MAGIC and Swift detection of renewed activity from the blazar PG1553+113 during a MWL campaign

ATel #3977; Juan Cortina (IFAE Barcelona) on behalf of the MAGIC Collaboration
on 19 Mar 2012; 18:00 UT
Credential Certification: Juan Cortina (cortina@ifae.es)

Tweet

The MAGIC telescopes have observed an increased VHE (E>~100 GeV) gamma-ray flux from the blazar PG 1553+113 in the course of a multi-wavelength campaign started on 2012 February 23.

PG 1553+113 (RA: 15h 55m 43.0s Dec: +11d 11m 24s) is a TeV BL-Lac object likely located at redshift 0.4-0.5 (Danforth et al. 2010).

Between February 23 and March 5 an extensive multi-wavelength campaign coordinated by the MAGIC Collaboration was performed, involving besides MAGIC, radio (Metsahovi), near-infrared (REM), optical (KVA), UV (Swift/UVOT), and X-ray (Swift/XRT) observations.

Preliminary analysis of MAGIC data shows a firm detection with significance around 15 sigma in 4 hours of good quality data. According to this analysis the estimated flux is above 15% Crab in the VHE range, one of the highest fluxes ever detected from PG 1553+113. The analysis of MAGIC data is still in progress.

Preliminary analysis of X-ray data taken with Swift/XRT reports a 2-10 keV flux rising from (16.8+/-1.0)E-12 erg/cm2/s to (23.0+/-1.3)E-12 erg/cm2/s between February 16 to March 2. The XRT spectrum obtained on single daily snapshots is fairly fitted with an absorbed power law with an absorbing column density fixed to the Galactic value of 3.65E20 cm-2 (Kalberla et al. 2005), yielding a photon index around 2.3+/-0.1, relatively stable within errors.

Swift/UVOT observed with all the optical and UV filters, providing the following preliminary averaged magnitudes, not corrected for absorption, and fluxes, corrected for Galactic extinctions due to reddening of E(B-V)=0.052 mag in the direction of the source (Schlegel et al 1998).

Filter	Obs.Mag	Flux (erg/cm2/s)
V	14.46+/-0.08	(3.90+/-0.27)E-11
B	14.75+/-0.05	(4.35+/-0.19)E-11
U	13.84+/-0.04	(4.51+/-0.18)E-11
W1	13.84+/-0.06	(4.12+/-0.22)E-11
M2	13.79+/-0.06	(4.92+/-0.28)E-11
W2	13.91+/-0.05	(4.17+/-0.18)E-11

The UV flux did not show strong variability during the campaign. Both X-ray and UV fluxes are a factor of three lower than the highest ever observed during the outburst in October 2005 (Tramacere et al. 2007).

In the optical band the flux has been increasing throughout the campaign with flux up to 12.2+/-0.2 mJy, but is still well below the historical maximum from Spring 2008 (~18mJy) http://users.utu.fi/kani/1m/PG_1553+113.html

Observations in the J,H and K bands with the REM telescope show a clear brightening of about 0.6 magnitude in K band on March, 1 (11.53+/-0.12 mag) with respect to February, 16 (12.19+/-0.16 mag).

Preliminary analysis of the Metsahovi 37 GHz observations indicates a possible detection on March 8. At other times during the campaign the source was not detected.

The reported activity motivated the extension of the multiwavelength campaign until end of March. Further MWL observations of PG 1553+113 are strongly encouraged.

For this MWL campaign the MAGIC contact persons are: E. Prandini (prandini@pd.infn.it), J. Cortina (cortina@ifae.es); for Swift observations: A. Stamerra (antonio.stamerra@pi.infn.it); for KVA observations: E. Lindfors (elilin@utu.fi), for REM observations: S.Covino (stefano.covino@brera.inaf.it).

We warmly thank N. Gehrels, the Science Planners, and the Observatory Duty Scientists for having scheduled Swift observations. We thank F. D'Ammando and A. Sandrinelli for their support in the analysis and coordination of REM telescope observations, and A. Lahteenmaki for the preliminary analysis of Metsahovi data.