column (see also Dai et al., 2008, ApJ 135, 333), as well as a hardening of the underlying power law continuum.

Further high spatial resolution monitoring of PKS 1830-211 is strongly encouraged.

The scientific results reported in this ATEL are based on observations made with the Chandra X-ray Observatory using Director's Discretionary Time. We thank the Chandra Director's office for granting the time and the CXC for the extra work necessitated by the planning and execution of the observations.