Time lag in transient cosmic accreting sources

¹ Physics Department, Sapienza University of Rome, Piazzale Aldo Moro 5, 00185 Rome, Italy
² National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), Kashirskoe Shosse 31, 115409 Moscow, Russia
³ Space Research Institute of Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia
e-mail: gkogan@iki.rssi.ru
⁴ INAF–Istituto di Astrofisica e Planetologia Spaziali, via del Fosso del Cavaliere, 100, 00133 Roma, Italy
e-mail: franco.giovannelli@iaps.inaf.it

Received: 28 April 2016
Accepted: 18 November 2016

Abstract

Context. We develop models for time lag between the maxima of the source brightness in different wavelengths during a transient flash of luminosity that is connected with a short-period increase of the mass flux onto the central compact object.

Aims. We derive a simple formula for finding the time delay among events in different wavelengths which is valid in general for all disk-accreting cosmic sources. We quantitatively also discuss a model for time-lag formation in active galactic nuclei (AGNs).

Methods. In close binaries with accretion disks, the time lag is connected with effects of viscosity that define a radial motion of matter in the accretion disk. In AGN flashes, the infalling matter has a low angular momentum, and the time lag is defined by the free-fall time to the gravitating center.

Results. We show the validity of these models by means of several examples of galactic and extragalactic accreting sources.