The short GRB 070707 afterglow and its very faint host galaxy^*

¹ INAF - Osservatorio Astronomico di Roma, via Frascati 33, Monteporzio-Catone (RM), 00040, Italy e-mail: piranomonte@oa-roma.inaf.it
² INAF - Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
³ Department of Physics and Astronomy, University of Leicester, Leicester, UK
⁴ Università degli Studi di Milano-Bicocca, Dipartimento di Fisica, piazza delle Scienze 3, 20126 Milano, Italy
⁵ INAF - Osservatorio Astronomico di Capodimonte, salita Moiariello 16, 80131 Napoli, Italy
⁶ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
⁷ International Center for Relativistic Astrophysics Network, P.le della Repubblica, 10, 65122, Pescara, Italy
⁸ JILA, University of Colorado, Boulder, CO 80309-0440, USA
⁹ Department of Physics, North Carolina State University, Box 8202, Raleigh, NC 27695, USA
¹⁰ Dark Cosmology Centre, Niels Bohr Instute, University of Copenhagen, Juliane Maries vej 30, 2100 Købenavn Ø, Denmark
¹¹ NASA/Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
¹² Dunsink Observatory, DIAS, Dunsink lane, Dublin 15, Ireland
¹³ School of Physical Sciences and NCPST, Dublin City University, Dublin 9, Ireland

Received: 8 July 2008
Accepted: 10 September 2008

Abstract

We present the results from an ESO/VLT campaign aimed at studying the afterglow properties of the short/hard gamma ray burst GRB 070707. Observations were carried out at ten different epochs from ~0.5 to ~80 days after the event. The optical flux decayed steeply with a power-law decay index greater than 3, later levelling off at  mag; this is likely the emission level of the host galaxy, the faintest yet detected for a short GRB. Spectroscopic observations did not reveal any line features/edges that could unambiguously pinpoint the GRB redshift, but set a limit . In the range of allowed redshifts, the host has a low luminosity, comparable to that of long-duration GRBs. The existence of such faint host galaxies suggests caution when associating short GRBs with bright, offset galaxies, where the true host might just be too dim for detection. The steepness of the decay of the optical afterglow of GRB 070707 challenges external shock models for the optical afterglow of short/hard GRBs. We argue that this behaviour might result from prolonged activity of the central engine or require alternative scenarios.