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Microquasars are potential $\gamma$-ray emitters. Indications of transientepisodes of $\gamma$-ray emission were recently reported in at least twosystems: Cyg X-1 and Cyg X-3. The identification of additional$\gamma$-ray-emitting microquasars is required to better understand how$\gamma$-ray emission can be produced in these systems. Theoretical models havepredicted very high-energy (VHE) $\gamma$-ray emission from microquasars duringperiods of transient outburst. Observations reported herein were undertakenwith the objective of observing a broadband flaring event in the $\gamma$-rayand X-ray bands. Contemporaneous observations of three microquasars, GRS1915+105, Circinus X-1, and V4641 Sgr, were obtained using the High EnergySpectroscopic System (H.E.S.S.) telescope array and the Rossi X-ray TimingExplorer (RXTE) satellite. X-ray analyses for each microquasar were performedand VHE $\gamma$-ray upper limits from contemporaneous H.E.S.S. observationswere derived. No significant $\gamma$-ray signal has been detected in any ofthe three systems. The integral $\gamma$-ray photon flux at the observationalepochs is constrained to be $I(> 560 {\rm\ GeV}) < 7.3\times10^{-13}$ cm$^{-2}$s$^{-1}$, $I(> 560 {\rm\ GeV}) < 1.2\times10^{-12}$ cm$^{-2}$ s$^{-1}$, and$I(> 240 {\rm\ GeV}) < 4.5\times10^{-12}$ cm$^{-2}$ s$^{-1}$ for GRS 1915+105,Circinus X-1, and V4641 Sgr, respectively. The $\gamma$-ray upper limitsobtained using H.E.S.S. are examined in the context of previous Cherenkovtelescope observations of microquasars. The effect of intrinsic absorption ismodelled for each target and found to have negligible impact on the flux ofescaping $\gamma$-rays. When combined with the X-ray behaviour observed usingRXTE, the derived results indicate that if detectable VHE $\gamma$-ray emissionfrom microquasars is commonplace, then it is likely to be highly transient.

astronomy and astrophysics

A search for very high-energy flares from the microquasars GRS 1915+105,  Circinus X-1, and V4641 Sgr using contemporaneous H.E.S.S. and RXTE  observations