6.9.2.1 Option 1: IRS Peak-up

The IRS peak-up array images in the IR, in contrast to using optical imaging as in the PCRS peak-up option. It provides two modes of operation. A bright, IR target whose absolute coordinates are known only to within $\sim 10-15$ arcseconds, but which is well isolated from other bright IR sources, can be located by the peak-up software and placed on the IRS slit(s). Alternatively, a faint IR target whose location relative to a bright, nearby (within $30^{\prime}$) IR source is accurately known can be observed by peaking up on the bright object and performing an offset to the science target. If an IRS peak-up is selected, then the proposer must specify which of the two peak-up sub-arrays to use. The IRS ``Blue'' peak-up sub-array covers $13.3-18.7 \; \mu {\rm m}$, while the IRS ``Red'' peak-up sub-array covers $18.5-26 \; \mu {\rm m}$. Only one peak-up sub-array (Blue or Red) can be selected per AOR. A Blue peak-up requires $\sim 2$ minutes less than a Red peak-up. On the other hand, Red can be used with brighter peak-up targets than Blue without saturating. Generally, Blue is preferable if a peak-up target can be found that works with either sub-array, and Red is a second-choice backup in the event that either the peak-up target is very much fainter in the Blue waveband than in the Red or the only available peak-up target saturates in the Blue, but not in the Red. In general, the following process is recommended for identifying a suitable IRS peak-up target.

Consider the science target itself: If the science target satisfies all of the following conditions, then it is probably a good IRS peak-up target.

(i) It is a point source.

(ii) It is brighter than 5 mJy (Blue) or 15 mJy (Red), but fainter than 150 mJy (Blue) or 340 mJy (Red).

(iii) It is not in a region of extended and/or bright IR emission.

3C 273 is effectively a point source, so satisfies condition (i). However, recall that the $12$ and $25 \; \mu {\rm m}$ IRAS flux densities of 3C 273 are 550 and 940 mJy (see §6.6), which violates condition (ii) by exceeding the allowed IRS peak-up flux limits for both the Blue and Red sub-arrays, respectively. Thus, we can already conclude that 3C 273 cannot itself be used as an IRS peak-up target. For the sake of completeness in this example, condition (iii) can be qualitatively checked by using the ``ISSA Image'' option in Spot's ``Images'' pull-down menu to download the $12$, $25$, and $100 \; \mu {\rm m}$ IRAS images of the region around the target (see Figure 6.4). This shows that 3C 273 is not located amidst bright, extended IR emission.

Figure 6.4: IRAS $12$ (left), $25$ (middle), and $100 \; \mu {\rm m}$ (right) images of a $1.5^{\circ }$ field around 3C 273 (bright white spot at the center of each image), displayed in Spot using the ``Images $\rightarrow$ ISSA Image'' menu.

A quantitative estimate of the local background level can be made using the ``Background'' option in the Spot Target window (see Figure 6.1). To access this feature, first click on the ``No Peak-Up'' tab^6.1 and then on ``OK'' in the IRS Staring Mode AOT entry window to dismiss it (this window can be recalled at any time by pressing the modify-AOR icon - a sheet of paper labeled ``AOR'' with an arrow - fifth from the left on the taskbar, or by selecting ``Modify current AOR'' from the ``Edit'' pull-down menu). Next, recall the Target window by pressing the modify-target icon (a bullseye with an arrow in it), ninth from the left on the taskbar, or by selecting ``Modify Target'' from the ``Targets'' pull-down menu. Finally, click on the ``Background'' button in the Target window. In the Background Estimate pop-up window, set a wavelength of $100 \; \mu {\rm m}$ and select the ``Calculate Range When Visible'' option, then click on the ``Do Estimate'' button (see Figure 6.5, left panel). This returns a table showing the breakdown of estimated $100 \; \mu {\rm m}$ background components during the range of target visibility windows (see Figure 6.5, right panel). The ISSA IRAS images should not be used to determine the background level, as the zodiacal light component has been removed from these images. In the case of 3C 273, Spot estimates the total local background surface brightness at $100 \; \mu {\rm m}$ to be between $\approx 8-11 \; {\rm MJy \; sr}^{-1}$.

Figure 6.5: (left) Spot Background Estimate entry window, which is available from the Target entry window. (right) Result of Spot's background estimate for 3C 273.

This can be compared to the plot of $100 \; \mu {\rm m}$ IR cirrus brightness vs. minimum peak-up source flux density in the IRS chapter of the SOM (Fig. 7.40), which shows that objects with flux densities of at least $\approx 5$ or $10 \; {\rm mJy}$ in the Blue or Red wavelength ranges, respectively, are acceptable against this level of cirrus background. The IR flux densities of 3C 273 (see above) would clearly satisfy condition (iii); however, 3C 273 has already been excluded from serving as its own peak-up target by exceeding the flux limits of condition (ii).

