---
abstract: 'We propose that the best sites to search for SGRs outside the Local group are galaxies with active massive star formation. Different possibilities to observe SGR activity from these sites are discussed. In particular we searched for giant flares from M82, M83, NGC 253, and NGC 4945 in the BATSE data. We present a list of potential candidates, however, in our opinion no good candidates alike giant SGR flares were found. Still, hyperflares similar to the one of 27 December 2004 can be observed from larger distances. From the BATSE data we select 5 candidates coincident with the galaxies Arp 299 and NGC 3256 which have very high rate of starformation, and propose that they can be examples of hyperflares; however, this result has low statistical significance.'
author:
- |
    S.B. Popov$^{1}$ [^1]\
    $^1$[*Sternberg Astronomical Institute, Universitetski pr. 13, Moscow 119992, Russia*]{}
date: 'Accepted ...... Received ......; in original form ...... '
title: Soft gamma repeaters and starforming galaxies
---

=-0.5 in

gamma rays: bursts —

Introduction
============

Sources of soft gamma repeaters (SGRs) are one of the most puzzling types of neutron stars. Now at least four of them are known in our Galaxy and in the Large Magellanic Cloud (we refer to @wt2004 for a recent summary of all properties of SGRs). SGRs show three main types of bursts:

-   weak bursts, $L\la 10^{41}$ erg s$^{-1}$;

-   intermediate bursts, $L\sim10^{41}$–$10^{43}$ erg s$^{-1}$;

-   giant flares, $L\la 10^{45}$ erg s$^{-1}$.

Weak bursts are relatively frequent. About several hundreds were detected from 4 sources during $\sim 25$ yrs, i.e. the average rate is about 1 per month per source. However, these bursts appear in groups during periods of activity of a SGR. Duration is very short, $\sim 0.1$ s. Intermediate bursts have typical durations $\sim$ few seconds and are much more rare. These two types of bursts will not be discussed below.

Giant flares (GFs) are very rare. Only four GFs were observed (however, some authors do not include the burst of SGR 1627-41 on June 18 1998 into this list as it is slightly weaker then the others and shows a different pulse structure). These flares are extremely energetic. Typical duration of the initial spike is about one second or smaller. The rate of GFs is very uncertain as only four were detected, usualy it is assumed to be just (1/20 - 1/30) yrs$^{-1}$ per source. The latest GF detected on 27 December 2004 was suggested to be a representative the forth class of bursts – “supergiant flares” or “hyperflares” (HFs). It exceeds previous GFs in the energy release by two orders of magnitude, and below we consider this kind of events separately as they can be observed from larger distances, and potentially can contribute to short-hard GRBs detected by BATSE and other satellites (see @ngpf2005 [@h2005] and references therein).

Being very interesting objects SGRs are very rare, probably due to a short life cycle, $10^4$ yrs, which can be connected with a short duration of a magnetar activity. It would be very important to detect these sources outside the Local group of galaxies to increase the sample. Especially it is interesting to understand the birth rate of SGRs and the fraction of NSs which appear as these sources. Any solid data (even upper limits) on the number of GFs and HFs detected from outside of the Local group can help here a lot.

Here we want to discuss a possibility to observe SGRs outside the Local group of galaxies (for previous discussions of extragalactic SGRs see @d2001 and recent e-prints by @ngpf2005 [@h2005]). Detection of such objects can give an opportunity to estimate a fraction of NSs which appear as SGRs on larger statistics. In this short note [^2] we mainly focus on regions of active starformation.

We will discuss two approaches to find GFs and/or HFs from sources outside the Local group:

-   Close-by ($\la$10 Mpc) galaxies with high starformation rate should give the main contribution for detection of GFs.

-   Few galaxies with extreme values of starformation rate (so-called “supernova factories”) are the best sights to search for rare HFs.

Giant flares from local galaxies in the BATSE catalogue
=======================================================

As it is discussed by Heckman (1998) inside 10 Mpc about 25% of starformation is due to only four well known galaxies: M82, NGC 253, NGC 4945, and M83 (see Table 1). As probably BATSE could detect GFs similar to the prototype event of 5 March 1979 [@m1979] only from short distances ($\la 3$-$5$ Mpc), the contribution of these four galaxies can be even more important. The main idea which we put forward here is the following: close-by galaxies with high present day star formation rate are the best sites to search for SGRs outside the Local group.

