Question: What date did Bagsecg die?

full_context: Document 1: Battle of Assandun: The Battle of Assandun (or Essendune) was fought between Danish and English armies on 18 October 1016. There is disagreement whether Assandun may be Ashdon near Saffron Walden in north Essex or, as long supposed, Ashingdon near Rochford in southeast Essex, England. It ended in victory for the Danes, led by Canute the Great, who triumphed over the English army led by King Edmund Ironside. The battle was the conclusion to the Danish reconquest of England.
Document 2: Bagsecg: Bagsecg (also known as Bægsecg or Bagsec) (died 8 January 871) was a Viking leader commonly referred to as a king. In 870 or 871 he led The Great Summer Army to England. His forces joined those of the Great Danish Army which had already overrun much of England. He and Halfdan Ragnarsson became the leaders of a joint invasion of the Kingdom of Wessex. He was killed at the Battle of Ashdown, fighting a West Saxon Army led by King Ethelred and his younger brother, the future Alfred the Great.
Document 3: Halfdan Ragnarsson: Halfdan Ragnarsson (Old Norse: "Hálfdan" ; or "Healfdene"; Old Irish: "Albann" ; died 877) was a Viking leader and a commander of the Great Heathen Army which invaded the Anglo-Saxon kingdoms of England, starting in 865. According to the tradition recorded in the Norse sagas he was one of the sons of Ragnar Lodbrok, and his brothers included Björn Ironside, Ivar the Boneless, Sigurd Snake-in-the-Eye and Ubba. He was the first Viking King of Northumbria and a pretender to the throne of Kingdom of Dublin. He died at the Battle of Strangford Lough in 877 trying to press his Irish claim.
Document 4: Battle of Ashdown: The Battle of Ashdown, in Berkshire (possibly the part now in Oxfordshire), took place on 8 January 871. Alfred the Great, then a prince of only 22, led the army of his brother, King Ethelred of Wessex, in a victorious battle against the invading Danes.
Document 5: Hvitserk: Hvitserk ("Hvitsärk") "White-Shirt" was one of the sons of the legendary 9th-century Norsemen Ragnar Lothbrok and his wife Aslaug. He is attested to by the Tale of Ragnar's Sons ("Ragnarssona þáttr"). He is not mentioned in any source which mentions Halfdan Ragnarsson, one of the leaders of the Great Heathen Army which invaded the Kingdom of East Anglia in 865, or vice versa which consequently led some scholars to suggest that they are the same individual with "Hvitserk" being only a nickname.
Document 6: Battle of Reading (871): The first Battle of Reading was a battle on 4 January 871 at Reading in what is now the English county of Berkshire. It was one of a series of battles, with honours to both sides, that took place following an invasion of the then kingdom of Wessex by an army of Danes led by Bagsecg and Halfdan Ragnarsson in an attempt to conquer Wessex. Both battle and campaign are described in the "Anglo-Saxon Chronicle", and this account provides the earliest known written record of the existence of the town of Reading.
Document 7: Battle of Basing: The Battle of Basing was a battle on 22 January 871 at Old Basing in what is now the English county of Hampshire. It was one of a series of battles that took place following an invasion of the then kingdom of Wessex by an army of Danes. Both battle and campaign are described in the "Anglo-Saxon Chronicle".
Document 8: Ivar the Boneless: Ivar the Boneless (Old Norse: "Ívarr hinn Beinlausi" ; ) was a Viking leader and a commander who invaded what is now England. According to the "Tale of Ragnar Lodbrok", he was the son of Ragnar Lodbrok and Aslaug. His brothers included Björn Ironside, Halfdan Ragnarsson, Hvitserk, Sigurd Snake-in-the-Eye and Ubba.
Document 9: Æthelwulf of Berkshire: Æthelwulf of Berkshire (before 825 – 4 January, 871) was a Saxon ealdorman. In 860 he and other men of Berkshire fought off a band of pirates near Winchester, Hampshire. Later he mustered a force of 1400 men against an army of Danes, won the 31 December 870 Battle of Englefield on behalf of the then kingdom of Wessex. He received a land grant in 843/44 from Brihtwulf, king of Mercia; and lost his life at the Battle of Reading.
Document 10: Battle of Marton: The Battle of Marton or Meretum took place on approximately 22 March 871 at a place recorded as Marton, perhaps in Wiltshire or Dorset. According to the Anglo-Saxon Chronicle, the battle took place "about two months" after a prior loss to the Danes at the Battle of Basing, in which both sides suffered heavy losses. King Æthelred of Wessex and Prince Alfred fought against the Danes, who were possibly under the leadership of King Halfdan Ragnarsson, who had commanded many of the earlier battles that year. The Chronicle tells us that the forces were split in two and that the Danes were initially put to flight, but at the end of the day and after "much slaughter" the Danes remained masters of the field. Among the fallen was Bishop Heahmund.
Document 11: Great Heathen Army: The Great Viking Army or Great Danish Army, known by the Anglo-Saxons as the Great Heathen Army (OE: "mycel hæþen here"), was a coalition of Norse warriors, originating from Denmark (and likely also from Sweden and Norway) who came together under a unified command to invade the four Anglo-Saxon kingdoms that constituted England in AD 865.
Document 12: Battle of Ashdown: The Battle of Ashdown, in Berkshire (possibly the part now in Oxfordshire), took place on 8 January 871. Alfred the Great, then a prince of only 22, led the army of his brother, King Ethelred of Wessex, in a victorious battle against the invading Danes.
Document 13: Ubba: Ubba, also known as Hubba, Ubbe, and Ubbi, was a mid-ninth-century Viking chieftain and one of the commanders of the Great Army, a coalition of Norse warriors that in AD 865 invaded the Anglo-Saxon kingdoms of Northumbria, Mercia, East Anglia and Wessex.
Document 14: Battle of Englefield: The Battle of Englefield was a battle on 31 December 870 at Englefield, near Reading in what is now the English county of Berkshire. It was one of a series of battles, with honours to both sides, that took place following an invasion of the then kingdom of Wessex by an army of Danes, during which the Danes had established a camp at Reading. Both battle and campaign are described in the "Anglo-Saxon Chronicle".
Document 15: The Strongbow Saga: The Strongbow Saga is a historical fiction series by Judson Roberts. The Saga tells the story of Halfdan Hroriksson, a young Viking trying to avenge his brother's murder. The series weaves the story of the fictional Halfdan into the lives of actual historical figures of the period, including famous Viking leaders Ragnar Lodbrok, Ivar the Boneless, Bjorn Ironside, and Hastein, as well as Frankish leaders King Charles the Bald and Count Robert the Strong. Actual historical events form a backdrop for parts of the story, including a Danish attack up the Seine River in the year 845 which was reported in several contemporary Frankish sources, including the Annals of St. Bertin.
ground_truth: 8 January 871