Context : Cassiopeia A is one of the best-studied supernova remnants .
Its bright radio and X-ray emission is due to shocked ejecta .
Cas A is rather unique in that the unshocked ejecta can also be studied : through emission in the infrared , the radio-active decay of ^ { 44 } Ti , and the low-frequency free-free absorption caused by cold ionised gas , which is the topic of this paper .

Aims : Free-free absorption processes are affected by the mass , geometry , temperature , and ionisation conditions in the absorbing gas .
Observations at the lowest radio frequencies can constrain a combination of these properties .

Methods : We used Low Frequency Array ( LOFAR ) Low Band Antenna observations at 30–77 MHz and Very Large Array ( VLA ) L-band observations at 1–2 GHz to fit for internal absorption as parametrised by the emission measure .
We simultaneously fit multiple UV-matched images with a common resolution of 17″ ( this corresponds to 0.25 pc for a source at the distance of Cas A ) .
The ample frequency coverage allows us separate the relative contributions from the absorbing gas , the unabsorbed front of the shell , and the absorbed back of the shell to the emission spectrum .
We explored the effects that a temperature lower than the \sim 100–500 K proposed from infrared observations and a high degree of clumping can have on the derived physical properties of the unshocked material , such as its mass and density .
We also compiled integrated radio flux density measurements , fit for the absorption processes that occur in the radio band , and considered their effect on the secular decline of the source .

Results : We find a mass in the unshocked ejecta of M = 2.95 \pm { 0.48 } M _ { \odot } for an assumed gas temperature of T = 100 K. This estimate is reduced for colder gas temperatures and , most significantly , if the ejecta are clumped .
We measure the reverse shock to have a radius of 114 ″ \pm 6″and be centred at 23:23:26 , +58:48:54 ( J2000 ) .
We also find that a decrease in the amount of mass in the unshocked ejecta ( as more and more material meets the reverse shock and heats up ) can not account for the observed low-frequency behaviour of the secular decline rate .

Conclusions : To reconcile our low-frequency absorption measurements with models that reproduce much of the observed behaviour in Cas A and predict little mass in the unshocked ejecta , the ejecta need to be very clumped or the temperature in the cold gas needs to be low ( \sim 10 K ) .
Both of these options are plausible and can together contribute to the high absorption value that we find .