First Trust Advisors, the Wheaton, Il-based money manager known for its niche strategies, has launched its fourth actively managed ETF. The First Trust Senior Loan ETF (FTSL) is expected to compete with the recently launched SPDR Blackstone/GSO Senior Loan ETF (SRLN). Debuted last month, SRLN is also actively managed and has already accumulated assets of more than $160 million. FTSL …