Supernova in M101

A somewhat rare occurrence in the local universe is a supernova. A supernova is the end stage of a star, typically when the star gravitationally collapses due to exhausting the matter that it is fusing within it. Stars are quite the balancing act, as the energy released by the fusion of matter within restricts the gravitational attraction of the matter itself. When massive stars run out of matter to fuse, typically at the iron stage, gravity is able to compress the iron in the core into pure neutrons, with a density of that of an atomic nucleus. If this core is massive enough, it can become a black hole. Otherwise, the neutron core is compressed to the limits of neutron degeneracy pressure and can have a temperature of 100,000,000,000 Celsius. The outer shells of the star, no longer supported by fusion in the core, collapse and “bounce” off of the neutron core, causing shockwaves that result in the explosion. A white dwarf will remain.

This supernova was discovered by amateur astronomer Koichi Itagawa on 2023-05-19 17:27:10UTC. It is named SN 2023ixf. The estimated eruption time is 2023-05-18 19:49:36UTC.

Location

The supernova is located in a star-forming region in an outer spiral arm of M101. This galaxy is a target that I recently imaged, and I took some more data on the galaxy to show the location of the supernova. 90 images of 45 second exposure were taken on May 21-22, 2023 from 11:41 to 00:56 UTC-5. Figure 1.1 shows the location of the supernova, and Figure 1.2 shows the subject compared to pre-discovery. It is located at 14 3 38.56 +54 18 42.0 (J2000).

Photometry

Supernovae are extremely powerful and luminescent events. Some of them at peak can outshine their entire host galaxy. With the data I took on this subject, I used the AperturePhotometry script in PixInsight to estimate the flux and brightness. Table 2.1 shows the G-band magnitudes of local stars from GAIA DR2 and the estimated apparent magnitude of SN 2023ixf by calculating the apparent magnitude as

m + 2.5\log_{10}\frac{b}{b_{sn}}

where b is the flux of the reference star, b_sn is the flux of the supernova, and m is the G-band apparent magnitude of the reference star. The measured flux of the supernova was 2.57289. The average of the magnitude conversions gives the supernova an estimated G-band apparent magnitude of 11.2. The accuracy of this is unknown since I am using a green channel from a OSC camera, and the green filter is likely to not match the Johnson/Cousins filters used by professional telescopes. However, other reports of the magnitude around this time that I have read match my estimate pretty well.

StarId	RA	Dec	Catalog_Gmag	PSF_Flux_1		Relative mag
SN 2023ixf	210.910658333333	54.3116666666667		2.57289
_1609307826153070464	211.102293452903	54.653578455938	10.1705	6.200219		11.1254645265555
_1609314938618876288	210.927451130937	54.844140289656	10.5602	4.301788		11.1182694618587
_1609308272828044288	210.972673735646	54.6384669038205	10.8203	3.189927		11.0536988138402
_1609321638767836288	210.432803601206	54.7384664523923	11.0086	3.294395		11.2769861278149
_1513187969896570880	210.272254268827	53.9241118478619	11.2303	2.413923		11.161055484317
_1609322051084697600	210.422496787806	54.7853797365819	11.2585	2.673311		11.3000706659911
_1609296040762764672	210.391997068185	54.5950413484867	11.3261	2.313922		11.2109187405554
_1609292256894781696	210.551504657823	54.5260337811976	11.6141	1.941964		11.3086498892932
_1609223984094652288	210.957784613434	54.1515141425831	11.6509	1.718367		11.2126367619499
_1609407980493607040	211.102190503551	54.844492324376	11.6603	1.741809		11.2367482780797
_1609317515599247104	210.541207701838	54.6664517415881	11.6865	1.728897		11.254869754256
_1609232543966385152	211.368446526044	54.1254867600814	11.8273	1.430443		11.1899233445035
_1609250441093191808	211.090333466939	54.3228979372932	11.932	1.373125		11.250222138144
					Mean	11.2076549220892

According to NED, M101 is at a distance of 17.4Mly (5.34Mpc) and has an apparent R-band magnitude of 7.76. The absolute magnitude, or the brightness of an object if viewed from a distance of 10 parsecs, is related to the apparent magnitude m_a and distance in parsecs d by

m_v = m_a - 2.5\log_{10} \left(\frac{d}{10} \right) ^ 2

This gives M101 an absolute magnitude of -20.88 and the supernova an absolute magnitude of -17.43. The supernova, at the time of observation, was about 3.45 magnitudes, or 24 times, dimmer than its host galaxy. With it estimated that M101 is composed of a trillion stars, one exploding star emits as much light as 40 billion stars!

Spectroscopy

Using my ALPY 600 spectroscope and ASI183MM camera, I took 140 minutes of data on this supernova on May 23, 2023, from 00:07 to 02:18 UTC-5. I used a common CFL lamp for calibration, a halogen lamp for flat correction, and the star Alkaid for instrument response correction. Figure 3.1 shows the synthetic spectra used for analysis.

Figure 3.2 shows the calibrated, flat- and response-corrected spectrum. The spectrum shows the Balmer series of hydrogen emission, classifying it as a Type-II supernova. This means that it is the result of the collapse of a massive star. A 17,000K Planck curve is included, giving an approximation to the temperature of the explosion.

Continuum removal and velocity correction reveals the emission lines, as shown in Figure 3.3. The largest spike is H-α at 656.3nm, which is present only in Type-II supernovae. Other elements of the Balmer series are prominent, as well as He-2 at 468nm. A carbon doublet is also present around 580nm. Absorption at the high end of the graph is due to tellurics. The H-β line shows what appears to be a P-Cygni profile, but it is not noticeable in the H-α line. This is due to circumstellar material being affected by the progenitor wind.

Data on the most prominent lines are given in Table 3.1. The mean of the observed velocities is 214km/s, which is pretty close to the accepted value for M101 at 243km/s. Using the FWHM of each emission line gives an average expansion velocity of 615km/s, which is in agreement with professional observations.

Element	Rest	Observed	z	Doppler Velocity	FWHM	Doppler Width
H-α	656.3	656.9	0.000914	274075.08	1.65	376509
H-β	486.1	486.5	0.000823	246692.00	2.01	619304
H-γ	434	434.1	0.000230	69076.60	2.71	935772
H-δ	410.2	410.6	0.000975	292337.84	0.75	273800
He II	468.5	468.8	0.000640	191969.56	2.72	869705
Mean			0.000717	214830	1.97	615018