Try a 2MASS Star: If the science target is not a good peak-up target, then another peak-up target within $30^{\prime}$ must be located. It is recommended that the proposer first approach this problem by using the Spot feature that searches for suitable IRS peak-up targets from the 2MASS Point Source Catalog (2MASS-PSC). In the IRS Staring Mode AOT entry window, click on the ``2MASS selection'' button under the ``IRS Peak-Up'' tab. This will display a window (see Figure 6.6) asking for the local surface brightness at $100 \; \mu {\rm m}$ (in order to facilitate selecting the largest possible number of 2MASS peak-up stars that are bright enough against the local background). Either enter a known maximum local surface brightness (e.g., as estimated previously - see Figure 6.5) or click on the ``Calculate Surface Brightness'' button (which performs the same operation as the ``Background'' option in the Spot Target window). When complete, click on ``OK'' to dismiss the Surface Brightness window and initiate the 2MASS search.

Warning: Clicking on ``OK'' in the Surface Brightness window while the surface brightness is still set to the default value of $0.00 \; {\rm MJy \; sr}^{-1}$ will cause Spot to return all available 2MASS stars brighter than 10 mJy. Many of these will not actually be usable against the true, non-zero background level. If a surface brightness larger than $50 \; {\rm MJy \; sr}^{-1}$ is entered (or calculated), then Spot displays a warning that the surface brightness must be between $0.0$ and $50.0 \; {\rm MJy \; sr}^{-1}$. This limit is imposed because non-uniform structure accompanying backgrounds higher than $50 \; {\rm MJy \; sr}^{-1}$ will confuse the peak-up algorithm. In this case, available peak-up stars can still be obtained by setting the surface brightness to exactly $50.0 \; {\rm MJy \; sr}^{-1}$; however, the success of the peak-up algorithm using any of these stars is not guaranteed. It is recommended that an initial ``Peak-up Only'' observation be used to confirm the success (or failure) of the peak-up before attempting the spectrum exposure on the science target.

Figure 6.6: Spot Surface Brightness window, which is the first step in performing a search for valid IRS peak-up stars from the 2MASS-PSC.

When the 2MASS search is complete (this can take a few seconds to several minutes), Spot returns a table of ``Peakup Candidates'' (see Figure 6.7). The candidates are noted as either ``GOOD for Blue'' or ``Acceptable.'' The former are optimum targets for use in an IRS Blue peak-up, while the latter are, as the label implies, acceptable (but not optimum) for peak-up. (``GOOD for Red'' candidates might also be listed, if available, but these should only be used if an IRS Red peak-up has been selected.) The remaining columns in the table give the 2MASS J2000 coordinates of the candidates, their spectral types (based on the 2MASS colors), their flux densities (in mJy) at $16$ and $22 \; \mu {\rm m}$ (estimated from the spectral type), and the 2MASS observation epoch.

Figure 6.7: Spot's recommended IRS peak-up point sources from the 2MASS database. The selected peak-up target is highlighted.

Among multiple ``GOOD'' peak-up targets, preference should be given to the target that is, first, closest to the science target and, second, brightest among approximately equidistant peak-up targets. In this example, the more nearby of the two ``GOOD'' peak-up targets available for 3C 273 has been assigned by selecting it in the ``Peakup Candidates'' window and then clicking on ``OK.'' When we do this, the coordinates, epoch and flux density are automatically transferred to the ``Peakup Target'' box in the ``Peak-Up Settings'' portion of the IRS Staring Mode AOT window (Figure 6.7). However, the 2MASS database does not contain proper motions. The observer must enter the proper motion of the peak-up star (see Figure 6.8). In general, selecting a peak-up star with small proper motion will lead to better results from the peak-up process (especially when a high accuracy peak-up is desired). In order to avoid using high proper motion stars for peak-up, the proper motion of the selected peak-up star should be verified. In all cases, the correct proper motion values should be entered into the ``Peak-up Settings'' section of the AOT entry window. Accurate proper motions for a large number of stars can be found, for example, in the Tycho-2 catalog, which is a new reduction of measurements obtained by the Hipparcos satellite combined with ground-based data. Tycho-2 can be searched interactively at the CDS-VizieR web site, at

