6.6 Saturation

The point source saturation levels for the IRS are as follows (Figure 7.23 of the SOM):

• Short-Low Module: $\sim 5.5 \; {\rm Jy}$ at $10 \; \mu {\rm m}$ in 6 seconds.
• Long-Low Module: $\sim 10 \; {\rm Jy}$ at $25 \; \mu {\rm m}$ in 6 seconds.
• Short-High Module: $\sim 65 \; {\rm Jy}$ ( $\sim 2.6 \times 10^{-14} \; {\rm W \; m}^{-2}$ for an unresolved emission line using equation 7.7 in the SOM) at $12.5 \; \mu {\rm m}$ in 6 seconds
• Long-High Module: $\sim 70 \; {\rm Jy}$ ( $\sim 1.4 \times 10^{-14} \; {\rm W \; m}^{-2}$ for an unresolved emission line) at $25 \; \mu {\rm m}$ in 6 seconds.

These saturation values assume that the incident light falls onto 4 pixles (in a $2 \times 2$ grid). Since, for example, the SL module pixels have linear sizes of $1{\hbox{$.\!\!^{\prime\prime}$}}8$, the SL module will saturate in 6 seconds when a total of 5.5 Jy illuminates 4 pixels, corresponding to an angular region of 13 square arcseconds. Pixel sizes for the other IRS modules are: $5{\hbox{$.\!\!^{\prime\prime}$}}1$ for Long-Low (LL) and LH (corresponding to 104 square arcseconds for a 4-pixel region), and $2{\hbox{$.\!\!^{\prime\prime}$}}3$ for SN (corresponding to 21.2 square arcseconds for a 4-pixel region). For the most up-to-date information about saturation of the IRS see:

http://ssc.spitzer.caltech.edu/IRS/sat.html.

The IRAS fluxes for a given target can be used to estimate if that target will saturate the IRS. IRAS fluxes (if available) can be found, for example, by using the SIMBAD or NED (for extragalactic objects) online databases, or by searching in the Infrared Science Archive (IRSA) database, at

http://irsa.ipac.caltech.edu/.

The target for this example, 3C 273, was observed with IRAS and has flux densities of 0.55 Jy and 0.94 Jy at $12$ and $25 \; \mu {\rm m}$, respectively. For the IRS high resolution modules, these flux density values are well below the saturation limits.