Suppose that $f$ is a weight $k$ newform for $Gamma_1(N)$ with attached $p$-adic Galois representation $rho_f$. Denote by $rho_{f,p}$ the restriction of $rho_f$ to a decomposition group at $p$. When is $rho_{f,p}$ semistable (as a representation of
$mathrm{Gal}(overline{mathbf{Q}}_p/mathbf{Q}_p)$?
To make things really concrete, I'm happy to assume that $k=2$ and that the $q$-expansion of $f$ lies in $mathbf{Z}[[q]]$.
Certainly if $N$ is prime to $p$ then $rho_{f,p}$ is in fact crystalline, while
if $p$ divides $N$ exactly once then $rho_{f,p}$ is semistable (just thinking about the Shimura construction in weight 2 here, and the corresponding reduction properties of $X_1(N)$
over $mathbf{Q}$ at $p$). For $N$ divisible by higher powers of $p$, we know that these representations are de Rham, hence potentially semistable. Can we say more? For example,
are there conditions on "numerical data" attached to $f$ (e.g. slope, $p$-adic valuation of $N$, etc.) which guarantee semistability or crystallinity over a specific
extension? Can we bound the degree and ramification of
the minimal extension over which $rho_{f,p}$ becomes semistable in terms of numerical
data attached to $f$? Can it happen that $N$ is highly divisible by $p$ and yet $rho_{f,p}$ is semistable over $mathbf{Q}_p$?
I feel like there is probably a local-Langlands way of thinking about/ rephrasing this question, which may be of use...
As a possible example of the sort of thing I have in mind: if $N$ is divisible by $p$ and $f$ is ordinary at $p$ then $rho_{f,p}$ becomes semistable over an abelian extension of
$mathbf{Q}p$
and even becomes crystalline over such an extension provided that the Hecke eigenvalues
of $f$ for the action of $mu_{p-1}subseteq (mathbf{Z}/Nmathbf{Z})^{times}$ via the diamond operators
are not all 1.
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