Let $k$ be a number field and denote by $H^i(k,-)$ the Galois cohomology functor $H^i(mathrm{Gal}(bar{k}/k),-)$. Let $X$ be a smooth geometrically integral curve over $k$. One can easily show that the map $H^1(k,mathrm{Pic}^0(X_{bar{k}})) rightarrow H^1(k,mathrm{Pic}(X_{bar{k}}))$ is surjective. Indeed, applying the functor $H^1(k,-)$ to the exact sequence $$0 rightarrow mathrm{Pic}^0(X_{bar{k}}) rightarrow mathrm{Pic}(X_{bar{k}}) rightarrow mathrm{NS}(X_{bar{k}}) rightarrow 0,$$ we get the exact sequence $$H^1(k,mathrm{Pic}^0(X_{bar{k}})) rightarrow H^1(k,mathrm{Pic}(X_{bar{k}})) rightarrow H^1(k,mathbb{Z}).$$ The last term is zero because $mathbb{Z}$ has trivial Galois action and so any 1-cocycle $f in H^1(k,mathbb{Z})$ is simply an element of $mathrm{Hom}(mathrm{Gal}(bar{k}/k),mathbb{Z})$. But $mathbb{Z}$ is torsion-free and thus all such maps are zero maps.

**Question 0.** Here I did not assume that $X$ is projective, but would all of the above still hold without this assumption? For example, I’ve never seen the Neron-Severi group of an affine curve discussed in any literature.

Now moving on, we have an exact sequence of Galois modules $$0 rightarrow mathrm{Pic}(X_{bar{k}})_{mathrm{tor}} rightarrow mathrm{Pic}(X_{bar{k}}) rightarrow mathrm{Pic}(X_{bar{k}})_{mathrm{free}} rightarrow 0$$ where $mathrm{Pic}(X_{bar{k}})_mathrm{tor}$ denotes the maximal torsion subgroup of $mathrm{Pic}(X_{bar{k}})$ and $mathrm{Pic}(X_{bar{k}})_mathrm{free} = mathrm{Pic}(X_{bar{k}})/mathrm{Pic}(X_{bar{k}})_mathrm{tor}$, the maximal free quotient.

**Question 1.** In this case $mathrm{Pic}(X_{bar{k}})_mathrm{free}$ certainly does not have trivial Galois action, so we cannot reduce 1-cocycles to group homomorphisms $mathrm{Gal}(bar{k}/k) rightarrow mathrm{Pic}(X_{bar{k}})_mathrm{free}$. Therefore how do we go about computing $H^1(k,mathrm{Pic}(X_{bar{k}})_mathrm{free})$?

I have thought about first studying $H^1(k,mathrm{Pic}(X_{bar{k}}))$ using a wild idea as follows:

We apply the (étale) cohomology functor $H^i(X_{bar{k}},-)$ to the Kummer sequence $$0 rightarrow mu_n rightarrow mathbb{G_m} rightarrow mathbb{G}_m rightarrow 0$$ to obtain the long cohomology sequence $$0 rightarrow mu_n(bar{k}) rightarrow bar{k}^* rightarrow bar{k}^* rightarrow H^1(X_bar{k},mu_n) rightarrow mathrm{Pic}(X_{bar{k}}) rightarrow mathrm{Pic}(X_{bar{k}}) rightarrow H^2(X_{bar{k}},mu_n) rightarrow H^2(X_{bar{k}},mathbb{G}_m)=0.$$

Then we apply $H^1(k,-)$ to $$H^1(X_bar{k},mu_n) rightarrow mathrm{Pic}(X_{bar{k}}) rightarrow mathrm{Pic}(X_{bar{k}}) rightarrow H^2(X_{bar{k}},mu_n)$$ and study the result. But I’m not sure if there are any spectral sequences we can use, or if this approach is even feasible.