What is Indra-Dahnushya? Is Indra-Dhanushya really Bow of Indra ?
The exact answer is found In Atharva-Veda - Kanda -15 –Sukta 1 - Adhyatma Prakaran Sukta – Verse -6
The Devata of this Sukta is Vrtaya. This entire Sukta Is revering deity Vratya.
स एकव्रात्यो s भवत् स धनुरादत्त तदेवेन्द्रधनु: ||6||
Sa Eka VraTayo Bhavat Sa DhanuRaDatta TaDevendraDhanuhu
He is singularly the leader of the Vratya’s group (Vratya Samuha) ,
He touched the BOW , which was called Indra-Dhanushya.
You can read the English translation of the verse - Here
Here it is said that the deity Vratya took a bow , touched it . And that is why the Bow is called Indra_Dhanushya.
This Vratya description resembles to Shiva , there are word like Nila-Lohita etc. in that sukta.Which are of Lord shiva.
Conclusion - It seems from Atharva-veda that Indra -Dhanushya is bow
of vedic deity Vratya.