The woman, identified as Jyoti Kamble, was detained for questioning and taken to Sangli, where police on Wednesday arrested Samir Gaikwad in connection with the case.
Suspected to be having close links with Sanatan Sansthan, Jyoti was said to be in constant touch with Gaikwad as could be gathered from call details, a senior officer of Mumbai police told PTI.
Gaikwad, an active member of Sanatan Sansthan since 1998, was arrested on Wednesday based on leads obtained through electronic surveillance by the special investigating team, set up to nab Pansare's assailants.
Inspector General of Police Sanjay Kumar Verma said the investigators are interrogating the woman on various aspects of the case.
Jyoti runs a mobile repair shop and has been a member of Sanatan Sanstha since long, said another senior police officer, who is part of the probe team.
Gaikwad was taken into custody after police went through his "innumerable" phone call records.
He was produced in a court at Kolhapur in western Maharashtra which remanded him in police custody till September 23.
Pansare, 81, was gunned down outside his home in Sagar Mala locality of Kolhapur city on February 16 and succumbed to his injuries on February 20 at Mumbai's BreachCandyHospital.
The senior CPI leader was a prominent social activist in Maharashtra and worked for the welfare of the poor and suppressed. He was also at the forefront of the toll agitation in Kolhapur.
His popular book Shivaji Kon Hota (Who was Shivaji), projected Shivaji as the people's king as against his portrayal as a Hindu bigot by certain right wing outfits and fringe groups.
The book sought to counter propaganda of the Hindutva outfits while noting that Shivaji was a secular ruler whose top generals and associates were Muslims.
Pansare's frequent speeches challenging the communal agenda of right-wing outfits angered them, his family said.
Another rationalist Narendra Dabholkar was killed in Pune in 2013. Dabholkar, who spent much of his life exposing sham rituals, miracles, black magic, and godmen, was murdered in Pune in 2013 and there has been no breakthrough in his case yet.
In the wake of the recent murder of Kannada scholar and social activist M M Kalburgi at Dharwar in Karnataka, Left parties and rationalist outfits have expressed serious concern and anguish over the delay in bringing to book culprits behind such murders.
They have also alleged that the perpetrators of such crimes are connected to rightwing and fundamentalist outfits.
According to government sources, there were specific phone calls between Gaikwad and his woman friend wherein he was bragging about having done away with Pansare.
"There are other conversations too (between the two) that were under surveillance. When alleged involvement of Sanatan Sanstha came up, he (Gaikwad stopped talking about this. We were waiting for some more days to see if he resumed talking and revealed more. But when he did not, we picked him up to find out more," they said.
However, it is yet to be established whether Gaikwad did anything on behalf of the right-wing organisation and had any role to play in the crime.
"If it is established that the organisation had some role (in the murder), action will be taken against it. We are aware that previous state government had made an attempt to ban the organisation. If a direct linkage is found, action may be contemplated," they added.
For its part, the Sanatan Sanstha had given a clean chit to Gaikwad, saying he was being falsely implicated by police in the case.
"Earlier, inquiry of many seekers of Sanatan was also conducted in case of the killing of Dr Dabholkar. Sanatan Sanstha also faced similar criticism on earlier occasions. However, no output came out of these inquiries," Sanatan Sanstha's managing trustee Virendra Marathe had said on Wednesday.
Name of the right-wing organisation had cropped up after blasts at Thane and Vashi in Maharashtra in 2008 and recovery of explosive material in Panvel.
The Mumbai ATS had chargesheeted six members of the Sanatan Sanstha and its front, the Hindu Janjagriti Samiti, in connection with the blasts.
In 2009, a low-intensity bomb went off in Goa for which members of the right-wing organisation were chargesheeted. The blast took place at Madgaon in Goa on October 16, 2009, on the eve of Diwali.
Two activists of the Sanstha had succumbed to injuries when the improvised explosive devices they were carrying in a scooter -- also owned by a Sanstha member -- exploded prematurely.