Let $L(s,\chi)$ be the $L$-function of a non-trivial Hecke character of a general number field $K$, so that $L(s,\chi)$ which has no pole or zero at $s=1$.


I am looking for a reference for upper bounds for $\alpha$ in the convexity estimate $$L(s,\chi)\ll\_{\epsilon,K,\chi}|\Im(s)|^{\alpha+\epsilon}, \qquad\frac{1}{2}<\Re(s)\leq 1,$$ for all $\epsilon>0$. I care only about the dependence on $s$ but not the dependence on other parameters (e.g.the conductor). I am aware that equation (5.20) of Iwaniec-Kowalski gives $$\alpha=[K:\mathbb{Q}]\frac{1-\Re(s)}{2},$$ but I was wondering if there is a better bound.



Subconvexity bound for Hecke $L$-functions in the $s$-aspect
You have to be careful: the implied constant also depends on $\chi$ (not just on $\epsilon$), in particular it depends on the number field $K$. (I edited your post to reflect this.)


There is a better (subconvex) bound available, namely there is an absolute constant $\lambda<1/2$ such that
$$ L(\sigma+it,\chi)\ll\_{\epsilon,K} C(\chi|\cdot|^{it})^{\lambda(1-\sigma+\epsilon)},\qquad 1/2\leq\sigma\leq 1, $$
where $C(\chi|\cdot|^{it})$ is the analytic conductor of the Hecke character $\chi|\cdot|^{it}$. Here $|\cdot|$ stands for the norm of ideals or ideles, depending on how you think of Hecke characters (Grössencharacters or idele class characters). This follows from the Phragmén-Lindelöf convexity principle combined with Theorem 5.1 in [this paper](http://arxiv.org/abs/0903.3591) of Michel and Venkatesh. In particular, 
$$\alpha=\lambda[K:\mathbb{Q}](1-\Re(s))$$ 
is admissible in your bound with the same absolute constant $\lambda<1/2$.


**Added 1.** The state-of-the art seems to be in [Han Wu's thesis](http://infoscience.epfl.ch/record/186378/files/EPFL_TH5623.pdf), see Theorem 0.3.4 there. According to this result, $\lambda=(5+2\theta)/12$ is available, where $\theta=7/64$ is the current record towards the Ramanujan-Selberg conjecture on $GL(2)$ over $K$. Perhaps the Burgess-like exponent $\lambda=(3+2\theta)/8$ is also within reach, especially in the light of the other main results of the thesis (for twists of cusp forms on $GL(2)$ over $K$), which have appeared in GAFA recently. See [here](http://arxiv.org/abs/1209.5950).


**Added 2.** Han Wu recently established $\lambda=(3+2\theta)/8$ in [this preprint](http://arxiv.org/abs/1604.08551).


