The S & P 500 has flashed one particular type of rare, bullish signal seen by technical analysts as an indicator that a big rally could be on the way. On Thursday, the broad market index formed what Wall Street calls a “golden cross,” which happens when a 50-day moving average crosses through and above the 200-day moving average. Moving averages are simply the average of the last 50, or 200, closing prices. Traders and analysts use the golden cross as an indicator that a market trend is about to turn more positive. The opposite, the so-called death cross, would indicate a bearish change. There have now been 37 golden crosses on the S & P 500 since 1950, according to Carson Group chief market strategist Ryan Detrick. One year later, the S & P has been higher 78% of the time, by a median of 12.7%. For some analysts, it’s only considered a golden cross if the 200-day is sloping upward. Thursday’s was the 20th time going back to 1928 that a golden cross has occurred with a declining 200-day moving average, according to Bank of America. Under this scenario, the S & P rises 74% of the time 260 days later, with average and median returns of 14.1% and 14.9%, respectively. JPMorgan hesitated to tell clients to chase the momentum, however, and is sticking to an outlook that favors a bearish reversal and a retest of the fourth-quarter stock market lows. “We strongly believe the low-frequency momentum-based buy signals that have or are close to triggering are likely to whipsaw,” said Jason Hunter, a technical strategist at the firm. “It is also important to note that near 30% of the signals since WWII have not resulted in a profit if held until the death cross (reverse of the moving average signal) and 40% of signals led to drawdowns within the first month after the bullish trigger.” “Despite the stronger than expected price action, we continue to think 4100-4200 will effectively cap the index in the early months of the year and risk-reward at these levels are dramatically skewed to the downside,” Hunter added. But golden crosses tend to shine when associated with recessions, according to Bank of America chart analyst Stephen Suttmeier. Going back to the late 1920s, the S & P has generated 14 golden crosses associated with 13 out of the last 15 recessions acknowledged by National Bureau of Economic Research, the unofficial arbiter of recessions . “Recession golden crosses indicate an improving and forward looking equity market during periods of economic stress,” Suttmeier said. “S & P 500 returns are strong after these signals across all time periods with the best returns 195 days after the signal.” After a 195-day period, the S & P tends to have advanced 93% of the time, with an average and median return of 22% when the golden cross takes place in a recession, according to Suttmeier. After a 260-day period, it historically has also been higher 93% of the time, with an average return of 21.9% and a median return of 18.4%. — CNBC’s Michael Bloom contributed reporting.