Swift confirms BL Lacertae in an historic high state in the Optical/UV

ATel #5598; Dirk Grupe (Swift MOC, PSU), Ann Wehrle (SSI), and Svetlana Jorstad (BU)
on 26 Nov 2013; 03:50 UT
Credential Certification: Dirk Grupe (dgrupe007@gmail.com)

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We report on an unprecedented high flux in the Optical/UV in BL Lacertae (22h 02m 43a, +42d 16m 40s) seen by Swift on 2013 November 22, confirming findings by Larionov et al. (ATEL #5597) in the R band. We are currently monitoring BL Lac with Swift twice a week. Swift UVOT observed BL Lacertae in all 6 filters. Over the last half year BL Lac has become increasingly brighter in all 6 UVOT filters. On 2013 November 22, we found that in all 6 filters BL Lac has exceeded its earlier high state as reported in Raiteri et al (2013, MNRAS, 436, 1530), Figure 3. We now measure the following flux densities (mJy, corrected for Galactic reddening and host galaxy contribution; observed values given in parenthesize: V: 46.56+/-1.65 (17.10+/-0.05) B: 35.68+/-0.97 (9.41+/-0.25) U: 24.86+/-0.99 (5.17+/-0.21) W1: 15.03+/-0.86 (1.64+/-0.09) M2: 13.29+/-0.53 (0.80+/-0.03) W2: 9.27+/-0.44 (0.70+/-0.03) In our most recent observation on 2013 November 25 19:35 UT (MJD 56621.816336) it appears to be slightly fainter with corrected flux densities of 39.15+/-1.10, 30.11+/-0.73, 20.49+/-0.99, 12.97+/-0.80, 11.41+/-0.53, and 8.21+/-0.44 mJy in V, B, U, W1, M2, and W2, respectively. In X-rays, however, BL Lac remains rather constant at a flux level of 1.4e-11 erg/s/cm2 (observed) in the 0.3 - 10 keV band (2.3e-11 erg/s/cm2 absorption corrected). Activity in gamma ray bands has been increasing since approximately MJD 56500, 2013 July 27, as seen on the Fermi Science Center public light curves for LAT monitored sources, http://fermi.gsfc.nasa.gov/ssc/data/access/lat/msl_lc/ . We will continue monitoring BL Lac with with Swift with daily cadence for the next 10 days in order to follow this bright state. We encourage ground-based observations of BL Lacertae in this unusual high state. We thank the Swift PI Neil Gehrels, the Observation Duty Scientists and the science planners. We also thank A. Falcone and M. Stroh (Penn State) for use of their Swift monitoring program webpage at http://www.swift.psu.edu/monitoring/. At Penn State we acknowledge support from the NASA Swift program through contract NAS5-00136.