http://vizier.u-strasbg.fr

(or one of the local mirror sites available at that site). SIMBAD also often provides proper motions, although they have sometimes been obtained from catalogs with larger uncertainties than Tycho-2. If a given peak-up star is not listed in the Tycho-2 catalog, then every effort should be made to obtain the best-known proper motion (and position - see below) information from another literature source. If high accuracy astrometric information is not available from other sources, then it might be prudent to select a different peak-up star that is listed in the Tycho-2 catalog, if available. Another important factor to consider is that the coordinates in the 2MASS-PSC, although given for the standard epoch J2000.0, do not include the cumulative displacement, due to proper motion between the time when the image of a given object was obtained and J2000.0. As a general rule of thumb when using an offset peak-up, the combined uncertainty in the coordinates of the science target and peak-up target (including any unaccounted proper motion between the epoch of the coordinates and J2000.0) should be no larger than 1/3 of the desired positional accuracy. While this does not present an especially rigorous challenge for a Low or Moderate accuracy peak-up, special care must be exercised when planning a High accuracy peak-up. The ``Peak-up Only'' observing mode for IRS allows testing and verification of the success of a questionable peak-up before the actual spectroscopic exposure is obtained. The trade-off for this, of course, is a potentially significant addition to the total time required to complete the observing program. Ideally, the coordinates used for the selected peak-up star should correspond precisely to the standard epoch J2000.0. Most often, the coordinates for a 2MASS peak-up star are not the standard epoch and possess an unaccounted proper motion offset. Unlike previous versions of Spot, the current Spot (v15) can handle any non-standard epoch value in the peak-up settings section of the AOR. It is no longer necessary manually to adjust the 2MASS coordinates to epoch J2000.0; this is now done automatically within Spot. The example IRS 2MASS peak-up star is HD 108473, a $V = 9.28$ and $K_{\rm s} = 6.84$ mag G5 star. This star was actually observed by 2MASS on epoch 2000.15, with the equinox 2000.0 coordinates, RA=12:27:42.32, DEC=+01:50:14.8, shown for it in Fig. 5.7. Although Spot corrects the star's epoch, you will still need to manually input the proper motions for the peak-up star. The Tycho-2 catalog gives proper motions of $\mu_{\alpha} = -22.9 \; {\rm mas \; yr}^{-1}$ and $\mu_{\delta} = +9.5 \; {\rm mas \; yr}^{-1}$. These proper motions can now be entered into the ``Peakup Target'' box, keeping in mind that Tycho-2 gives proper motions in mas per year, but Spot requires them to be entered in arcseconds per year. It should be pointed out that the RA proper motions in arcsec/yr from Tycho-2 have already been multiplied by cosine of the declination as Spot requires. However, if an observer got an estimate of the proper motion from some other source in seconds/yr, then they would have to convert it to arcseconds/yr using the following formula:

$\mu_{\alpha} {\rm [arcseconds \; year^{-1}]} = 15.0 * \cos ({\rm DEC}) * \mu_{\alpha} {\rm [seconds \; year^{-1}]}$.

Proper motion uncertainties from Tycho-2 are $2.5 \; {\rm mas \; year}^{-1}$, so we will typically not have to worry about this correction over the Spitzer mission lifetime (e.g., the maximum combined positional error accumulated in 7 years amounts to only $0\hbox{$.\!\!^{\prime\prime}$}025$). Figure 6.8 shows the completed ``Peak-Up Settings'' section of the IRS Staring Mode AOT window configured for a High accuracy, Blue IRS peak-up on HD 108473, which was selected from the 2MASS-PSC.

Figure 6.8: Completed ``Peak-Up Settings'' portion of the IRS Staring Mode AOT for an IRS peak-up using a star selected from the 2MASS-PSC.

The peak-up algorithm assumes that the brightest object within the peak-up array field-of-view is the intended peak-up target, so it is also a good idea, after selecting a peak-up target, to examine the DSS, 2MASS, and/or IRAS images around the peak-up target to ensure that this condition is met (a reminder to do this is given in the ``Peakup Candidates'' window - see Figure 6.7). Both SIMBAD (via Aladin) and Spot (via the ``Images'' menu) offer means of doing this. Recall that the Blue IRS peak-up operates in the $13.3-18.7 \; \mu {\rm m}$ range, so it is useful to examine the IRAS $12 \; \mu {\rm m}$ image. Figure 6.9 shows the IRAS $12 \; \mu {\rm m}$, 2MASS $K_{\rm s}$, and DSS optical images of the field around the selected peak-up star, obtained using Spot's ``Images'' menu (this was accomplished by creating a new ``dummy'' target using the coordinates of the selected peak-up star - see §6.4).

Figure 6.9: IRAS 12 micron (left), 2MASS $K_{\rm s}$ (middle), and DSS optical (right) images of the peak-up star HD 108473, obtained using Spot's ``Images'' menu. The 2MASS and DSS images are displayed in reverse greyscale.

The 2MASS and DSS images shown in Figure 6.9 cover $1^{\prime} \times 1^{\prime}$, which is slightly larger than the size of the IRS peak-up array field-of-view in acquisition mode, and clearly show that there is no brighter object within this area. The IRAS image covers $1^{\circ} \times 1^{\circ}$. The peak-up star (located in the small box at the image center) is too faint to have been detected by IRAS (and the IRAS spatial resolution is too poor to show a 1 square arcminute field) but, at least, there is no overwhelmingly bright IR source (like the distant objects near the top of the IRAS image) located close to the peak-up star that would confuse the peak-up algorithm.

Find another peak-up star: If the science target is not a good peak-up target, and there is no ``GOOD'' or ``Acceptable'' star available from 2MASS, then another usable IRS peak-up object must be manually located. Care should be taken to ensure that the selected target meets all of the criteria described above; namely, that it is located within $30^{\prime}$ of the science target, is the brightest object within $1^{\prime}$ of itself in the selected peak-up passband, has flux levels within the acceptable range for the selected peak-up, and is also bright enough against the background of local IR cirrus. Although Spot now excludes stars as candidate peak-ups if their neighbors are too bright, it cannot do this for neighbor stars that are exceedingly bright. So, it is very important that observers visualize their peak-ups! Only when all attempts to locate a point source peak-up target have failed should an extended source be used for the peak-up target.