Following the increase in gamma-ray, optical and near-infrared activity of BL Lacertae reported in ATel #3368, #3371, and #3375 we have initiated Swift monitoring observations of the source with 1ks-long pointings planned once every 3 days until 2011 June 04. The first observation of the series was conducted on May 23 16:55 UT and is described here.

The 0.3-10 keV X-ray spectrum of BL Lacertae observed by Swift/XRT can be described by an absorbed (N_HI = 3.6x10^21 cm^-2, Ravasio et al. 2003, 408, 479) power law model with the photon index of 1.59 +/-0.16 and a corresponding 2-10 keV (unabsorbed) flux of (1.74+/-0.27)x10^-11 erg cm^-2 s^-1. This spectrum is harder than that found in any previous Swift/XRT observation (Falcone & Stroh, the Swift Monitoring Program). The X-ray flux level is a factor of 2-3 higher with respect to the Swift/XRT and RXTE/PCA observations during the low activity state of the source in August - October 2008 (Abdo et al. 2011, ApJ, 730, 101). The current 2-10 keV flux is 1.5 times lower than the historical maximum observed for BL Lacertae in July 1997 by RXTE (Madejski et al. 1999, ApJ, 521, 145).

The source was observed at V=14.15 +/-0.01 during simultaneous Swift/UVOT observations on JD(TT)2455705.2084. The magnitude calibration in B and V bands was performed using comparison stars from Gonzalez-Perez et al. 2001, AJ, 122, 2055 (for the other UVOT bands the standard calibration by Poole et al. 2008, MNRAS, 383, 627 was applied). Recent observations reported in vsnet-alert 13337 and the American Association of Variable Star Observers (AAVSO) database (see the AAVSO lightcurve generator) show a gradual brightness increase during the last two months and the UVOT data point appears to be a continuation of that general trend. However, the current V-band brightness is about 1 mag. lower than the peak values observed in 1997-2004. This may be the ending of the fast flare reported in ATel #3371 or an indication of fast irregular variability.

We thank the Swift team for their rapid scheduling of this observation.

Results of the upcoming Swift observations will be posted on this webpage.

Swift/XRT spectral modeling:

   obsid       JD        Gamma      Flux(2-10keV, unabsorbed)
00035028063 2455705.2 1.59 +/-0.16 (1.69+/-0.14)x10^-11 erg cm^-2 s^-1
00035028064 2455707.8 1.89 +/-0.17 (1.67+/-0.23)x10^-11 erg cm^-2 s^-1
00035028065 2455710.8 1.98 +/-0.14 (1.78+/-0.22)x10^-11 erg cm^-2 s^-1
00035028066 2455713.6 2.04 +/-0.11 (1.17+/-0.10)x10^-11 erg cm^-2 s^-1
00035028067 2455714.7 1.97 +/-0.10 (1.36+/-0.11)x10^-11 erg cm^-2 s^-1
00035028068 2455717.4 1.99 +/-0.21 (1.23+/-0.20)x10^-11 erg cm^-2 s^-1
00035028070 2455726.7 1.80 +/-0.26 (1.31+/-0.26)x10^-11 erg cm^-2 s^-1
00035028071 2455730.6 2.02 +/-0.13 (1.17+/-0.12)x10^-11 erg cm^-2 s^-1
00035028072 2455738.3 2.11 +/-0.24 (8.68+/-1.55)x10^-12 erg cm^-2 s^-1
00035028073 2455742.9 2.35 +/-0.16 (6.65+/-0.73)x10^-12 erg cm^-2 s^-1
00035028074 2455743.9 2.15 +/-0.18 (7.91+/-1.09)x10^-12 erg cm^-2 s^-1
00035028075 2455744.9 2.21 +/-0.21 (7.06+/-1.10)x10^-12 erg cm^-2 s^-1
00035028076 2455745.9 2.15 +/-0.15 (9.28+/-1.00)x10^-12 erg cm^-2 s^-1
00035028077 2455746.9 2.17 +/-0.17 (9.78+/-1.27)x10^-12 erg cm^-2 s^-1
00035028078 2455747.9 2.07 +/-0.16 (9.44+/-1.24)x10^-12 erg cm^-2 s^-1

