By Dave Warner

PHILADELPHIA (Reuters) - A Pennsylvania mother was charged on Friday with the murder of her 11-month-old son who authorities say died of acute heroin intoxication.

Christine Rivero, 29, is accused of third degree murder, criminal homicide, involuntary manslaughter, aggravated assault and endangering the welfare of children. She was being held without bail.

Investigators said Rivero, of Collingdale, southwest of Philadelphia, told them she left her two children, Niccolo, 11 months, and Luciano, 5, in a bathtub on July 14 while she went outside and snorted a bag of heroin.

According to the investigators' affidavit, she said Luciano emerged a few minutes later to say Niccolo had fallen and hit his head. The next morning, Niccolo was unresponsive and she called police, according to the affidavit.

The baby was taken to a hospital where he was declared dead. A medical examiner ruled that Niccolo died from acute heroin intoxication. Authorities said they did not know how the heroin got into the baby's system.

The baby's father, Victor Varner, 48, told police that Niccolo was born addicted to heroin, the affidavit said.

The baby had been hospitalized for two or three months after he was born and was under the supervision of the local Children and Youth Services, the affidavit said.

(Editing by Ellen Wulfhorst and Claudia Parsons)