  Name         Distance, Mpc   SN rate per year
  ---------- --------------- ------------------
  M 82                   3.4            0.1-0.6
  NGC 253                2.5            0.1-0.3
  NGC 4945               3.7            0.1-0.5
  M 83                   3.7            0.1-0.5

  : Close galaxies with high starformation rate

Supernova (SN) rates presented in the table are approximate ones as there are no estimates better than a factor of 2-3 precision. [^3] We collected data from different papers. For example, the rate for NGC253 is taken from @e1998 (see also @p2001). In the following we will use the conservative intermediate values: 0.4, 0.2, 0.2, and 0.1 for M82, NGC 253, NGC 4945, and M83 correspondently. In comparison with the Galactic rate of SN, these galaxies have significant enhancement (roughly 12, 6, 6, and 3 times correspondently), so we can expect proportionally higher number of SGRs (and GFs from them). With the galactic rate $\sim $ 4 flares in 25 years, for BATSE (4.75 years equivalent of all-sky coverage) we can expect roughly 8-9 GFs from M82, 4-5 GFs from each of NGC 253 and NGC 4945, and 2 GFs from M83 (it total about 20 GFs from four galaxies during the BATSE life cycle).

It is useful to check if in the BATSE catalogue[^4] there are potential SGR-candidates in these four galaxies. We have to look for short bursts with $T_{90}$ at least less than 2 seconds (the burst from SGR 1627-41 was longer than initial strong spikes from three other SGRs). Another criterium is fluence. Taking into account large distances to host galaxies we do not expect SGR candidates to be bright (expected fluences $\la 10^{-7}$ erg cm$^{-2}$). Then we have to select only relatively soft bursts, as GFs are softer than typical GRBs[^5].

All GRBs for which at least one of the four galaxies appears inside the error box are given in Table 2. For each burst we give its number, coordinates, error box radius, $T_{50}$ and $T_{90}$, maximum fluence (maximum among four channels), and softness (the ratio of the 1st to the 3rd channel). Coordinates and error box radii are given in degrees.

  ---------- ---------- ---------- ------- ----------- ----------- ------------ ----------
  Trigger      $\alpha$   $\delta$   Error   $T_{50}$,   $T_{90}$,      Maximum   Softness
  number                               box           s           s      fluence 
                                                                                
                                                                                
  M82                                                                           
  2054           164.33      66.15   17.91           —           —            —          —
  2160           150.84      68.81    2.15      11.072     123.136            —          —
  2660           157.28      70.08    3.02       6.464      16.896   79.451e-08      0.613
  2821           124.84      60.59   13.53       0.152       0.392   67.355e-09      0.122
  3118           117.57      80.37   34.15       0.136       0.232   18.628e-08      0.059
  3915            96.66      65.27   61.91       0.080       0.200            —          —
  6219           170.66      70.18    9.54       1.856       2.752   1.1506e-07      0.014
  6255           148.68      60.79   12.71           —           —            —          —
  6488           155.76      76.41    9.94       1.152       2.240   6.3462e-07      0.333
  6547           155.18      62.23   13.58       0.029       0.097   1.0350e-07      0.129
  7297           140.07      76.39    9.53       0.438       1.141   6.5662e-07      0.050
  7552           137.56       65.9    6.15      11.648      63.296   5.8479e-07      1.122
  7970           136.87      64.49    8.48       0.157       0.387   6.0876e-08      0.343
                                                                                
  M83                                                                           
  1485           202.01     -29.98    9.65           —           —   50.284e-08      0.501
  1510           198.84     -34.35    7.29           —           —            —          —
  2349           203.15     -31.88    6.19       1.696       4.032   21.409e-08      0.426
  2384            203.8     -18.21   17.81       0.128       0.192   84.657e-09      0.309
  2596           211.51     -27.07   19.74           —           —            —          —
  2756           209.39     -23.16    9.65           —           —            —          —
  5444           199.44     -31.51    4.94           —           —            —          —
  6447           191.44      -36.6   14.77       0.256       1.024   6.0229e-08      0.159
  6708           206.73      -29.7    3.02       3.904      12.160   1.0452e-06      0.051
  7361           204.17     -28.29    7.28       0.960       1.856   7.8501e-08      0.121
  7385           203.02     -27.81    3.59           —           —            —          —
  8076           199.39     -29.98    7.39       0.075       0.218   3.1075e-07      0.072
                                                                                
