[vsnet-grb-info 5509] XRF 080109 in NGC2770: Detectable Superluminal Motion

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  7174
SUBJECT: XRF 080109 in NGC2770: Detectable Superluminal Motion
DATE:    08/01/12 17:14:24 GMT
FROM:    Arnon Dar at Technion-Israel Inst. of Tech  <arnon at physics.technion.ac.il>

Shlomo Dado (Technion), Arnon Dar (Technion and CERN) and A. De Rujula (CERN) 
Report:

The source of XRF 080109 in NGC2770 (GCN 7159) may, for months or even 
years, be moving at a constant superluminal angular velocity of the order 
of ~ 10 mas / month , should XRF 080109 be an ordinary long GRB viewed 
off-axis (DDD2004). The Doppler factor, delta, the Lorentz factor, gamma, 
and the viewing angle, theta, for XRFs and GRBs are related by,
delta ~ 2 gamma / [ 1 + (gamma theta)^2 ].

For the average GRB gamma*theta ~ 1 and the isotropic energy is,
E_{iso} ~ 8 E52 erg. For GRBs and XRFs, E_{iso} scales with delta^3 (DD2004).
For XRF 080109, E_{iso} ~ E45 erg (GCN 7159) , resulting in delta ~ 2, and 
gamma*theta ~ 30, for a typical gamma ~ 500 to 1000. The angular velocity 
cited above is v/d, with v ~ gamma*delta*theta*c, and d = 27 Mpc.

Interpreted in the same way, GRB 980425 had delta ~ 20 and gamma*theta ~ 10.
Its X-ray plateau (which we attributed to the GRB's jetted source and 
NOT to SN1998bw, DDD2002,3,7) was flat for a few months, implying a 
constant gamma, delta and v for that period. For XRF 080109 this 
approximate result may be valid for ten times a longer period, the ratio 
of Doppler factors. For single-pulse events, this ratio is also an 
estimate of the ratio of pulse durations, ~ 50 s in the 1.5-10 keV X-ray 
band for GRB 980425, and ~ 500 s for XRF 080109. If XRF 080109 and
GRB  980425 are identical, but for distance and viewing angle, the prompt 
peak X-ray luminosity of GRB 980425 (Pian et al. 2000) can also be scaled
as in the above to predict 5 E42 erg/s in the Swift XRT band, in agreement
with the observed ~ 4 E52 erg/s (GCN 7159).

The energy flux of the X-ray plateau of XRF 080901 is expected to be 
suppressed relative to that of GRB 980425 by the ratio (10) of Doppler 
factors to the power 4.1 (DDD2007) and may be hard to detect, unless it is 
compensated by a large ISM density around SN2007D in NGC2770.

If the radio emission of the GRB980425/SN1998bw pair was dominated by the jet 
which produced its off-axis GRB (DDD2003), scaling the observed peak flux 
density at t ~ 10 days of GRB 980425 (F_p = 50 mJy at 3.5 cm, Kulkarni et al. 
1998) yields, for XRF 080109a in a similar environment, a rough estimate of a 
peak date at ~100 days and a 35-cm peak flux of ~1.4 mJy, which may be 
observable at this wavelenght and nearby ones. Radio observations could even 
resolve two separated radio sources, SN2008D and the moving source of XRF 
090109. It is also not excluded that there is more than one moving source.

DDD2002: A&A, 388, 1079 (2002)
DDD2003: A&A, 401, 243 (2003)
DDD2004: A&A, 422, 381 (2004)
DDD2007: arXiv0712.1527 (2007)
Kulkarni, S. R., et al. 1998, astro-ph/9807001
Pian, E., et al. 2000, ApJ, 536, 778