Swift/UVOT lightcurve:

   JD(TT)   band  mag. err. 
2455705.20851  V 14.15 0.03
2455707.83079  V 14.10 0.03
2455710.84028  V 13.53 0.02
2455713.64631  V 13.66 0.02
2455714.65479  V 14.11 0.03
2455714.71823  V 14.12 0.02
2455714.78596  V 14.20 0.02
2455717.39593  V 14.37 0.03
2455726.62861  V 14.83 0.04
2455727.16343  V 14.57 0.04
2455730.50586  V 14.57 0.03
2455730.57329  V 14.56 0.03
2455730.70360  V 14.66 0.04
2455738.32865  V 14.00 0.02
2455742.86146  V 14.21 0.03
2455742.92758  V 14.36 0.03
2455743.94060  V 14.54 0.01
2455744.94329  V 14.78 0.03
2455745.87903  V 14.17 0.03
2455745.94676  V 14.25 0.02
2455746.88060  V 14.20 0.02
2455746.94192  V 13.96 0.03
2455747.88359  V 14.09 0.02
2455747.94428  V 14.10 0.03
2455754.05344  V 13.82 0.02
2455754.11949  V 13.87 0.02

2455705.20376  B 15.14 0.02
2455707.82564  B 15.06 0.02
2455710.83517  B 14.47 0.02
2455713.63816  B 14.53 0.01
2455714.65167  B 14.97 0.03
2455714.71047  B 15.04 0.02
2455714.77935  B 15.14 0.02
2455717.39084  B 15.23 0.02
2455726.62578  B 15.71 0.04
2455727.16095  B 15.46 0.04
2455730.50147  B 15.48 0.03
2455730.56916  B 15.60 0.03
2455730.70095  B 15.59 0.04
2455738.32309  B 14.91 0.02
2455742.85671  B 15.11 0.02
2455742.92284  B 15.21 0.02
2455743.93030  B 15.50 0.02
2455744.93524  B 15.73 0.02
2455745.87620  B 15.14 0.03
2455745.93870  B 15.12 0.02
2455746.87429  B 15.13 0.02
2455746.93909  B 14.86 0.03
2455747.87715  B 14.96 0.02
2455747.94134  B 14.99 0.03
2455754.04914  B 14.74 0.02
2455754.11434  B 14.72 0.02

2455705.20202  U 14.72 0.03 
2455707.82382  U 14.53 0.02 
2455710.83337  U 13.88 0.02 
2455713.63574  U 13.95 0.01 
2455714.65025  U 14.60 0.03 
2455714.70814  U 14.60 0.02 
2455714.77724  U 14.60 0.02 
2455717.38903  U 14.74 0.03 
2455726.62442  U 15.28 0.04 
2455727.15966  U 14.98 0.04 
2455730.49981  U 15.04 0.03 
2455730.56755  U 15.07 0.03 
2455730.69964  U 15.16 0.04 
2455738.32119  U 14.43 0.02 
2455742.85497  U 14.55 0.02 
2455742.92111  U 14.72 0.03 
2455743.92766  U 15.02 0.02 
2455744.93285  U 15.28 0.03 
2455745.87484  U 14.60 0.03 
2455745.93631  U 14.61 0.02 
2455746.87225  U 14.50 0.02 
2455746.93773  U 14.26 0.03 
2455747.87509  U 14.44 0.02 
2455747.93995  U 14.41 0.03 
2455754.04748  U 14.11 0.02 
2455754.11252  U 14.23 0.02 

