Rafecas, whose studies have earned him awards from three Argentine Jewish groups, takes over after revelations that Nisman had wanted President Cristina Fernandez de Kirchner arrested for allegedly trying to shield Iranian bombing suspects.
Kirchner, who's due to complete her trade trip to China on Thursday, raised eyebrows on Wednesday with a controversial tweet in which she mimicked Chinese accent stereotypes.
Many recipients accused her of making a racist tweet.
She later apologized, saying that she currently faced "ridiculousness" that could only be "digested with humor."
Draft warrant found
While arriving in China on Tuesday, the Argentinean prosecutor looking into Nisman's mysterious death, Viviana Fein, admitted that a draft warrant for Kirchner's arrest, dated June 2014, had been found in Nisman's apartment.
Nisman, 51, was found dead in his apartment bathroom in Buenos Aires on January 18, hours before he was due to brief Argentine lawmakers on the 1994 bombing that left 85 people dead and 300 wounded.
Four days before that briefing he had filed a 300-page report accusing Iran of ordering the attack via the Lebanon-based group Hezbollah and claiming that Kirchner and other senior Argentine officials had shielded Iranian suspects in exchange for oil.
Fein told a press conference on Wednesday that she had cancelled a planned vacation from mid-February to proceed with her probe uninterrupted.
Kirchner, who has been president since 2007, has vehemently denied accusations in the widening affair.
Two Argentine nuclear plants planned
In Beijing on Wednesday, Argentine and Chinese officials signed 15 trade agreements.
Kirchner said two nuclear power plants would be built in Argentina with transfer of technology from China.
China is also helping Argentina build infrastructure such as dams and railways.
Beijing recently helped Buenos Aires stabilize its peso with a currency swap as Argentina struggles to shore up its foreign reserves.
China has pledged multibillion investments in Latin America over the next five years as part of its drive to boost its influence in the region.
ipj/bk (AP, AFP, dpa)