  NGC 253                                                                       
  2140             9.08     -24.03    4.34       6.784      19.072   48.766e-08      1.070
  2312            14.72     -33.56    8.93       0.112       0.272   45.113e-08      0.082
  2325             12.7     -24.78   13.55      16.448      22.528   39.642e-08      0.218
  3908            11.56     -27.25    7.96       5.344      13.920   59.805e-08      0.203
  6135             9.06     -22.84    8.79       1.856       4.032   1.4354e-06      0.119
  6648            10.26     -26.97    2.52      24.512      88.384   1.7639e-06      0.061
  6918             4.86     -30.74   13.24           —           —            —          —
  7551            13.78     -24.45    3.75      53.248     119.616   6.8409e-07      1.665
                                                                                
  NGC 4945                                                                      
  108            201.31     -45.41   13.78       1.280       3.136   14.604e-07      0.259
  1167            209.4     -45.74   10.04       7.552      24.576   22.754e-08      0.330
  2405           197.74     -44.32   11.28      25.088      64.014   83.646e-08      0.155
  2800           200.29     -47.94   15.92       0.320       0.448   35.153e-08      0.072
  3895           189.39     -47.72    6.99       0.384       0.768   42.483e-09      0.135
  6447           191.44      -36.6   14.77       0.256       1.024   6.0229e-08      0.159
                                                                                
  ---------- ---------- ---------- ------- ----------- ----------- ------------ ----------

There are many short burst coincident with one of the four selected galaxies. (still, a significant number can be projected onto the region around a particular galaxy by chance). Only one of them – 7970 – is soft. So, only this burst can be considered as a reasonable candidate. However, $T_{90}=0.387$ s seems to be long for a typical GF. The main conclusion of this analysis can be such that we found no good candidates GFs from hypothetical SGRs in M82, M83, NGC 253, and NGC 4945. However, if our assumption about softness of GFs can be soften several candidates can be proposed. This result can be used to put constraints on the number of SGR sources in these galaxies, on the frequency of GFs or/and on the maximum flux during a GF (Popov, Stern in prep.).

[@lrrrrrrr@]{} Trigger &$\alpha$ & $\delta$ & Error & $T_{50}$,& $T_{90}$, & Maximum &\
number & & & box & s & s & fluence\
&&&&&&\
&&&&&&\
Arp 299 &&&&&&\
2265 & 180.2 & 59.57 & 8.32 & 0.256 & 0.456 &10.557e-08\
3118 & 117.57 & 80.37 & 34.15& 0.136 & 0.232 & 18.628e-08\
6547 & 155.18 & 62.23 & 13.58 & 0.029 & 0.097 & 1.0350e-07\
&&&&&&\

NGC 3256 &&&&&&\
2372 & 161.1& -36.01& 8.84& 0.072 & 0.256& 64.178e-09\
2485 & 173.45 & -40.09 & 18.39& 0.128 & 0.176 & 34.750e-09\
\
&&&&&&\

Hyperflares from Virgo and from galaxies with extreme starformation rates
=========================================================================

The situation with HFs (like the 27 Dec 2004 one) is quite different as the BATSE maximum detection distance for such events is larger by about an order of magnitude. Having a limiting distance $\approx 50$ Mpc we can roughly estimate a number of GFs and HFs from this volume. We assume that this number can be scaled from the galactic one using the number of galaxies or the starformation rate (SFR). Inside 50 Mpc we can use several estimates. For example, @d2001 uses the following expression to obtain an estimate of a number of galaxies similar to the Milky Way: $
N_{Gal}=0.0117 \,  h_{65}^3 \, R_{Mpc}^3. $ For $R=50$ Mpc we obtain about 1500 galaxies. So, for 4.5 years of observation we can expect nearly 800 GFs and about 200 HFs assuming 3 GFs and 1 HF observed in the Milky Way in 25 years. Similar estimates can be obtained using estimates of @bcw2004 and @gza1995. @bcw2004 provide the following value for SFR density at $z=0.1$: $0.01915\, M_{\odot}/{\rm yr}/{\rm Mpc}^3$. Inside 50 Mpc it gives $\approx 10^4\, M_{\odot}/{\rm yr}/{\rm Mpc}^3$. SFR for the Milky way is estimated to be few solar masses per year. So, the ratio is about few thousands. @gza1995 estimate SFR in star-forming galaxies for $z\la0.045$ as $0.013 \, M_{\odot}/{\rm yr}/{\rm Mpc}^3$. It gives $\approx 6800 \,
M_{\odot}/{\rm yr}/{\rm Mpc}^3$ inside 50 Mpc. All three estimates are in good correspondence. So, having one HF in $\sim$30 years in our Galaxy we can expect few hundreds HFs during the BATSE lifetime potentially detectable by this satellite.

