Central Bureau for Astronomical Telegrams
IAUC 6169: PKS 2155-304; GRAVITATIONAL MICROLENSING EVENTS IN PROGRESS
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Circular No. 6169
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
Postal Address: Central Bureau for Astronomical Telegrams
Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions)
Phone 617-495-7244/7440/7444 TWX 710-320-6842 ASTROGRAM CAM
MARSDEN@CFA.HARVARD.EDU or GREEN@CFA.HARVARD.EDU (science)
PKS 2155-304
W. T. Vestrand and J. G. Stacy, University of New Hampshire;
and P. Sreekumar, Goddard Space Flight Center, NASA, report: "Our
analysis of data taken during 1994 Nov. 15-29 by the Energetic
Gamma Ray Experiment Telescope (EGRET) aboard the Compton Gamma Ray
Observatory has resulted in the first detection of high-energy
gamma-ray emission from the prototypical, x-ray-selected BL Lac
object PKS 2155-304. The photon power-law index derived for the
30-MeV to 10-GeV emission has an index of -1.69 +/- 0.26 and an
integral flux above 100 MeV of (2.7 +/- 0.7) x 10E-7 photon cmE-2.
The spectral hardness of the source and its proximity (z = 0.117)
make it an ideal candidate for measurements at TeV gamma-ray
energies. Since a detection of this object at TeV energies will
place important constraints on intergalactic infrared radiation
fields, we urge observers of TeV gamma-ray sources to search for
emission from PKS 2155-304."
GRAVITATIONAL MICROLENSING EVENTS IN PROGRESS
The MACHO collaboration (cf. IAUC 6155, plus D. Welch) reports
the probable discovery of two more gravitational microlensing
events in progress toward the galactic bulge: "The stars in question
are events 95-7, R.A. = 18h13m29s.3, Decl. = -26o13'58" (equinox
2000.0); and 95-8, 18h16m46s.0, -26o11'43". These stars were
constant in brightness during 1993, at R about 17.5 (V about 18.2)
and 16.3 (17.1), respectively, and both have brightened by 0.9 mag
as of 1995 May 3. Microlensing fits to data obtained with the Mt.
Stromlo 1.3-m reflector indicate that event 95-7 is currently near
its peak amplification, while event 95-8 should reach its peak
between May 7 and 13. Accurate photometry near the peak of event
95-8 may yield valuable information about the nature of the
microlensing object. The MACHO collaboration will henceforth maintain
an online catalogue of ongoing candidate microlensing events (in
lieu of announcing via IAUC) for each detected candidate event.
The MACHO collaboration is also working to establish a world-wide
network to provide continuous photometry of microlensing events to
enhance the probability of detecting departures from simple
microlensing, in particular those indicating the existence of
extrasolar planets. Further information is available via anonymous ftp
from darkstar.astro.washington.edu in the macho/Alert directory,
and at the WWW site http://darkstar.astro.washington.edu/. Questions
or comments should be directed to macho@astro.washington.edu."
1995 May 5 (6169) Daniel W. E. Green
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