2455705.20059 W1 15.37 0.04 
2455707.82226 W1 15.15 0.03 
2455710.83182 W1 14.44 0.02 
2455713.63327 W1 14.57 0.02
2455714.64931 W1 15.08 0.04 
2455714.70580 W1 15.19 0.03 
2455714.77525 W1 15.32 0.03 
2455717.38747 W1 15.41 0.03 
2455726.62356 W1 15.88 0.06 
2455727.15891 W1 15.69 0.06 
2455730.49847 W1 15.72 0.04 
2455730.56630 W1 15.75 0.04 
2455738.32028 W1 15.19 0.05
2455738.39776 W1 15.24 0.09
2455742.85354 W1 15.10 0.03 
2455742.91967 W1 15.23 0.03 
2455743.92482 W1 15.75 0.03 
2455743.99078 W1 15.70 0.04 
2455744.93042 W1 15.91 0.03 
2455745.93388 W1 15.31 0.03 
2455746.87111 W1 15.25 0.05
2455746.93764 W1 14.93 0.05
2455747.87314 W1 15.14 0.03 
2455747.93905 W1 14.98 0.04 
2455754.04619 W1 14.78 0.03 
2455754.11097 W1 14.85 0.03 

2455705.21001 M2 16.14 0.05 
2455707.83192 M2 15.73 0.04 
2455710.84153 M2 15.03 0.03 
2455713.64887 M2 15.21 0.03
2455714.65605 M2 15.91 0.08
2455714.72057 M2 15.92 0.04 
2455714.78770 M2 15.97 0.04 
2455717.39712 M2 16.15 0.05 
2455726.62975 M2 16.79 0.10
2455727.16449 M2 16.47 0.09
2455730.50691 M2 16.43 0.06 
2455730.57429 M2 16.30 0.06 
2455738.33094 M2 15.81 0.06
2455742.86640 M2 15.84 0.04
2455742.92947 M2 16.09 0.07
2455743.94348 M2 16.36 0.09
2455744.94638 M2 16.70 0.07
2455745.87995 M2 15.89 0.09
2455745.94985 M2 16.07 0.06
2455746.88300 M2 15.91 0.06
2455746.94299 M2 15.53 0.07
2455747.88624 M2 15.92 0.06
2455747.94557 M2 15.91 0.07
2455754.05508 M2 15.53 0.06
2455754.12149 M2 15.57 0.06

2455705.20538 W2 16.19 0.04 
2455707.82746 W2 15.88 0.03 
2455710.83697 W2 15.20 0.02 
2455713.64148 W2 15.33 0.02
2455714.65248 W2 15.92 0.04 
2455714.71360 W2 16.09 0.03 
2455714.78190 W2 16.13 0.03 
2455717.39267 W2 16.12 0.04 
2455726.62644 W2 16.68 0.06 
2455727.16143 W2 16.70 0.07 
2455730.57047 W2 16.59 0.05 
2455730.70152 W2 16.54 0.06 
2455738.32512 W2 15.83 0.03
2455742.85833 W2 15.82 0.03 
2455742.92446 W2 16.00 0.03 
2455743.93425 W2 16.43 0.03 
2455744.93851 W2 16.73 0.04 
2455745.87686 W2 15.98 0.04 
2455745.94197 W2 16.06 0.03 
2455746.87670 W2 15.86 0.03 
2455746.93975 W2 15.56 0.04 
2455747.87962 W2 15.91 0.03 
2455747.94206 W2 15.94 0.04 
2455754.05053 W2 15.50 0.03 
2455754.11616 W2 15.56 0.03 

External links
Some external links to pages with (more or less) recent observations of BL Lacertae (B2200+420, J2202+4216) across the electromagnetic spectrum:
Fermi/LAT monitored sources page
Swift/BAT Monitoring of BL Lacertae
Swift/XRT Monitoring of BL Lacertae
Steward Observatory observations of Fermi blazars
AAVSO database
SMA submillimeter calibrator monitoring page
F-GAMMA radio monitoring of AGN
UMRAO radio monitoring of AGN
Boston University 43 GHz VLBA monitoring of gamma-ray blazars
MOJAVE 15 GHz VLBA monitoring of AGN
(If your favorite blazar monitoring program which observes BL Lac is not in the list and you would like to include it, just drop me a line)