The largest structure up to $R \sim 50$ Mpc is the Virgo cluster of galaxies (see @bts1987 for all details about the cluster). It includes about 1300 galaxies (including 130 spirals). BATSE should be able to detect HFs from Virgo cluster as fairly strong bursts. It is important to estimate an expected number of GFs and HFs from Virgo. However, there are several galaxies with significantly enhanced starformation. So, roughly we can estimate that the SFR in Virgo is about few hundreds time larger than in our galaxy. We can expect up to one hundred HFs during the BATSE lifetime if we assume the rate in the Galaxy about 1 in 30-40 years.

Despite these optimistic predictions no anisotropy in distribution of short GRBs or any correlations with known type of objects were found. Here we want to adress another possibility — observations of HFs by BATSE from particular galaxies outside the Virgo cluster. Of course, this large volume ($R\la 50$ Mpc) cannot be dominated by few starforming galaxies, nevertheless, some peculiar objects can be considered as important targets to search for HFs from extragalactic SGRs.

In the Universe there is a small amount of galaxies with extreme SFR – “supernova factories”. They have core collapse SN rate up to two orders of magnitude higher than in the Milky Way. Up to the limiting distance of detection of a HF by BATSE there are two prominent objects of that kind: Arp 299 [@n2004] and NGC 3256 [@l2004]. We propose to look for HF candidates in the direction of these two galaxies. [^6]

Knowing a SN rate in a given galaxy we can obtain an estimate of a HF rate. In the Galaxy only one HF in 30 years was observed. (By the way it is roughly coincedent with the core collapse SN rate.) The SN rates in Arp 299 and NGC 3256 are about 1 per year. So, we can expect about 1 HF per year from each of them. During the BATSE lifetime few such events can be expected.

The fluence for the 27 Dec event was estimated in @h2005 as 1.36 erg cm$^{-2}$. The distance is about 15 kpc. For 40 Mpc (distance to Arp 299 and NGC 3256) we can expect fluences about $10^{-7}$ erg cm$^{-2}$.

In the BATSE catalogue [^7] we looked for short GRBs with known timing and fluences coincident with Arp 299 and NGC 3256. On the whole (among 2704 GRB coordinates of which we used) error boxes of 12 appeared to be coincident with Arp 299 and 6 with NGC 3256. From these set five short hard bursts with known fluences were selected (Table 3): three from the direction of Arp 299 (for one another very short burst – the trigger number 3915 – fluences are not given) and two from NGC 3256 (however, another burst with the trigger number 6278 can be a possible candidate). All five GRBs look like short spikes in the BATSE data. We propose that these 5 GRBs can be good candidates to be HFs. Of course, some amount of events can be coincident with these galaxies by chance, and such a probability is not low.[^8] Still, we think that our finding is worth discussing. Future and present day missions with better angular resolustion (like [*Swift*]{} and, probably, [*Integral*]{} and [*HETE*]{}) can shed light on the association of short GRBs with galaxies with high SFR.

Conclusions
===========

We discussed the connection between SGRs and starforming galaxies. Our suggestion is that few well know galaxies with large SFR are the best candidate sites to look for SGR flares. For the case of GFs we especially mention such close-by galaxies as M82, M83, NGC 253, NGC 4945. For HFs we point to “supernova factories” Arp 299 and NGC 3256.

We found 5 candidates (3 in the direction of Arp 299 and 2 in the direction of NGC 3256) which can be HFs.

Acknowledgments {#acknowledgments .unnumbered}
===============

I want to thank Drs. B.E. Stern and M.E. Prokhorov for many discussions and suggestions. The work was supported by the RFBR grants 04-02-16720 and 03-02-16068.

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[^1]: E-mail: polar@sai.msu.ru

[^2]: This is just a brief discussion note, it is submitted only to the ArXiv, and should be refered by its astro-ph number. More elaborated results will be presented elsewhere (Popov, Stern in prep.).

[^3]: Note, that distances are also uncertain, but the precision for them is much better.

[^4]: http://cossc.gsfc.nasa.gov/batse/

[^5]: A hard tail in the spectrum of 5 March event starts at $E>430$ keV [@g1979], so it doesn’t influence the third channel with $100<E<300$ keV; see also fig. 14.8 in [@wt2004]

[^6]: @h2005 suggested that flares even stronger then the one of Dec 27 can be expected from younger SGRs. Without any doubts the best place to look for them are these two galaxies (and objects similar to them).

[^7]: http://cossc.gsfc.nasa.gov/batse/

[^8]: For example, three bursts with large error boxes – 3118, 3915 and 6547 – appeared to be coincident both with M82 and Arp 299; and 6447 with M83 and NGC 4945.
