On the nature of the microquasar V4641 Sagittarii

We present photometric and spectroscopic optical and near-infrared (NIR) observations¹ taken during the outburst of the recently flaring source V4641 Sgr = SAX J1819.3-2525 (in't Zand et al. 2000), on September 1999. This source was independently detected as the RXTE source XTE J1819-254 (Markwardt et al. 1999), and afterwards identified with the variable object V4641 Sgr (Kato et al. 1999). It underwent a bright optical outburst on 1999 Sept. 15.7 UT, going from magnitude 14 to 8.8 in the V-band (Stubbings 1999), and reaching 12.2 Crab in the X-rays (Smith et al. 1999) and K s ≃ 13. This outburst was therefore bright, but very brief, with an e-fold decay time of 0.6 days. A radio source was resolved, making of V4641 Sgr a new microquasar (Hjellming et al. 2000). We discuss the nature of this system, showing that our observations suggest a distance farther than previously derived from the radio observations (Hjellming et al. 2000). The distance of the system would be between 3 and 8 kpc, the companion star being a B3-A2 main sequence star. Another possibility is that the companion star is crossing the Hertzsprung gap (type B3-A2 IV), and in this case the distance cited above would be the minimum distance of the system. The system is therefore an Intermediate or High Mass X-ray Binary System (IMXB or HMXB). The inconsistency regarding the distance between the radio and optical/NIR observations could be explained by the detection of an interaction between matter ejected before the X-ray outburst and the surrounding medium of the source. If this is confirmed, this source could be added to the short list of microquasars where such an interaction has been detected.